Psd Ceu 169julyaug10

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169

Recircling Domestic Hot Water Systems

Continuing Education from Plumbing Systems & Design

JULY/AUGUST 2010

PSDMAGAZINE.ORG

INTRODUCTIONIt has been determined through field studies that the correct sizing and operation of water heaters depend on the appropriateness of the hot water maintenance system. If the hot water maintenance system is inadequate, the water heater sizing criteria are wrong and the temperature of the hot water distributed to the users of the plumb-ing fixtures is below acceptable standards. Additionally, a poorly designed hot water maintenance system wastes large amounts of energy and potable water and creates time delays for those using the plumbing fixtures. This chapter addresses the criteria for establish-ing an acceptable time delay in delivering hot water to fixtures and the limitations of the length between a hot water recirculation system and plumbing fixtures. It also discusses the temperature drop across a hot water supply system, types of hot water recirculation system, and pump selection criteria, and gives extensive information on the insulation of hot water supply and return piping.

BaCkgROUNDIn the past, the plumbing engineering community considered the prompt delivery of hot water to fixtures either a requirement for a project or a matter of no concern. The plumbing engineer’s decision was based primarily on the type of facility under consideration and the developed length from the water heater to the farthest fixture. Previous reference material and professional common practices have indicated that, when the distance from the water heater to the farthest fixture exceeds 100 ft (30.48 m) water should be circulated. However, this recommendation is subjective, and, unfortunately, some engineers and contractors use the 100-ft (30.48-m) criterion as the maximum length for all uncirculated, uninsulated, dead-end hot water branches to fixtures in order to cut the cost of hot water distribution piping. These long, uninsulated, dead-end branches to fixtures create considerable problems, such as a lack of hot water at fixtures, inadequately sized water heater assemblies, and thermal temperature escalation in showers.

The 100-ft (30.48-m) length criterion was developed in 1973 after the Middle East oil embargo, when energy costs were the paramount concern and water conservation was given little consideration. Since the circulation of hot water causes a loss of energy due to radiation and convection in the circulated system and such energy losses have to be continually replaced by water heaters, the engineering commu-nity compromised between energy loss and construction costs and developed the 100-ft (30.48-m) maximum length criterion.

LeNgTh aND TIme CRITeRIaRecently, due to concern about not only energy conservation but also the extreme water shortages in parts of the country, the 100-ft (30.48-m) length criteria has changed. Water wastage caused by the long delay in

obtaining hot water at fixtures has become as critical an issue as the energy losses caused by hot water temperature maintenance systems. To reduce the wasting of cooled hot water significantly, the engineer-ing community has reevaluated the permissible distances for uncir-culated, dead-end branches to periodically used plumbing fixtures. The new allowable distances for uncirculated, dead-end branches represent a trade-off between the energy utilized by the hot water maintenance system and the cost of the insulation, on the one hand, and the cost of energy to heat the excess cold water makeup, the cost of wasted potable water, and extra sewer surcharges, on the other hand. Furthermore, engineers should be aware that various codes now limit the length between the hot water maintenance system and plumbing fixtures. They also should be aware of the potential for liability if an owner questions the adequacy of their hot water system design.

What are reasonable delays in obtaining hot water at a fixture? For anything beside very infrequently used fixtures (such as those in industrial facilities or certain fixtures in office buildings), a delay of 0 to 10 sec is normally considered acceptable for most residential occu-pancies and public fixtures in office buildings. A delay of 11 to 30 sec is marginal but possibly acceptable, and a time delay longer than 31 sec is normally considered unacceptable and a significant waste of water and energy. Therefore, when designing hot water systems, it is prudent for the designer to provide some means of getting hot water to the fixtures within these acceptable time limits. Normally this means that there should be a maximum distance of approximately 25 ft (7.6 m) between the hot water maintenance system and each of the plumbing fixtures requiring hot water, the distance depending on the water flow rate of the plumbing fixture at the end of the line and the size of the line. (See Tables 1, 2, and 3.) The plumbing designer may want to stay under this length limitation because the actual installation in the field may differ slightly from the engineer’s design, and additional delays may be caused by either the routing of the pipe or other problems. Furthermore, with the low fixture discharge rates now mandated by national and local laws, it takes considerably longer to obtain hot water from non-temperature maintained hot water lines than it did in the past, when fixtures had greater flow rates. For example, a public lava-tory with a 0.50 or 0.25 gpm (0.03 or 0.02 L/sec) maximum discharge rate would take an excessive amount of time to obtain hot water from 100 ft (30.48 m) of uncirculated, uninsulated hot water piping. (See Table 3.) This table gives conservative approximations of the amount of time it takes to obtain hot water at a fixture. The times are based on the size of the line, the fixture flow rate, and the times required to replace the cooled off hot water, to heat the pipe, and to offset the convection energy lost by the insulated hot water line.

Recirculating Domestic Hot Water Systems

Domestic Water Heating Design Manual II, Chapter 14: “Recirculating Domestic Hot Water Systems,” © American Society of Plumbing Engineers, 2006

Note: All decimal equivalencies in the metric calculations are rounded. Therefore, the metric conversions shown in the text may vary slightly from the answers shown in the metric equations.

WWW.PSDMAGAZINE.ORG2 Plumbing Systems & Design JULY/AUGUST 2010

CONTINUINg eDUCaTION

Table 3(M) Approximate Time Required to Get Hot Water to a Fixture

Delivery Time (sec)Fixture Flow 0.03 0.10 0.16 0.25 Rate (L/sec)

Piping 3.1 7.6 3.1 7.6 3.1 7.6 3.1 7.6 Length (m)

Copper DN15 25 63a 8 21 5 13 3 8 Pipe DN22 48a 119a 16 40a 10 24 6 15

Steel Pipe DN15 63a 157a 21 52a 13 31a 8 20 Sched. 40 DN20 91a 228a 30 76a 18 46a 11 28

CPVC Pipe DN15 64a 159a 21 53a 13 32a 8 20 Sched. 40 DN20 95a 238a 32 79a 19 48a 12 30

Note: Table based on various fixture flow rates, piping materials, and dead-end branch lengths. Calculations are based on the amount of heat required to heat the piping, the water in the piping, and the heat loss from the piping. Based on water temperature of 60°C and an air temperture of 21.1°C.

a Delays longer than 30 sec are not acceptable.

ResULTs Of DeLays IN DeLIveRINg hOT WaTeR TO fIxTURes

As mentioned previously, when there is a long delay in obtaining hot water at the fixture, there is significant wastage of potable water as the cooled hot water supply is simply discharged down the drain unused. Furthermore, plumbing engineers concerned about total system costs should realize that the cost of this wasted, previously heated water must include: the original cost for obtaining potable water, the cost of previously heating the water, the final cost of the waste treatment of this excess potable water, which results in larger sewer surcharges (source of supply to end disposal point), and the cost of heating the new cold water to bring it up to the required tem-perature. Furthermore, if there is a long delay in obtaining hot water at the fixtures, the faucets are turned on for long periods of time to bring the hot water supply at the fixture up to the desired tempera-ture. This can cause the water heating system to run out of hot water and make the heater sizing inadequate, because the heater is unable to heat all the extra cold water brought into the system through the wastage of the water discharged down the drain. In addition, this extra cold water entering the hot water system reduces the hot water supply temperature. This exacerbates the problem of insufficient hot water because to get a proper blended temperature more lower tem-perature hot water will be used to achieve the final mixed water tem-perature. (See Chapter 1, Table 1.1.) Additionally, this accelerates the downward spiral of the temperature of the hot water system.

Another problem resulting from long delays in getting hot water to the fixtures is that the fixtures operate for longer than expected peri-ods of time. Therefore, the actual hot water demand is greater than the demand normally designed for.

Therefore, when sizing the water heater and the hot water piping distribution system, the designer should be aware that the lack of a proper hot water maintenance system can seriously impact the required heater size.

meThODs Of DeLIveRINg ReasONaBLy PROmPT hOT WaTeR sUPPLy Hot water maintenance systems are as varied as the imaginations of the plumbing engineers who create them. They can be grouped into three basic categories, though any actual installation may be a com-bination of more than one of these types of system. The three basic categories are1. Circulation systems.2. Self-regulating heat trace systems.3. Point-of-use water heaters (include booster water heaters).

Table 1 Water Contents and Weight of Tube or Piping per Linear Foot Copper Copper Steel Pipe CPVC Pipe Nominal Pipe Pipe Schedule Schedule Diameter Type L Type M 40 40 Water Wgt. Water Wgt. Water Wgt. Water Wgt. (in.)a (gal/ft) (lb/ft) (gal/ft) (lb/ft) (gal/ft) (lb/ft) (gal/ft) (lb/ft)½ 0.012 0.285 0.013 0.204 0.016 0.860 0.016 0.210¾ 0.025 0.445 0.027 0.328 0.028 1.140 0.028 0.2901 0.043 0.655 0.045 0.465 0.045 1.680 0.045 0.4201¼ 0.065 0.884 0.068 0.682 0.077 2.280 0.078 0.5901½ 0.093 1.14 0.100 0.940 0.106 2.720 0.106 0.710

a Pipe sizes are indicated for mild steel pipe sizing.

Table 1(M) Water Contents and Weight of Tube or Piping per Meter

Copper Copper Steel Pipe CPVC Pipe Nominal Pipe Pipe Schedule Schedule Diameter Type L Type M 40 40 Water Wgt. Water Wgt. Water Wgt. Water Wgt. (mm)a (L) (kg) (L) (kg) (L) (kg) (L) (kg)DN15 0.045 0.129 0.049 0.204 0.061 0.390 0.061 0.099DN20 0.095 0.202 0.102 0.328 0.106 0.517 0.106 0.132DN25 0.163 0.297 0.170 0.465 0.170 0.762 0.170 0.191DN32 0.246 0.401 0.257 0.682 0.291 1.034 0.295 0.268DN40 0.352 0.517 0.379 0.940 0.401 1.233 0.401 0.322

a Pipe sizes are indicated for mild steel pipe sizing.

Table 2 Approximate Fixture and Appliance Water Flow Rates

Maximum Flow RatesaFittings GPM L/SecLavatory faucet 2.0 1.3

Public non-metering 0.5 0.03Public metering 0.25 gal/cycle 0.946 L/cycle

Sink faucet 2.5 0.16Shower head 2.5 0.16Bathtub faucets

Single-handle 2.4 minimum 0.15 minimumTwo-handle 4.0 minimum 0.25 minimum

Service sink faucet 4.0 minimum 0.25 minimumLaundry tray faucet 4.0 minimum 0.25 minimumResidential dishwasher 1.87 aver 0.12 averResidential washing machine 7.5 aver 0.47 aver

a Unless otherwise noted.

Table 3 Approximate Time Required to Get Hot Water to a Fixture

Delivery Time (sec)Fixture Flow 0.5 1.5 2.5 4.0 Rate (gpm) Piping 10 25 10 25 10 25 10 25 Length (ft)

Copper ½ in. 25 63a 8 21 5 13 3 8 Pipe ¾ in. 48a 119a 16 40a 10 24 6 15

Steel Pipe ½ in. 63a 157a 21 52a 13 31a 8 20 Sched. 40 ¾ in. 91a 228a 30 76a 18 46a 11 28

CPVC Pipe ½ in. 64a 159a 21 53a 13 32a 8 20 Sched. 40 ¾ in. 95a 238a 32 79a 19 48a 12 30

Note: Table based on various fixture flow rates, piping materials, and dead-end branch lengths. Calculations are based on the amount of heat required to heat the piping, the water in the piping, and the heat loss from the piping. Based on water temperature of 140°F and an air temperture of 70°F.

a Delays longer than 30 sec are not acceptable.

JULY/AUGUST 2010 Plumbing Systems & Design 3

Circulation Systems for Commercial, Industrial, and Large Residential ProjectsA circulation system is a system of hot water supply pipes and hot water return pipes with appropriate shutoff valves, balancing valves, circulating pumps, and a method of controlling the circulating pump. The diagrams for six basic circulating systems are shown in Figures 1 through 6.


CONTINUING EDUCATION: Recirculating Domestic Hot Water Systems

Fixture 1 Upfeed Hot Water System with Heater at Bottom of System.* See text for requirements for strainers.

Figure 2 Downfeed Hot Water System with Heater at Top of System.* See text for requirements for strainers.


Figure 3 Upfeed Hot Water System with Heater at Bottom of System.* See text for requirements for strainers.

Figure 5 Combination Upfeed and Downfeed Hot Water System with Heater at Bottom of System.Note: This piping system increases the developed length of the HW system over the upfeed systems shown in Figures 14.1 and 14.3.* See text for requirements for strainers.

Figure 4 Downfeed Hot Water System with Heater at Top of System.* See text for requirements for strainers.

Self-Regulating Heat Trace

Over approximately the last 20 years, self-regulating heat trace has come into its own because of the problems of balancing circulated hot water systems and energy loss in the return piping. For further discussion of this topic, see Chapter 15.

Point-of-Use HeatersThis concept is applicable when there is a single fixture or group of fixtures that is located far from the temperature maintenance system. In such a situation, a small, instantaneous, point-of-use water heat-er—an electric water heater, a gas water heater, or a small under-fixture storage type water heater of the magnitude of 6 gal (22.71 L)—can be provided. (See Figure 7.) The point-of-use heater will be very cost-effective because it will save the cost of running hot water piping to a fixture that is a long distance away from the temperature maintenance system. The plumbing engineer must remember, how-ever, that when a water heater is installed there are various code and installation requirements that must be complied with, such as those pertaining to T & P relief valve discharge. Instantaneous electric heaters used in point-of-use applications can require a considerable amount of power, and may require 240 or 480 V service.

POTeNTIaL PROBLems IN CIRCULaTeD hOT WaTeR maINTeNaNCe sysTemsThe following are some of the potential problems with circulated hot water maintenance systems that must be addressed by the plumbing designer.

Figure 7 Instantaneous Point-of-Use Water Heater Piping Dia-gram.Source: Courtesy of Chronomite Laboratories, Inc.



Figure 6 Combination Downfeed and Upfeed Hot Water System with Heater at Top of System.Note: This piping system increases the developed length of the HW system over the downfeed systems shown in Figures 14.2 and 14.4.* See text for requirements for strainers.

Water Velocities in Hot Water Piping SystemsFor copper piping systems, it is very important that the circulated hot water supply piping and especially the hot water return piping be sized so that the water is moving at a controlled velocity. High veloci-ties in these systems can cause pinhole leaks in the copper piping in as short a period as six months or less.

Balancing SystemsIt is extremely important that a circulated hot water system be bal-anced for its specified flows, including all the various individual loops within the circulated system. Balancing is required even though an insulated circulated line usually requires very little flow to main-tain satisfactory system temperatures. If the individual hot water circulated loops are not properly balanced, the circulated water will tend to short-circuit through the closest loops, creating high veloci-ties in that piping system. Furthermore, the short-circuiting of the circulated hot water will result in complaints about the long delays in getting hot water at the remotest loops. If the hot water piping is copper, high velocities can create velocity erosion which will destroy the piping system.

Because of the problems inherent in manually balancing hot water circulation systems, many professionals incorporate factory preset flow control devices in their hot water systems. While the initial cost of such a device is higher than the cost of a manual balancing valve, a preset device may be less expensive when the field labor cost for balancing the entire hot water system is included. When using a preset flow con-trol device, however, the plumbing designer has to be far more accu-rate in selecting the control device’s capacity as there is no possibility of field adjustment. Therefore, if more or less hot water return flow is needed during the field installation, a new flow control device must be installed and the old one must be removed and discarded.

Isolating Portions of Hot Water SystemsIt is extremely important in circulated systems that shutoff valves be provided to isolate an entire circulated loop. This is done so that if individual fixtures need modification, their piping loop can be iso-lated from the system so the entire hot water system does not have to be shut off and drained. The location of these shutoff valves should be given considerable thought. The shutoff valves should be accessible at all times, so they should not be located in such places as the ceil-ings of locked offices or condominiums.

Maintaining the Balance of Hot Water SystemsTo ensure that a balanced hot water system remains balanced after the shutoff valves have been utilized, the hot water return system must be provided with a separate balancing valve in addition to the shutoff valve or, if the balancing valve is also used as the shutoff valve, the balancing valve must have a memory stop. (See the discussion of “balancing valves with memory stops” below.) With a memory stop on the valve, plumbers can return a system to its balanced position after working on it rather than have the whole piping system remain unbalanced, which would result in serious problems.

Providing Check Valves at the Ends of Hot Water LoopsThe designer should provide a check valve on each hot water return line where it joins other hot water return lines. This is done to ensure that a plumbing fixture does not draw hot return water instead of hot supply water, which could unbalance the hot water system and cause delays in obtaining hot water at some fixtures.

a DeLay IN OBTaININg hOT WaTeR aT DeaD-eND LINesKeep the delay in obtaining hot water at fixtures to within the time (and branch length) parameters given previously to avoid unhappy users of the hot water system and to prevent lawsuits.

fLOW BaLaNCINg DevICesThe following are the more common types of balancing device.

Fixed Orifices and VenturisThese can be obtained for specific flow rates and simply inserted into the hot water return piping system. (See Figure 8.) However, extreme care should be taken to locate these devices so they can be removed and cleaned out, as they may become clogged with the debris in the water. It is recommended, therefore, that a strainer with a blow-down valve be placed ahead of each of these devices. Additionally, a strainer with a fine mesh screen can be installed on the main water line coming into the building to help prevent debris buildup in the individual strainers. Also, a shutoff valve should be installed before and after these devices so that an entire loop does not have to be drained in order to service a strainer or balancing device.

Figure 8 Fixed Orifices and Venturi Flow Meters.Source: Courtesy of Gerand Engineering Co.


Factory Preset Automatic Flow Control ValvesThe same admonition about strainers and valves given for “fixed ori-fices and venturis” above applies to the installation and location of these devices. (See Figure 9.)

Flow Regulating ValvesThese valves can be used to determine the flow rate by reading the pressure drop across the valve. They are available from various man-ufacturers. (See Figure 10.)

Balancing Valves with Memory StopsThese valves can be adjusted to the proper setting by installing insert-able pressure measuring devices (Pete’s Plugs, etc.) in the piping system, which indicate the flow rate in the pipe line. (See Figure 11.)

sIzINg hOT WaTeR ReTURN PIPINg sysTems aND ReCIRCULaTINg PUmPs

The method for selecting the proper size of the hot water return piping system and the recirculating pump is fairly easy, but it does require engineering judgment. First, the plumbing engineer has to design the hot water supply and hot water return piping systems, keeping in mind the parameters for total developed length1, prompt delivery of hot water to fixtures, and velocities in pipe lines. The plumbing engi-neer has to make assumptions about the sizes of the hot water return piping.

Figure 9 Preset Self-Limiting Flow Control Cartridge.Source: Courtesy of Griswold Controls.

Figure 10 Adjustable Orifice Flow Control Valve.Source: ITT Industries. Used with permission.

Figure 11 Adjustable Balancing Valve with Memory Stop.Source: Courtesy of Milwaukee Valve Co.

1See American Society of Plumbing Engineers, 2000, Cold-water sys-tems, Chapter 5 in ASPE Data Book, Volume 2, for piping sizing meth-ods.



After the hot water supply and hot water return systems are designed, the designer should make a piping diagram of the hot water supply system and the assumed return system showing piping sizing and approximate lengths. From this piping diagram the hourly heat loss occurring in the circulated portion of the hot water supply and return systems can be determined. (See Table 4 for minimum required insula-tion thickness and Table 5 for approximate piping heat loss.)

Next determine the heat loss in the hot water storage tank if one is provided. (See Table 6 for approximate tank heat loss.) Calculate the total hot water system energy loss (tank heat loss plus piping heat loss) in British thermal units per hour (watts). This total hot water system energy loss is represented by q in Equation 1 below. Note: Heat losses from storage type water heater tanks are not normally included in the hot water piping system heat loss because the water heater capacity takes care of this loss, whereas pumped hot water has to replace the piping convection losses in the piping system.

Figure 10 Adjustable Orifice Flow Control Valve.Source: ITT Industries. Used with permission.

Figure 11 Adjustable Balancing Valve with Memory Stop.Source: Courtesy of Milwaukee Valve Co.

Table 4 Minimum Pipe Insulation Thickness Required Insulation Thickness for Piping (in.)

Runouts 2 in. or 1 in. or Less 1¼–2 in. 2½–4 in. 5 & 6 in. 8 in. or Lessa Larger

½ 1 1 1½ 1½ 1½

Note: Data based on fiberglass insulation with all-service jacket. Data will change depending on actual type of insulation used. Data apply to recirculating sections of hot water systems and the first 3 ft from the storage tank of uncirculated systems.

aUncirculated pipe branches to individual fixtures (not exceeding 12 ft in length). For lengths longer than 12 ft, use required insulation thickness shown in table.

Table 4(M) Minimum Pipe Insulation Thickness Required Insulation Thickness for Piping (mm)

Runouts DN32 or DN25 or DN32–DN50 DN65–DN100 DN125 & DN150 DN200 or Lessa Less Larger

13 25 25 40 40 40

Note: Data based on fiberglass insulation with all-service jacket. Data will change depending on actual type of insulation used. Data apply to recirculating sections of hot water systems and the first 0.9 m from the storage tank of uncirculated systems.

aUncirculated pipe branches to individual fixtures (not exceeding 3.7 m in length). For lengths longer than 305 mm, use required insulation thickness shown in table.

Table 5 Approximate Insulated Piping Heat Loss and Surface Temperature

Nominal Insulation Heat Loss Surface Pipe Size Thickness (Btu/h/ Temperature (in.) (in.) linear ft) (°F) ½ 1 8 68 ¾ 1 10 69 1 1 10 69 1¼ 1 13 70 1½ 1 13 69 2 or less ½a 24 or less 74 2 1 16 70 2½ 1½ 12 67 3 1½ 16 68 4 1½ 19 69 6 1½ 27 69 8 1½ 32 69 10 1½ 38 69

Note: Figures based on average ambient temperature of 65°F and annual average wind speed of 7.5 mph.aUncirculating hot water runout branches only.

Table 5(M) Approximate Insulated Piping Heat Loss and Surface Temperature

Nominal Insulation Heat Loss Surface Pipe Size Thickness (W/m) Temperature (mm) (mm) (°C) DN15 25 7.7 20 DN20 25 9.6 21 DN25 25 9.6 21 DN32 25 12.5 21 DN40 25 12.5 21 DN50 or less 13a 23.1 or less 23 DN50 25 15.4 21 DN65 38 11.5 19 DN80 38 15.4 20 DN100 38 18.3 21 DN150 38 26.0 21 DN200 38 30.8 21 DN250 38 36.5 21

Note: Figures based on average ambient temperature of 18°C and annual average wind speed of 12 km/h.aUncirculating hot water runout branches only.

Table 6 Heat Loss from Various Size Tanks with Various Insulation Thicknesses

Insulation Tank Approx. Energy Loss Thickness Size from Tank at Hot (in.) (gal) Water Temperature 140°F (Btu/h)a

1 50 468 1 100 736 2 250 759 3 500 759 3 1000 1273Source: From Sheet Metal and Air Conditioning Contractors National Association (SMACNA) Table 2 data.aFor unfired tanks, federal standards limit the loss to no more than 6.5 Btu/h/ft2 of tank surface.

Table 6(M) Heat Loss from Various Size Tanks with Various Insulation Thicknesses

Insulation Tank Approx. Energy Loss Thickness Size from Tank at Hot (mm) (L) Water Temperature 60°C (W)a

25.4 200 137 25.4 400 216 50.8 1000 222 76.2 2000 222 76.2 4000 373Source: From Sheet Metal and Air Conditioning Contractors National Association (SMACNA) Table 2 data.aFor unfired tanks, federal standards limit the loss to no more than 1.9 W/m2 of tank surface.


(Equation 1) q = 60rwc∆T

[q = 3600rwc∆T]

where 60 = min/h 3600 = sec/h q = piping heat loss, Btu/h (kJ/h) r = flow rate, gpm (L/sec) w = weight of heated water, lb/gal (kg/L) c = specific heat of water, Btu/lb/°F (kJ/kg/K) ∆T = change in heated water temperature (temperature of

leaving water minus temperature of incoming water, represented in this manual as Th – Tc, °F [K])

Therefore q = c (gpm × 8.33 lb/gal)(60 min/h)(°F temperature drop) = 1(gpm) × 500 × °F temperature drop [q = c(L/sec•1kg/L)(3600sec/h)(Ktemperaturedrop) = 1(L/sec)•15077kJ/L/sec/K•Ktemperaturedrop]

(Equation 2) gpm ≈ system heat loss (Btu/h) 500 × °F temperature drop

[L/sec ≈ system heat loss (kJ/h) ] 15077•Ktemperaturedrop

In sizing hot water circulating systems, the designer should note that the greater the temperature drop across the system, the less water is required to be pumped through the system and, therefore, the greater the savings on pumping costs. However, if the domestic hot water supply starts out at 140°F (60°C) with, say, a 20°F (6.7°C) temperature drop across the supply system, the fixtures near the end of the circulat-ing hot water supply loop could be provided with a hot water supply of only 120°F (49°C). In addition, if the hot water supply delivery tem-perature is 120°F (49°C) instead of 140°F (60°C), the plumbing fixtures will use greater volumes of hot water to get the desired blended water temperature (see Chapter 1, Table 1.1). Therefore, the recommended hot water system temperature drop should be of the magnitude of 5°F (3°C). This means that if the hot water supply starts out from the water heater at a temperature between 135 and 140°F (58 and 60°C), the lowest hot water supply temperature provided by the hot water supply system could be between 130 and 135°F (54 and 58°C). With multiple temperature distribution systems, it is recommended that the recircu-lation system for each temperature distribution system be extended back to the water heating system separately and have its own pump.Using Equation 2, we determine that, if there is a 5°F (3°C) tempera-ture drop across the hot water system, the number to divide into the hot water circulating system heat loss (q) to obtain the minimum required hot water return circulation rate in gpm (L/sec) is 2500 (500 ×5°F),(45213[15071•3°C]).

For a 10°F (6°C) temperature drop that number is 5000 (from Equa-tion2,500×10°F=5000)(90426[fromEquation2,15071•6°C=90426]). However, this 10°F (6°C) temperature drop may produce hot water supply temperatures that are lower than desired.

After Equation 2 is used to establish the required hot water return flow rate, in gpm (L/sec), the plumbing designer can size the hot water return piping system based on piping flow rate velocities and the avail-

able pump heads. It is quite common that a plumbing designer will make wrong initial assumptions about the sizes of the hot water return lines to establish the initial heat loss figure (q). If that is the case, the plumbing engineer will have to correct the hot water return pipe sizes, redo the calculations using the new data based on the correct pipe sizing, and verify that all the rest of the calculations are now correct.

EXAMPLE 1—CALCULATION TO DETERMINE REQUIRED CIRCULATION RATE1. Assume that the hot water supply piping system has 800 ft (244

m) of average size 1 ¼ in. (DN32) pipe. From Table 5, determine the heat loss per linear foot (meter). To find the total heat loss, multiply length times heat loss per foot (meter):

800 ft × 13 Btu/h/ft = 10,400 Btu/h supply piping losses

(244m•12.5W= 3050Wsupplypipinglosses)

2. Assume that the hot water return piping system for the system in no. 1 above has 100 ft (30.5 m) of average ½ in. (DN15) piping and 100 ft (30.5 m) of average ¾ in. (DN20) pipe. From Table 5 determine the heat loss per linear foot (meter):

100 ft × 8 Btu/h/ft = 800 Btu/h piping loss

(30.5m•7.7W/m = 235Wpipingloss)

100 ft × 10 Btu/h/ft = 1000 Btu/h piping loss 1800 Btu/h piping loss

(30.5m•9.6W/m =293 W piping loss) 528 W piping loss

3. Determine the hot water storage tank heat loss. Assume the system in no. 1 above has a 200-gal (757-L) hot water storage tank. From Table 6 determine the heat loss of the storage tank @ 759 Btu/h (222 W).

4. Determine the hot water system’s total heat losses by totaling the various losses:

A. Hot water supply piping losses 10,400 Btu/h

B. Hot water return piping losses 1,800 Btu/h

C. Hot water storage tank losses 759 Btu/h

Total system heat losses 12,959 Btu/h

Total system piping heat losses (A + B) = 12,200 Btu/h

[A. Hot water supply piping losses 3050 W

B. Hot water return piping losses 527 W

C. Hot water storage tank losses 222 W

Total system heat losses 3799 W

Total system piping heat losses (A + B) = 3577 W]

From Equation 2, using a system piping loss of 12,200 Btu/h (3577 W) and a 5°F (3°C) temperature drop,

12,200 Btu/h = 4.88 gpm (say 5 gpm) 5°F temperature difference × 500 required hot water return circulation rate



3577 W = 0.29 (say 0.3) L/sec 3°Ctemp.difference•4188.32kJ/m3 requiredhotwater return circulation rate

Recalculation of Hot Water System Losses1. Assume that the hot water supply piping system has 800 ft

(244 m) of average size 1¼ in. (DN32) pipe. From Table 5 determine the heat loss per linear foot (meter):

800 ft × 13 Btu/h/ft = 10,400 Btu/h piping loss

(244m•12.5W/m = 3050Wpipingloss)

2. Assume that the hot water return piping system for the system in no. 1 above has 100 ft (30.5 m) of average ½ in. (DN15) pipe, 25 ft (7.6 m) of average ¾ in. (DN22) pipe, and 75 ft (22.9 m) of average 1 in. (DN28) pipe. From Table 5, determine the heat loss per linear foot (meter):




1800 Btu/h piping loss

[30.5m•7.7W/m =235Wpipingloss

7.6m•9.6W/m = 73Wpipingloss

22.9m•9.6W/m = 220 W piping loss

528 W piping loss]

3. Determine the hot water storage tank heat loss. Assume the system in no. 1 above has a 200-gal (757-L) hot water storage tank. From Table 6 determine the heat loss of the storage tank @ 759 Btu/h (222 W).

4. Determine the system’s total heat losses:

A. Hot water supply losses 10,400 Btu/h

B. Hot water return losses 1,800 Btu/h

C. Hot water storage tank losses 759 Btu/h

Total system heat losses 12,959 Btu/h

Total system piping heat losses (A + B) = 12,200 Btu/h

[A. Hot water supply losses 3050 W

B. Hot water return losses 528 W

C. Hot water storage tank losses 222 W

Total system heat losses 3800 W

Total system piping heat losses (A + B) = 3578 W]

Note: The recalculation determined that the hot water system heat losses remained unchanged and that 4.88 (say 5) gpm (0.29 [say 0.3] L/sec) is the flow rate that is required to maintain the 5°F (3°C) tempera-ture drop across the hot water supply system.

It should be stated that engineers use numerous rules of thumb to size hot water return systems. These rules of thumb are all based on assumptions, however, and are not recommended. It is recommended that the engineer perform the calculations for each project to establish the required flow rates because, with all the various capacities of the pumps available today, exact sizing is possible, and any extra circulated flow caused by the plumbing engineer using a rule of thumb equates to higher energy costs, to the detriment of the client.

esTaBLIshINg The heaD CaPaCITy Of The hOT WaTeR CIRCULaTINg PUmPThe hot water return circulating pump is selected based on the required hot water return flow rate (in gpm [L/sec]), calculated using Equation 2, and the system’s pump head. The pump head is normally determined by the friction losses through only the hot water return piping loops and any losses through balancing valves. The hot water return piping friction losses usually do not include the friction losses that occur in the hot water supply piping. The reason for this is that the hot water return circulation flow is needed only to keep the hot water supply system up to the desired temperature when there is no flow in the hot water supply piping. When people use the hot water at the fixtures, there is usually sufficient flow in the hot water supply piping to keep the system hot water supply piping up to the desired tempera-ture without help from the flow in the hot water return piping.

The only exception to the rule of ignoring the friction losses in the hot water supply piping is a situation where a hot water return pipe is connected to a relatively small hot water supply line. “Relatively small” here means any hot water supply line that is less than one pipe size larger than the hot water return line. The problems created by this condition are that the hot water supply line will add additional friction to the head of the hot water circulating pump, and the hot water circu-lating pump flow rate can deprive the last plumbing fixture on this hot water supply line from obtaining its required flow. It is recommended, therefore, that in such a situation the hot water supply line supplying each hot water return piping connection point be increased to pre-vent these potential problems, i.e., use ¾ in. (DN22) hot water supply piping and ½ in. (DN15) hot water return piping, or 1 in. (DN28) hot water supply piping and ¾ in. (DN22) hot water return piping, etc.

When selecting the hot water circulating pump’s head, the designer should be sure to calculate only the restrictions encountered by the circulating pump. A domestic hot water system is normally consid-ered an open system (i.e., open to the atmosphere). When the hot water circulating pump is operating, however, it is assumed that the piping is a closed system. Therefore, the designer should not include static heads where none exists. For example, in Figure 1, the hot water circulating pump has to overcome only the friction in the hot water return piping not the loss of the static head pumping the water up to the fixtures because in a closed system the static head loss is offset by the static head gain in the hot water return piping.

hOT WaTeR CIRCULaTINg PUmPs

Most hot water circulating pumps are of the centrifugal type and are available as either in-line units for small systems or base-mounted units for large systems. Because of the corrosiveness of hot water systems, the pumps should be bronze, bronze fitted, or stainless steel. Conventional, iron bodied pumps, which are not bronze fitted, are not recommended.

CONTROL fOR hOT WaTeR CIRCULaTINg PUmPs

There are three major methods commonly used for controlling hot water circulating pumps: manual, thermostatic (aquastat), and time clock control. Sometimes more than one of these methods are used on a system.

1. A manual control runs the hot water circulating pump con-tinuously when the power is turned on. A manual control


should be used only when hot water is needed all the time, 24 h a day, or during all the periods of a building’s operation. Otherwise, it is not a cost-effective means of controlling the circulating pump because it will waste energy.

Note: The method for applying the “on demand” concept for control-ling the hot water circulating pump is a manual control. It can be used very successfully for residential and commercial applications.

2. A thermostatic aquastat is a device that is inserted into the hot water return line. When the water in the hot water return system reaches the distribution temperature, it shuts off the circulating pump until the hot water return system tempera-ture drops by approximately 10°F [5.5°C]. With this method, when there is a large consumption of hot water by the plumbing fixtures, the circulating pump does not operate.

3. A time clock is used to turn the pump on during specific hours of operation when people are using the fixtures. The pump would not operate, for example, at night in an office building when nobody is using the fixtures.

4. Often an aquastat and a time clock are used in conjunction so that during the hours a building is not operating the time clock shuts off the circulating pump, and during the hours the building is in use the aquastat shuts off the pump when the system is up to the desired temperature.

aIR eLImINaTIONIn any hot water return circulation system it is very important that there be a means of eliminating any entrapped air from the hot water return piping. Air elimination is not required in the hot water supply piping because the discharge of water from the fixtures will eliminate any entrapped air. If air is not eliminated from the hot water return lines, however, it can prevent the proper circulation of the hot water system. It is imperative that a means of air elimination be provided at all high points of a hot water return system. The plumbing engineer must always give consideration to precisely where the air elimination devices are to be located and drained. For example, they should not be located in the unheated attics of buildings in cold climates. If the plumbing engineer does not consider the location of these devices and where they will drain, the result may be unsightly piping in a building or extra construction costs.

INsULaTIONThe use of insulation is very cost-effective. It means paying one time to save the later cost of significant energy lost by the hot water supply and return piping system. Also, insulation decreases the stresses on the piping due to thermal expansion and contraction caused by changes in water temperature. Furthermore, the proper use of insu-lation eliminates the possibility of someone getting burned by a hot, uninsulated water line. See Table 5 for the surface temperatures of insulated lines (versus 140°F [60°C] for bare piping).

It is recommended that all hot water supply and return piping be insulated. This recommendation exceeds some code requirements.

See Table 4 for the minimum required insulation thicknesses for all systems.

If the insulated piping is installed in a location where it is subjected to rain or other water, the insulation must be sealed with a watertight covering that will maintain its tightness over time. Wet insulation not only does not insulate, it also releases considerable heat energy from the hot water piping, thus wasting energy. Furthermore, the insulation on any outdoor lines that is not sealed watertight can be plagued by birds or rodents, etc., pecking at the insulation to use it for their nests. In time, the entire hot water supply and/or return piping will have no insulation. Such bare hot water supply and/or return piping will waste considerable energy and can seriously affect the operation of the hot water system and water heaters.

The minimum required insulation thicknesses given in Table 4 are based on insulation having thermal resistivity (R) in the range of 4.0 to4.6ft2×h×(°F/Btu)×in.(0.028to0.032m2•[°C/W]•mm)onaflat surface at a mean temperature of 75°F (24°C). Minimum insulation thickness shall be increased for materials having R values less than 4.0 ft2×h×(°F/Btu)×in.(0.028m2•[°C/W]•mm)ormaybereducedformaterials having R values greater than 4.6 ft2 × h × (°F/Btu) × in. (0.032 m2•[°C/W]•mm).

1. For materials with thermal resistivity greater than 4.6 ft2 × h×(°F/Btu)×in.(0.032m2•[°C/W]•mm),theminimuminsulation thickness may be reduced as follows:

4.6 × Table 4 thickness = New minimum thickness

Actual R

(0.032•Table4thickness = New minimum thickness)

Actual R

2. For materials with thermal resistivity less than 4.0 ft2 × h × (°F/Btu)×in.(0.028m2•[°C/W]•mm),theminimuminsu-lation thickness shall be increased as follows:

4.0 × Table 4 thickness = New minimum thickness

Actual R

(0.028 •Table4thickness = New minimum thickness)

Actual R

CONCLUsIONIn conclusion, an inappropriate hot water recirculation system can have serious repercussions for the operation of the water heater and the sizing of the water heating system. In addition, it can cause the wastage of vast amounts of energy, water, and time. Therefore, it is incumbent upon the plumbing designer to design a hot water recir-culation system so that it conserves natural resources and is in accor-dance with the recommendations given in this chapter.



BIBLIOgRaPhy1. American Society of Heating, Refrigerating, and Air Condi-

tioning Engineers. 1993. Pipe sizing. Chapter 33 in Funda-mentals Handbook.

2. American Society of Heating, Refrigerating, and Air Condi-tioning Engineers. 1993. Thermal and water vapor transmis-sion data. Chapter 22 in Fundamen tals Handbook.

3. American Society of Heating, Refrigerating, and Air Condi-tioning Engineers. 1995. Service water heating. Chapter 45 in Applications Handbook.

4. American Society of Heating, Refrigerating, and Air Con-ditioning Engineers. Energy conservation in new building design. ASHRAE Standards, 90A–1980, 90B–1975, and 90C–1977.

5. American Society of Heating, Refrigerating, and Air Condi-tioning Engineers. Energy efficient design of new low rise residential buildings. ASHRAE Standards, 90.2–1993.

6. American Society of Heating, Refrigerating, and Air Condi-tioning Engineers. New information on service water heat-ing. Technical Data Bulletin. Vol. 10, No. 2.

7. American Society of Mechanical Engineers. Plumbing fixture fittings. ASME A112.18.1M–1989.

8. American Society of Plumbing Engineers. 2000. Cold water systems. Chapter 5 in ASPE Data Book, Volume 2.

9. American Society of Plumbing Engineers. 1989. Piping sys-tems. Chapter 10 in ASPE Data Book.

10. American Society of Plumbing Engineers. 1989. Position paper on hot water temperature limitations.

11. American Society of Plumbing Engineers. 1989. Service hot water systems. Chapter 4 in ASPE Data Book.

12. American Society of Plumbing Engineers. 1990. Insulation. Chapter 12 in ASPE Data Book.

13. American Society of Plumbing Engineers. 1990. Pumps. Chapter 11 in ASPE Data Book.

14. American Society of Plumbing Engineers. 2000. Energy conservation in plumbing systems. Chapter 7 in ASPE Data Book, Volume 1.

15. American Water Works Association. 1985. Internal corrosion of water distribution systems. Research Foundation coopera-tive research report.

16. Cohen, Arthur. Copper Development Association. 1978. Copper for hot and cold potable water systems. Heating/Piping/Air Conditioning Magazine. May.

17. Cohen, Arthur. Copper Development Association. 1993. His-torical perspective of corrosion by potable waters in building systems. Paper no. 509 presented at the National Association of Corrosion Engineers Annual Conference.

18. Copper Development Association. 1993. Copper Tube Hand-book.

19. International Association of Plumbing and Mechanical Officials. 1985. Uniform Plumbing Code Illustrated Training Manual.

20. Konen, Thomas P. 1984. An experimental study of competing systems for maintaining service water temperature in resi-dential buildings. In ASPE 1984 Convention Proceedings.

21. Konen, Thomas P. 1994. Impact of water conservation on interior plumbing. In Technical Proceedings of the 1994 ASPE Convention.

22. Saltzberg, Edward. 1988. The plumbing engineer as a foren-sic engineer. In Technical Proceedings of the 1988 ASPE Convention.

23. Saltzberg, Edward. 1993. To combine or not to combine: An in–depth review of standard and combined hydronic heat-ing systems and their various pitfalls. Paper presented at the American Society of Plumbing Engineers Symposium, Octo-ber 22–23.

24. Saltzberg, Edward. 1996. The effects of hot water circulation systems on hot water heater sizing and piping systems. Tech-nical presentation given at the American Society of Plumb-ing Engineers convention, November 3–6.

25. Saltzberg, Edward. 1997. In press. New methods for analyz-ing hot water systems. Plumbing Engineer Magazine.

26. Saltzberg, Edward. 1997. In press. Prompt delivery of hot water at fixtures. Plumbing Engineer Magazine.

27. Sealine, David A., Tod Windsor, Al Fehrm, and Greg Wilcox. 1988. Mixing valves and hot water temperature. In Technical Proceedings of the 1988 ASPE Convention.

28. Sheet Metal and Air Conditioning Contractors National Association. 1982. Retrofit of Building Energy Systems and Processes.

29. Steele, Alfred. Engineered Plumbing Design. 2d ed.

30. Steele, Alfred. 1988. Temperature limits in service hot water systems. In Technical Proceedings of the 1988 ASPE Conven-tion.

31. Wen-Yung, W. Chan, and Milton Meckler. 1983. Pumps and pump systems. In American Society of Plumbing Engineers Handbook.


About This Issue’s ArticleThe July/August 2010 continuing education article is “Re-circulating Domestic Hot Water Systems,” Chapter 14 from Domestic Water Heating Design Manual II.

This chapter addresses the criteria for establishing an ac-ceptable time delay in delivering hot water to fixtures and the limitations of the length between a hot water recircu-lation system and plumbing fixtures. It also discusses the temperature drop across a hot water supply system, types of hot water recirculation systems, and pump selection criteria and provides extensive information on the insula-tion of hot water supply and return piping.

You may locate this article at www.psdmagazine.org. Read the article, complete the following exam, and submit your answer sheet to the ASPE office to potentially receive 0.1 CEU.

PSD

169

Continuing Education from Plumbing Systems & Design

CE Questions — “Recirculating Domestic Hot Water Systems” (PSD 169) 1. What aspect of a circulated system causes energy loss in

the circulation of hot water?a. convectionb. pressurec. radiationd. both a and c

2. What delay period in obtaining hot water at a fixture is considered most acceptable?a. zero to 10 secondsb. 11 to 30 secondsc. more than 30 secondsd. no delay is acceptable

3. What is the approximate time required to deliver hot water to a 1.5-gpm fixture 10 feet from the hot water maintenance system using ½-inch Schedule 40 steel pipe?a. 8 secondsb. 16 secondsc. 21 secondsd. 30 seconds

4. Which of the following is a type of hot water maintenance system?a. self-regulating heat trace systemb. circulation systemc. point-of-use water heaterd. all of the above

5. High velocities in copper piping systems can cause ________ in less than six months.a. corrosionb. pinhole leaksc. water hammerd. decreased flow

6. A ________ should be provided in circulated systems to isolate an entire loop.a. balancing valveb. control valvec. shutoff valved. check valve

7. Which of the following is a common type of balancing device?a. automatic flow control valveb. flow-regulating valvec. pressure-regulating valved. both a and b

8. What is the required insulation thickness for a 3-inch runout?a. 0.5 inchb. 1 inchc. 1.5 inchesd. 2 inches

9. What is the approximate heat loss for a 1½-inch pipe with 1 inch of insulation?a. 8 Btuh/linear footb. 10 Btuh/linear footc. 13 Btuh/linear footd. 16 Btuh/linear foot

10. In the total hot water system energy loss calculation, what does r stand for?a. piping heat lossb. flow ratec. weight of waterd. specific heat of water

11. What is the maximum recommended hot water system temperature drop?a. 1°Fb. 5°Fc. 10°Fd. 15°F

12. What is the recommended material for a hot water circulating pump?a. bronzeb. ironc. stainless steeld. both a and c

Do you find it difficult to obtain continuing education units (CEUs)? Through this special section in every issue of PS&D, ASPE can help you accumulate the CEUs required for maintaining your Certified in Plumbing Design (CPD) status.

Now Online!The technical article you must read to complete the exam is located at www.psdmagazine.org. Just click on “Plumbing Systems & Design Continuing Education Article and Exam” at the top of the page. The following exam and application form also may be downloaded from the website. Reading the article and completing the form will allow you to apply to ASPE for CEU credit. If you earn a grade of 90 percent or higher on the test, you will be notified that you have logged 0.1 CEU, which can be applied toward CPD renewal or numerous regulatory-agency CE programs. (Please note that it is your responsi-bility to determine the acceptance policy of a particular agency.) CEU information will be kept on file at the ASPE office for three years.

Note: In determining your answers to the CE questions, use only the material presented in the corresponding continuing education article. Using informa-tion from other materials may result in a wrong answer.



PS&D Continuing education answer sheetRecirculating Domestic Hot Water Systems (PSD 169)

Questions appear on page 14. Circle the answer to each question. Q 1. a B C D Q 2. a B C D Q 3. a B C D Q 4. a B C D Q 5. a B C D Q 6. a B C D Q 7. a B C D Q 8. a B C D Q 9. a B C D Q 10. a B C D Q 11. a B C D Q 12. a B C D

Plumbing Systems & Design Continuing Education Application FormThis form is valid up to one year from date of publication. The PS&D Continuing Education program is approved by ASPE for up to one contact hour (0.1 CEU) of credit per article. Participants who earn a passing score (90 percent) on the CE questions will receive a letter or certification within 30 days of ASPE’s receipt of the application form. (No special certificates will be issued.) Participants who fail and wish to retake the test should resubmit the form along with an additional fee (if required).1. Photocopy this form or download it from www.psdmagazine.org.2. Print or type your name and address. Be sure to place your ASPE membership number in the appropriate space.3. Answer the multiple-choice continuing education (CE) questions based on the corresponding article found on

www.psdmagazine.org and the appraisal questions on this form.4. Submit this form with payment ($35 for nonmembers of ASPE) if required by check or money order made payable to ASPE or credit

card via mail (ASPE Education Credit, 2980 S. River Road, Des Plaines, IL 60018) or fax (847-296-2963).

Please print or type; this information will be used to process your credits.

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Title _________________________________________________ ASPE Membership No. ____________________________________

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PE State _____________________________________________ PE No. _________________________________________________

appraisal QuestionsRecirculating Domestic Hot Water Systems (PSD 169)

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Expiration date: Continuing education credit will be givenfor this examination through august 31, 2011.

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