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PSD

1

43

RecirculatingDomestic

Hot Water

SystemsContinuing Education from Plumbing Systems & DesignHaig Demergian, PE, CPD

DECEMBER 2007

PSDMAGAZINE.ORG

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INTRODUCTIONIt has been determined through field studies that the correct sizing

and operation of water heaters depend on the appropriateness of thehot water maintenance system. If the hot water maintenance systemis inadequate, the water heater sizing criteria are wrong and thetemperature of the hot water distributed to the users of the plumb-ing fixtures is below acceptable standards. Additionally, a poorlydesigned hot water maintenance system wastes large amounts ofenergy and potable water and creates time delays for those usingthe plumbing fixtures. Tis chapter addresses the criteria for estab-lishing an acceptable time delay in delivering hot water to fixturesand the limitations of the length between a hot water recirculationsystem and plumbing fixtures. It also discusses the temperaturedrop across a hot water supply system, types of hot water recircula-tion system, and pump selection criteria, and gives extensive infor-mation on the insulation of hot water supply and return piping.

BACKGROUNDIn the past, the plumbing engineering community considered theprompt delivery of hot water to fixtures either a requirement for aproject or a matter of no concern. Te plumbing engineers decisionwas based primarily on the type of facility under consideration andthe developed length from the water heater to the farthest fixture.Previous reference material and professional common practiceshave indicated that, when the distance from the water heater to thefarthest 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) criterionas the maximum length for all uncirculated, uninsulated, dead-endhot water branches to fixtures in order to cut the cost of hotwater distribution piping. Tese long, uninsulated, dead-endbranches to fixtures create considerable problems, such as alack of hot water at fixtures, inadequately sized water heaterassemblies, and thermal temperature escalation in showers.

Te 100-ft (30.48-m) length criterion was developed in 1973after the Middle East oil embargo, when energy costs were theparamount concern and water conservation was given littleconsideration. Since the circulation of hot water causes a lossof energy due to radiation and convection in the circulatedsystem and such energy losses have to be continually replacedby water heaters, the engineering community compromisedbetween energy loss and construction costs and developedthe 100-ft (30.48-m) maximum length criterion.

LENGTH AND TIME CRITERIARecently, due to concern about not only energy conservationbut also the extreme water shortages in parts of the country, the100-ft (30.48-m) length criteria has changed. Water wastagecaused by the long delay in obtaining hot water at fixtures hasbecome as critical an issue as the energy losses caused by hotwater temperature maintenance systems. o reduce the wast-ing of cooled hot water significantly, the engineering com-munity has reevaluated the permissible distances for uncir-

culated, dead-end branches to periodically used plumbing fixturTe new allowable distances for uncirculated, dead-end branc

represent a trade-off between the energy utilized by the hot wamaintenance system and the cost of the insulation, on the ohand, and the cost of energy to heat the excess cold water makethe cost of wasted potable water, and extra sewer surcharges, on other hand. Furthermore, engineers should be aware that variocodes now limit the length between the hot water maintenansystem and plumbing fixtures. Tey also should be aware of potential for liability if an owner questions the adequacy of thhot water system design.

What are reasonable delays in obtaining hot water at a fixtuFor anything beside very infrequently used fixtures (such as thoseindustrial facilities or certain fixtures in office buildings), a delay0 to 10 sec is normally considered acceptable for most residenoccupancies and public fixtures in office buildings. A delay of 11

30 sec is marginal but possibly acceptable, and a time delay lonthan 31 sec is normally considered unacceptable and a significwaste of water and energy. Terefore, when designing hot wasystems, it is prudent for the designer to provide some meansgetting hot water to the fixtures within these acceptable time limNormally this means that there should be a maximum distanceapproximately 25 ft (7.6 m) between the hot water maintenansystem and each of the plumbing fixtures requiring hot water, tdistance depending on the water flow rate of the plumbing fixtat the end of the line and the size of the line. (See ables 1, 2, andTe plumbing designer may want to stay under this length limition because the actual installation in the field may differ slighfrom the engineers design, and additional delays may be caus

Recirculating Domestic HotWater Systems

Reprinted from Domestic Water Heating Design Manual, Second Edition,Chapter 14: Recirculating Domestic Hot Water Systems.

American Society of Plumbing Engineers , 2003.

2 Plumbing Systems & Design DECEMBER 2007 PSDMAGAZINE

CONTINUING EDUCAT

Table 1 Water Contents and Weight of Tube or Piping per Linear Foot

NominalDiameter

Copper PipeType L

Copper PipeType M

Steel PipeSchedule 40

CPVC PipeSchedule 40

(in.)aWater

(gal/ft)Wgt.

(lb/ft)Water

(gal/ft)Wgt.

(lb/ft)Water

(gal/ft)Wgt.

(lb/ft)Water

(gal/ft)Wg

(lb/

0.012 0.285 0.013 0.204 0.016 0.860 0.016 0.2 0.025 0.445 0.027 0.328 0.028 1.140 0.028 0.29

1 0.043 0.655 0.045 0.465 0.045 1.680 0.045 0.421 0.065 0.884 0.068 0.682 0.077 2.280 0.078 0.591 0.093 1.14 0.100 0.940 0.106 2.720 0.106 0.7

a Pipe sizes are indicated for mild steel pipe sizing.

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

NominalDiameter Copper PipeType L Copper PipeType M Steel PipeSchedule 40 CPVC PipeSchedule 40

(mm)aWater

(L)Wgt.(kg)

Water(L)

Wgt.(kg)

Water(L)

Wgt.(kg)

Water(L)

Wg(kg

DN15 0.045 0.129 0.049 0.204 0.061 0.390 0.061 0.09

DN20 0.095 0.202 0.102 0.328 0.106 0.517 0.106 0.13DN25 0.163 0.297 0.170 0.465 0.170 0.762 0.170 0.19DN32 0.246 0.401 0.257 0.682 0.291 1.034 0.295 0.26DN40 0.352 0.517 0.379 0.940 0.401 1.233 0.401 0.32

a Pipe sizes are indicated for mild steel pipe sizing.

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by either the routing of the pipe or other problems. Furthermore,with the low fixture discharge rates now mandated by national andlocal laws, it takes considerably longer to obtain hot water fromnon-temperature maintained hot water lines than it did in the past,when fixtures had greater flow rates. For example, a public lavatorywith a 0.50 or 0.25 gpm (0.03 or 0.02 L/sec) maximum dischargerate would take an excessive amount of time to obtain hot waterfrom 100 ft (30.48 m) of uncirculated, uninsulated hot water piping.(See able 3.) Tis table gives conservative approximations of theamount of time it takes to obtain hot water at a fixture. Te timesare based on the size of the line, the fixture flow rate, and the timesrequired to replace the cooled off hot water, to heat the pipe, and tooffset the convection energy lost by the insulated hot water line.

RESULTS OF DELAYS IN DELIVERING HOT WATER TO

FIXTURESAs mentioned previously, when there is a long delay in obtainhot water at the fixture, there is significant wastage of potable waas the cooled hot water supply is simply discharged down the drunused. Furthermore, plumbing engineers concerned about tosystem costs should realize that the cost of this wasted, previouheated water must include: the original cost for obtaining potawater, the cost of previously heating the water, the final cost of waste treatment of this excess potable water, which results in lar

sewer surcharges (source of supply to end disposal point), and cost of heating the new cold water to bring it up to the required teperature. Furthermore, if there is a long delay in obtaining hot waat the fixtures, the faucets are turned on for long periods of timebring the hot water supply at the fixture up to the desired tempe

ture. Tis can cause the water heating system to run outhot water and make the heater sizing inadequate, becauthe heater is unable to heat all the extra cold water brouinto the system through the wastage of the water dischargdown the drain. In addition, this extra cold water enterthe hot water system reduces the hot water supply tempeture. Tis exacerbates the problem of insufficient hot wabecause to get a proper blended temperature more low

temperature hot water will be used to achieve the final mixwater temperature. (See Chapter 1, able 1.1.) Additionathis accelerates the downward spiral of the temperaturethe hot water system.

Another problem resulting from long delays in getting water to the fixtures is that the fixtures operate for longer thexpected periods of time. Terefore, the actual hot wademand is greater than the demand normally designed fo

Terefore, when sizing the water heater and the hot wapiping distribution system, the designer should be awthat the lack of a proper hot water maintenance system cseriously impact the required heater size.

METHODS OF DELIVERING REASONABLY PROMP

HOT WATER SUPPLYHot water maintenance systems are as varied as the imagintions of the plumbing engineers who create them. Tey cbe grouped into three basic categories, though any actinstallation may be a combination of more than one of thtypes of system. Te three basic categories are1. Circulation systems.2. Self-regulating heat trace systems.3. Point-of-use water heaters (include booster water hea

ers).

CIRCULATION SYSTEMS FOR COMMERCIAL, INDUSTRI

AND LARGE RESIDENTIAL PROJECTSA circulation system is a system of hot water supply pi

and hot water return pipes with appropriate shutoff valvbalancing valves, circulating pumps, and a method of controllthe circulating pump. Te diagrams for six basic circulating systeare shown in Figures 1 through 6.

SELFREGULATING HEAT TRACEOver approximately the last 20 years, self-regulating heat trace hcome into its own because of the problems of balancing circulahot water systems and energy loss in the return piping. For furthdiscussion of this topic, see Chapter 15.

DECEMBER 2007 Plumbing Systems & Design

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

Delivery Time (sec)

Fixture FlowRate (gpm) 0.5 1.5 2.5 4.0

PipingLength (ft) 10 25 10 25 10 25 10 25

Copper in. 25 63a

8 21 5 13 3 8 Pipe in. 48a 119a 16 40a 10 24 6 15Steel Pipe in. 63a 157a 21 52a 13 31a 8 20 Sched. 40 in. 91a 228a 30 76a 18 46a 11 28CPVC Pipe in. 64a 159a 21 53a 13 32a 8 20

Sched. 40 in. 95a 238a 32 79a 19 48a 12 30Note: Table based on various fixture flow rates, piping materials, and dead-end branch lengths. Calculations are basedon the amount of heat required to heat the piping, the water in the piping, and the heat loss from the piping. Based onwater temperature of 140F and an air temperture of 70F.a Delays longer than 30 sec are not acceptable.

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

Delivery Time (sec)

Fixture FlowRate (L/sec) 0.03 0.10 0.16 0.25

PipingLength (m) 3.1 7.6 3.1 7.6 3.1 7.6 3.1 7.6

Copper DN15 25 63a 8 21 5 13 3 8 Pipe DN22 48a 119a 16 40a 10 24 6 15Steel Pipe DN15 63a 157a 21 52a 13 31a 8 20 Sched. 40 DN20 91a 228a 30 76a 18 46a 11 28CPVC 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 basedon the amount of heat required to heat the piping, the water in the piping, and the heat loss from the piping. Based onwater temperature of 60C and an air temperture of 21.1C.a Delays longer than 30 sec are not acceptable.

Table 2 Approximate Fixture and Appliance Water Flow Rates

Fittings

Maximum Flow Ratesa

GPM L/Sec

Lavatory faucet 2.0 1.3 Public non-metering 0.5 0.03 Public 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 minimum Two-handle 4.0 minimum 0.25 minimumService 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.

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POINTOFUSE HEATERSTis concept is applicable when there is a singlefixture or group of fixtures that is located farfrom the temperature maintenance system. Insuch a situation, a small, instantaneous, point-of-use water heateran electric water heater, agas water heater, or a small under-fixture stor-age type water heater of the magnitude of 6 gal(22.71 L)can be provided. (See Figure 7.) Tepoint-of-use heater will be very cost-effective

because it will save the cost of running hot waterpiping to a fixture that is a long distance awayfrom the temperature maintenance system. Teplumbing engineer must remember, however,that when a water heater is installed there arevarious code and installation requirements thatmust be complied with, such as those pertain-ing to & P relief valve discharge.

Instantaneous electric heaters used in point-of-use applications can require a considerableamount of power, and may require 240 or 480V service.

POTENTIAL PROBLEMS IN CIRCULATED

HOT WATER MAINTENANCE SYSTEMSTe following are some of the potential prob-lems with circulated hot water maintenancesystems that must be addressed by the plumb-ing designer.

WATER VELOCITIES IN HOT WATER PIPINGSYSTEMSFor copper piping systems, it is very importantthat the circulated hot water supply piping andespecially the hot water return piping be sizedso that the water is moving at a controlled veloc-ity. High velocities in these systems can causepinhole leaks in the copper piping in as short aperiod as six months or less.

BALANCING SYSTEMSIt is extremely important that a circulated hotwater system be balanced for its specified flows,including all the various individual loops withinthe circulated system. Balancing is requiredeven though an insulated circulated line usu-ally requires very little flow to maintain satis-factory system temperatures. If the individualhot water circulated loops are not properly bal-anced, the circulated water will tend to short-circuit through the closest loops, creating highvelocities in that piping system. Furthermore,the short-circuiting of the circulated hot waterwill result in complaints about the long delaysin getting hot water at the remotest loops. If thehot water piping is copper, high velocities cancreate velocity erosion which will destroy thepiping system.

Because of the problems inherent in manu-ally balancing hot water circulation systems,many professionals incorporate factory presetflow control devices in their hot water systems.While the initial cost of such a device is higherthan the cost of a manual balancing valve, a


CONTINUING EDUCATION: Recirculating Domestic Hot Water Syste

* See text for requirements for strainers.

Fixture 1 Upfeed Hot Water System with Heater at Bottom of System.

Figure 2 Downfeed Hot Water System with Heater at Top of System.


Figure 3 Upfeed Hot Water System with Heater at Bottom of System.


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preset device may be less expensive when field labor cost for balancing the entire hwater system is included. When using a preflow control device, however, the plumbdesigner has to be far more accurate in seleing the control devices capacity as there ispossibility of field adjustment. Tereforemore or less hot water return flow is needduring the field installation, a new flow condevice must be installed and the old one m

be removed and discarded.

ISOLATING PORTIONS OF HOT WATER

SYSTEMSIt is extremely important in circulated systethat shutoff valves be provided to isolate entire circulated loop. Tis is done so thaindividual fixtures need modification, thpiping loop can be isolated from the systemthe entire hot water system does not havebe shut off and drained. Te location of thshutoff valves should be given considerathought. Te shutoff valves should be accsible at all times, so they should not be loca

in such places as the ceilings of locked officescondominiums.

MAINTAINING THE BALANCE OF HOT

WATER SYSTEMSo ensure that a balanced hot water systremains balanced after the shutoff valves habeen utilized, the hot water return system mbe provided with a separate balancing valveaddition to the shutoff valve or, if the balaing valve is also used as the shutoff valve, balancing valve must have a memory st(See the discussion of balancing valves wmemory stops below.) With a memory s

on the valve, plumbers can return a systemits balanced position after working on it raththan have the whole piping system remunbalanced, which would result in serioproblems.

PROVIDING CHECK VALVES AT THE ENDS HOT WATER LOOPSTe designer should provide a check valveeach hot water return line where it joins othot water return lines. Tis is done to ensthat a plumbing fixture does not draw hot retuwater instead of hot supply water, which counbalance the hot water system and cadelays in obtaining hot water at some fixture

A DELAY IN OBTAINING HOT WATER ATDEADEND LINESKeep the delay in obtaining hot water at tures to within the time (and branch lengparameters given previously to avoid unhapusers of the hot water system and to prevlawsuits.

FLOW BALANCING DEVICESTe following are the more common typesbalancing device.


Figure 4 Downfeed Hot Water System with Heater at Top of System.


Note:This piping system increases the developed length of the HW system over the upfeed systems shown in Figures 1 and 3.* 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 downfeed systems shown in Figures 2 and 4.* See text for requirements for strainers.

Figure 6 Combination Downfeed and Upfeed Hot Water System with Heater at Top of System.

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FIXED ORIFICES AND VENTURISTese can be obtained for specific flow rates andsimply inserted into the hot water return piping system.(See Figure 8.) However, extreme care should be takento locate these devices so they can be removed andcleaned out, as they may become clogged with thedebris in the water. It is recommended, therefore, thata strainer with a blowdown valve be placed ahead ofeach of these devices. Additionally, a strainer with afine mesh screen can be installed on the main water

line coming into the building to help prevent debrisbuildup in the individual strainers. Also, a shutoff valveshould be installed before and after these devices sothat an entire loop does not have to be drained in orderto service a strainer or balancing device.

FACTORY PRESET AUTOMATIC FLOW CONTROLVALVESTe same admonition about strainers and valvesgiven for fixed orifices and venturis above appliesto the installation and location of these devices. (SeeFigure 9.)

FLOW REGULATING VALVES

Tese valves can be used to determine the flow rateby reading the pressure drop across the valve. Teyare available from various manufacturers. (See Figure10.)

BALANCING VALVES WITH MEMORY STOPSTese valves can be adjusted to the proper setting by installinginsertable pressure measuring devices (Petes Plugs, etc.) in thepiping system, which indicate the flow rate in the pipe line. (SeeFigure 11.)

SIZING HOT WATER RETURN PIPING SYSTEMS AND

RECIRCULATING PUMPSTe method for selecting the proper size of the hot water returnpiping system and the recirculating pump is fairly easy, but it doesrequire engineering judgment. First, the plumbing engineer hasto design the hot water supply and hot water return piping sys-tems, keeping in mind the parameters for total developed length,1prompt delivery of hot water to fixtures, and velocities in pipe lines.Te plumbing engineer has to make assumptions about the sizes ofthe hot water return piping.

After the hot water supply and hot water return systems aredesigned, the designer should make a piping diagram of the hotwater supply system and the assumed return system showingpiping sizing and approximate lengths. From this piping diagramthe hourly heat loss occurring in the circulated portion of the hotwater supply and return systems can be determined. (See able 4for minimum required insulation thickness and able 5 for approx-

imate piping heat loss.)Next determine the heat loss in the hot water storage tank if one

is provided. (See able 6 for approximate tank heat loss.) Calculatethe total hot water system energy loss (tank heat loss plus pipingheat loss) in British thermal units per hour (watts). Tis total hotwater system energy loss is represented by q in Equation 1 below.Note: Heat losses from storage type water heater tanks are not nor-mally included in the hot water piping system heat loss becausethe water heater capacity takes care of this loss, whereas pumpedhot water has to replace the piping convection losses in the pipingsystem.(1) q = 60rwc

[q = 3600rwc]

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) = change in heated water temperature (tem-

perature of leaving water minus temperature oincoming water, represented in this manual as c, F [K])

Terefore q =c (gpm 8.33 lb/gal)(60 min/h)(F temperaturedrop)

=1(gpm) 500 F temperature drop [q =c (L/sec 1kg/L)(3600 sec/h)(K temperature dro =1(L/sec) 15 077 kJ/L/sec/K K temperature dr

(2) gpm system heat loss (Btu/h)500 F temperature drop

[L/sec system heat loss (kJ/h) ]15 077 K temperature dropIn sizing hot water circulating systems, the designer should n

that the greater the temperature drop across the system, the lwater is required to be pumped through the system and, therefothe greater the savings on pumping costs. However, if the domehot water supply starts out at 140F (60C) with, say, a 20F (6.7temperature drop across the supply system, the fixtures near end of the circulating hot water supply loop could be providwith a hot water supply of only 120F (49C). In addition, if the water supply delivery temperature is 120F (49C) instead of 14(60C), the plumbing fixtures will use greater volumes of hot wato get the desired blended water temperature (see Chapter 1, a1.1). Terefore, the recommended hot water system temperatdrop should be of the magnitude of 5F (3C). Tis means thathe hot water supply starts out from the water heater at a tempeture between 135 and 140F (58 and 60C), the lowest hot wasupply temperature provided by the hot water supply system co



Figure 7 Instantaneous Point-of-Use Water Heater Piping Diagram.

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be between 130 and 135F (54 and 58C). With multiple tempera-ture distribution systems, it is recommended that the recirculationsystem for each temperature distribution system be extended backto the water heating system separately and have its own pump.

Using Equation 2, we determine that, if there is a 5F (3C) tem-

perature drop across the hot water system, the number to divideinto the hot water circulating system heat loss (q) to obtain theminimum required hot water return circulation rate in gpm (L/sec)is 2500 (500 5F), (45 213 [15 071 3C]).

For a 10F (6C) temperature drop that number is 5000 (fromEquation 2, 500 10F = 5000) (90 426 [from Equation 2, 15 071 6C= 90 426]). However, this 10F (6C) temperature drop may producehot water supply temperatures that are lower than desired.

After Equation 2 is used to establish the required hot water returnflow rate, in gpm (L/sec), the plumbing designer can size the hotwater return piping system based on piping flow rate velocitiesand the available pump heads. It is quite common that a plumb-ing designer will make wrong initial assumptions about the sizes

of the hot water return lines to establish the iniheat loss figure (q). If that is the case, the plumbengineer will have to correct the hot water retupipe sizes, redo the calculations using the new dbased on the correct pipe sizing, and verify thatthe rest of the calculations are now correct.

EXAMPLE 1 CALCULATION TO

DETERMINE REQUIRED CIRCULATION RAT1. Assume that the hotwater supply piping system has 800 ft (244 m) ofaverage size 1 in. (DN32) pipe. From able 5,determine the heat loss per linear foot (meter). find the total heat loss, multiply length times healoss per foot (meter):

800 ft 13 Btu/h/ft = 10,400 Btu/h supply piping losses(244 m 12.5 W = 3050 W supply piping losses)

2. Assume that the hot water return piping system for thesystem in no. 1 above has 100 ft (30.5 m) of average in.

(DN15) piping and 100 ft (30.5 m) of average in. (DN20)pipe. From able 5 determine the heat loss per linear foot(meter):

100 ft 8 Btu/h/ft = 800 Btu/h piping loss(30.5 m 7.7 W/m = 235 W piping loss)

100 ft 10 Btu/h/ft = 1000 Btu/h piping loss1800 Btu/h piping loss

(30.5 m 9.6 W/m = 293 W piping loss )528 W piping loss3. Determine the hot water storage tank heat loss. Assume the

system in no. 1 above has a 200-gal (757-L) hot water storag


Source:CourtesyofGerandEngin

eeringCo.

Figure 8 Fixed Orifices and Venturi Flow Meters.

Source:CourtesyofGriswold

Controls

.

Figure 9 Preset Self-Limiting Flow Control Cartridge.

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tank. From able 6 determine the heat loss of the storage tank@ 759 Btu/h (222 W).

4. Determine the hot water systems total heat losses by totalingthe 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

otal system heat losses 12,959 Btu/h otal 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 otal system heat losses 3799 W otal 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 5F (3C) temperature drop,

12,200 Btu/h = 4.88 gpm (say 5 gpm)required hot water returncirculation rate

5F temperature difference 500

3577 W = 0.29 (say 0.3) L/sec

required hot water returncirculation

3C temp. difference

4188.32 kJ/m3

RECALCULATION OF HOT WATER SYSTEM LOSSES1. Assume that the hot water supply piping system has 800 ft

(244 m) of average size 1 in. (DN32) pipe. From able 5determine the heat loss per linear foot (meter):

800 ft 13 Btu/h/ft = 10,400 Btu/h piping loss(244 m12.5 W/m = 3050 W piping loss)

2. Assume that the hot water return piping system for thesystem in no. 1 above has 100 ft (30.5 m) of average in.(DN15) pipe, 25 ft (7.6 m) of average in. (DN22) pipe, and

75 ft (22.9 m) of average 1 in. (DN28) pipe. From able 5,determine the heat loss per linear foot (meter):

100 ft 8 Btu/h/ft = 800 Btu/h piping loss 25 ft 10 Btu/h/ft = 250 Btu/h piping loss 75 ft 10 Btu/h/ft = 750 Btu/h piping loss 1800 Btu/h piping loss

[30.5 m 7.7 W/m = 235 W piping loss



Source:ITTIndustries

.Usedwithpermission.

Figure 10 Adjustable Orifice Flow Control Valve.

Source:Courtesy

ofMilwaukeeValveCo.

Figure 11 Adjustable Balancing Valve with Memory Stop.

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7.6 m 9.6 W/m = 73 W piping loss 22.9 m 9.6 W/m = 220 W piping loss 528 W piping loss]

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

4. Determine the systems 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 otal system heat losses 12,959 Btu/h otal 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 otal system heat losses 3800 W otal system piping heat losses (A + B) = 3578 W]

Note: Te recalculation determined that the hot water systheat losses remained unchanged and that 4.88 (say 5) gpm (0[say 0.3] L/sec) is the flow rate that is required to maintain the (3C) temperature drop across the hot water supply system.

It should be stated that engineers use numerous rules of thuto size hot water return systems. Tese rules of thumb are all baon assumptions, however, and are not recommended. It is recomended that the engineer perform the calculations for each projto establish the required flow rates because, with all the variocapacities of the pumps available today, exact sizing is possible, aany extra circulated flow caused by the plumbing engineer usinrule of thumb equates to higher energy costs, to the detrimentthe client.

ESTABLISHING THE HEAD CAPACITY OF THE HOT WAT

CIRCULATING PUMP

Table 4 Minimum Pipe Insulation Thickness

Required Insulation Thickness for Piping (in.)

Runouts 2in. or Lessa 1 in. or Less 12 in. 24 in. 5 & 6 in.

8 in. orLarger

1 1 1 1 1Note: Data based on fiberglass insulation with all-service jacket. Data will change depending on actual typeof insulation used. Data apply to recirculating sections of hot water systems and the first 3 ft from the storagetank of uncirculated systems.a Uncirculated pipe branches to individual fixtures (not exceeding 12 ft in length). For lengths longer than 12ft, use required insulation thickness shown in table.

Table 4(M) Minimum Pipe Insulation Thickness

Required Insulation Thickness for Piping (mm)

RunoutsDN32 or

LessaDN25 or

LessDN32DN50

DN65DN100

DN125 &DN150

DN200 orLarger

13 25 25 40 40 40Note: 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 thestorage tank of uncirculated systems.

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

Table 5 Approximate Insulated Piping Heat Loss and SurfaceTemperature

Nominal PipeSize (in.)

InsulationThickness (in.)

Heat Loss(Btu/h/ linear ft)

SurfaceTemperature

(F)

1 8 68 1 10 69

1 1 10 69

1 1 13 701 1 13 692 or less a 24 or less 742 1 16 702 1 12 673 1 16 684 1 19 69

6 1 27 698 1 32 69

10 1 38 69Note: Figures based on average ambient temperature of 65F and annual average wind

speed of 7.5 mph.a Uncirculating hot water runout branches only.

Table 5(M) Approximate Insulated Piping Heat Loss and SurfaceTemperature

Nominal PipeSize (mm)

InsulationThickness (mm)

Heat Loss(W/m)

SurfaceTemperature

(C)

DN15 25 7.7 20DN20 25 9.6 21DN25 25 9.6 21

DN32 25 12.5 21DN40 25 12.5 21DN50 or less 13a 23.1 or less 23DN50 25 15.4 21DN65 38 11.5 19DN80 38 15.4 20DN100 38 18.3 21DN150 38 26.0 21DN200 38 30.8 21

DN250 38 36.5 21Note: Figures based on average ambient temperature of 18C and annual average wind

speed of 12 km/h.a Uncirculating hot water runout branches only.

Table 6 Heat Loss from Various Size Tanks withVarious Insulation Thicknesses

InsulationThickness

(in.)Tank Size

(gal)

Approx. Energy Lossfrom Tank at Hot Water

Temperature 140F (Btu/h)a

1 50 4681 100 7362 250 7593 500 7593 1000 1273

Source: From Sheet Metal and Air Conditioning Contractors NationalAssociation (SMACNA) Table 2 data.a For unfired tanks, federal standards limit the loss to no more than 6.5Btu/h/ft2of tank surface.

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

InsulationThickness

(mm)Tank Size

(L)

Approx. Energy Lossfrom Tank at Hot WaterTemperature 60C (W)a

25.4 200 13725.4 400 21650.8 1000 222

76.2 2000 22276.2 4000 373

Source: From Sheet Metal and Air Conditioning Contractors NationalAssociation (SMACNA) Table 2 data.

a For unfired tanks, federal standards limit the loss to no more than 1.9W/m2of tank surface.


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Te hot water return circulating pump is selected based on therequired hot water return flow rate (in gpm [L/sec]), calculated usingEquation 2, and the systems pump head. Te pump head is nor-mally determined by the friction losses through only the hot waterreturn piping loops and any losses through balancing valves. Te hotwater return piping friction losses usually do not include the frictionlosses that occur in the hot water supply piping. Te reason for this isthat the hot water return circulation flow is needed only to keep thehot water supply system up to the desired temperature when thereis 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 watersupply piping to keep the system hot water supply piping up to thedesired temperature without help from the flow in the hot waterreturn piping.

Te only exception to the rule of ignoring the friction losses in thehot water supply piping is a situation where a hot water return pipeis connected to a relatively small hot water supply line. Relativelysmall here means any hot water supply line that is less than onepipe size larger than the hot water return line. Te problems cre-ated by this condition are that the hot water supply line will addadditional friction to the head of the hot water circulating pump,and the hot water circulating pump flow rate can deprive the lastplumbing fixture on this hot water supply line from obtaining its

required flow. It is recommended, therefore, that in such a situationthe hot water supply line supplying each hot water return pipingconnection point be increased to prevent these potential problems,i.e., use in. (DN22) hot water supply piping and in. (DN15) hotwater return piping, or 1 in. (DN28) hot water supply piping and in. (DN22) hot water return piping, etc.

When selecting the hot water circulating pumps head, thedesigner should be sure to calculate only the restrictions encoun-tered by the circulating pump. A domestic hot water system is nor-mally considered an open system (i.e., open to the atmosphere).When the hot water circulating pump is operating, however, it isassumed that the piping is a closed system. Terefore, the designershould not include static heads where none exists. For example, inFigure 1, the hot water circulating pump has to overcome only the

friction in the hot water return piping not the loss of the static headpumping the water up to the fixtures because in a closed systemthe static head loss is offset by the static head gain in the hot waterreturn piping.

HOT WATER CIRCULATING PUMPSMost hot water circulating pumps are of the centrifugal type and areavailable as either in-line units for small systems or base-mountedunits for large systems. Because of the corrosiveness of hot watersystems, the pumps should be bronze, bronze fitted, or stainlesssteel. Conventional, iron bodied pumps, which are not bronzefitted, are not recommended.

CONTROL FOR HOT WATER CIRCULATING PUMPS

Tere are three major methods commonly used for controlling hotwater circulating pumps: manual, thermostatic (aquastat), andtime clock control. Sometimes more than one of these methods areused on a system.1. A manual control runs the hot water circulating pump contin-

uously when the power is turned on. A manual control shouldbe used only when hot water is needed all the time, 24 h a day,or during all the periods of a buildings operation. Otherwise,it is not a cost-effective means of controlling the circulatingpump because it will waste energy.

Note: Te method for applying the on demand concept forcontrolling the hot water circulating pump is a manual control.

It can be used very successfully for residential and commercapplications.2. A thermostatic aquastat is a device that is inserted into the

hot water return line. When the water in the hot water retursystem reaches the distribution temperature, it shuts off thecirculating pump until the hot water return system temperature drops by approximately 10F [5.5C]. With this methodwhen there is a large consumption of hot water by the pluming fixtures, the circulating pump does not operate.

3. A time clock is used to turn the pump on during specific ho

of operation when people are using the fixtures. Te pumpwould not operate, for example, at night in an office buildinwhen nobody is using the fixtures.

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

AIR ELIMINATIONIn any hot water return circulation system it is very important ththere be a means of eliminating any entrapped air from the water return piping. Air elimination is not required in the hot wasupply piping because the discharge of water from the fixtures w

eliminate any entrapped air. If air is not eliminated from the hwater return lines, however, it can prevent the proper circulatof the hot water system. It is imperative that a means of air elimnation be provided at all high points of a hot water return systeTe plumbing engineer must always give consideration to preciswhere the air elimination devices are to be located and drained. example, they should not be located in the unheated attics of buiings in cold climates. If the plumbing engineer does not considthe location of these devices and where they will drain, the resmay be unsightly piping in a building or extra construction cost

INSULATIONTe use of insulation is very cost-effective. It means paying one tito save the later cost of significant energy lost by the hot water supand return piping system. Also, insulation decreases the streson the piping due to thermal expansion and contraction causby changes in water temperature. Furthermore, the proper useinsulation eliminates the possibility of someone getting burneda hot, uninsulated water line. See able 5 for the surface tempetures of insulated lines (versus 140F [60C] for bare piping).

It is recommended that all hot water supply and return pipinginsulated. Tis recommendation exceeds some code requiremenSee able 4 for the minimum required insulation thicknesses forsystems.

If the insulated piping is installed in a location where it is sujected to rain or other water, the insulation must be sealed witwatertight covering that will maintain its tightness over time. W

insulation not only does not insulate, it also releases consideraheat energy from the hot water piping, thus wasting energy. Fthermore, the insulation on any outdoor lines that is not seawatertight can be plagued by birds or rodents, etc., pecking at insulation to use it for their nests. In time, the entire hot wasupply and/or return piping will have no insulation. Such bare hwater supply and/or return piping will waste considerable eneand can seriously affect the operation of the hot water system awater heaters.

Te minimum required insulation thicknesses given in ablare based on insulation having thermal resistivity (R) in the rang4.0 to 4.6 ft2 h (F/Btu) in. (0.028 to 0.032 m2[C/W] mm) oflat surface at a mean temperature of 75F (24C). Minimum insu



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tion thickness shall be increased for materials having R values lessthan 4.0 ft2 h (F/Btu) in. (0.028 m2 [C/W] mm) or may bereduced for materials 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. (0.032 m2[C/W] mm), the minimuminsulation thickness may be reduced as follows:

4.6 able 4 thickness = New minimum thicknessActual R

(0.032 able 4 thickness

= New minimum thickness)Actual R2. For materials with thermal resistivity less than 4.0 ft2 h (F/Btu) in. (0.028 m2[C/W] mm), the minimum insulationthickness shall be increased as follows:

4.0 able 4 thickness = New minimum thickness)Actual R( 0.028 able 4 thickness = New minimum thickness)Actual R

CONCLUSIONIn conclusion, an inappropriate hot water recirculation system canhave serious repercussions for the operation of the water heaterand the sizing of the water heating system. In addition, it can causethe wastage of vast amounts of energy, water, and time. Terefore,

it is incumbent upon the plumbing designer to design a hot waterrecirculation system so that it conserves natural resources and is inaccordance with the recommendations given in this chapter.

BIBLIOGRAPHY1. American Society of Heating, Refrigerating, and Air Condi-

tioning Engineers. 1993. Pipe sizing. Chapter 33 in Fundamen-tals Handbook.

2. American Society of Heating, Refrigerating, and Air Condi-tioning Engineers. 1993. Termal and water vapor transmis-sion data. Chapter 22 inFundamentals Handbook.

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

4. American Society of Heating, Refrigerating, and Air Con-ditioning Engineers. Energy conservation in new build-ing design.ASHRAE Standards,90A1980, 90B1975, and90C1977.

5. American Society of Heating, Refrigerating, and Air Condi-tioning Engineers. Energy efficient design of new low riseresidential buildings.ASHRAE Standards, 90.21993.

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

7. American Society of Mechanical Engineers. Plumbing fixturefittings. ASME A112.18.1M1989.

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 inASPE 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 inASPE Data Book.12. American Society of Plumbing Engineers. 1990.Insulation.

Chapter 12 inASPE Data Book.13. American Society of Plumbing Engineers. 1990. Pumps. Chap-

ter 11 inASPE Data Book.

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

15. American Water Works Association. 1985. Internal corrosioof water distribution systems. Research Foundation coopertive 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 buildinsystems. Paper no. 509 presented at the National Associatioof Corrosion Engineers Annual Conference.

18. Copper Development Association. 1993. Copper Tube Handbook.

19. International Association of Plumbing and Mechanical Offi-cials. 1985. Uniform Plumbing Code Illustrated Training Manu

20. Konen, Tomas P. 1984. An experimental study of competinsystems for maintaining service water temperature in residetial buildings. InASPE 1984 Convention Proceedings.

21. Konen, Tomas P. 1994. Impact of water conservation oninterior plumbing. In Technical Proceedings of the 1994 ASPEConvention.

22. Saltzberg, Edward. 1988. Te plumbing engineer as a forensengineer. In Technical Proceedings of the 1988 ASPE Conventio23. Saltzberg, Edward. 1993. o combine or not to combine: An

indepth review of standard and combined hydronic heat-ing systems and their various pitfalls. Paper presented at thAmerican Society of Plumbing Engineers Symposium, October 2223.

24. Saltzberg, Edward. 1996. Te effects of hot water circulationsystems on hot water heater sizing and piping systems. echnical presentation given at the American Society of PlumbinEngineers convention, November 36.

25. Saltzberg, Edward. 1997. In press. New methods for analyzihot 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., od Windsor, Al Fehrm, and Greg Wilcox

1988. Mixing valves and hot water temperature. In TechnicaProceedings of the 1988 ASPE Convention.

28. Sheet Metal and Air Conditioning Contractors National Assciation. 1982.Retrofit of Building Energy Systems and Processe

29. Steele, Alfred.Engineered Plumbing Design. 2d ed.30. Steele, Alfred. 1988. emperature limits in service hot water

systems. In Technical Proceedings of the 1988 ASPE Conventio31. Wen-Yung, W. Chan, and Milton Meckler. 1983. Pumps and

pump systems. InAmerican Society of Plumbing EngineersHandbook.


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