Summary - Energy Design Resources · Central DHW Systems in Multifamily Buildings design brief...

Central DHW Systems in Multifamily Buildings

design brief

Summary

Domestic water heating (DHW) accounts for a significant share of

multifamily building energy use, especially in California’s coastal

towns. While each new construction project will have a different ratio

of energy end uses, one can look at the existing building stock to get

an idea of how much is attributable to DHW systems. According to

the Department of Energy’s Residential Energy Consumption Survey

(RECS), DHW accounted for around 32% of overall energy use for

existing multifamily units in the late 1990’s.1

DHW can be installed on a unit-by-unit level through individual water

heaters or a building-wide basis using a single water heating device,

such as a boiler, to provide hot water for multiple dwelling units. This

second category is known as a central DHW system.

The following design guide, intended for building owners, developers,

and designers, provides an overview and introduction to central DHW

systems by:

1. Highlighting the basics of the regulatory context in California

(Title 24)

2. Discussing the major opportunities for energy savings and

improved service to tenants through improved systems, many

of which have arisen from recent advances in heater and boiler

technology and advanced control systems

3. Identifying common design problems and mistakes

1 O’Brien, Eileen M., comp. United States. Energy Information Administration. Department of Energy. Residential Energy Consumption Survey. Nov. 1999. 2 July 2008 http://www.eia.doe.gov/emeu/recs.

Central DHW Heater Options 2

Instantaneous (Tankless) vs. Storage Tank 2

Condensing vs. Non-Condensing 3

Performance Specifications 4

Central DHW Recirculation Loops 5

Recirculation Loop Controls 8

Commissioning, Performance Monitoring, Fault Detection and Diagnosis 10

Code Provisions Set Baseline for Energy Performance 11

Mandatory Requirements 11

Prescriptive Requirements 12

Performance Requirements 12

Resources for Designers and Contractors 13

contents

Design Brief on Central DHW Systems in Multifamily Buildings

Central DHW Heater Options

There are a number of factors to consider when selecting a central water heater or

boiler: fuel type, maintenance cost, service life, and equipment efficiency. Below

we discuss issues that primarily affect equipment efficiency.

Typically, a DHW system serving 20 units or fewer would require a recovery capacity of 4-5 gallons per hour per apartment, whereas a system serving 100 units would only require 2-3 gallons per hour per apartment.

Figure 1: Representative Graphic of a Central DHW System Including Components

Hot WaterBoiler

RECIRCPump

RECIRCPump

Cold

Wat

erM

ake-

Up

Boile

r Roo

m

Demand Flow Sensor

Representative Setup forBoiler with Storage Tank

StorageWater Tank

Hot Water Supply

Hot Water Return

Conditioned

Semi-conditioned

Unconditioned

Source: Heschong Mahone Group, Inc.

Instantaneous (Tankless) vs. Storage Tank

Water heaters with storage tanks work by maintaining a prescribed hot water

temperature in the tank, which is then distributed to end users. Because storage

tanks constantly lose energy by conduction and convection, it is important to

insulate and minimize the size of the storage tank to decrease energy consumption.

Tank sizing guidelines are available from Section 49.9 of the 2003 ASHRAE

Applications Handbook. not be designated as the Field Technician or

Responsible Person.


Instantaneous heaters are designed to heat water very quickly as it passes

through the combustion chamber, allowing them to meet the peak load on the

system without using a storage tank. Because there is no storage tank, there is no

standby heat loss, resulting in higher efficiency than a typical non-condensing

storage heater.

Figure 2: Schematic of a Residential Tankless Gas Water Heater

Source: California Energy Commission

Fan

Gas

Cold Water InHot Water Out

BurnerVa

lve

Heat Exchanger

FlowSensor

Multiple instantaneous boilers can be connected in parallel to achieve needed throughout.

Condensing vs. Non-Condensing

The combustion of natural gas produces water vapor (steam) as a by-product,

which contains a large amount of heat. In conventional systems, this heat is wasted

up the flue. Condensing boilers recapture this lost energy by cooling the flue gases

to the point where the water vapor re-condenses and releases its energy. Because


Additionally, condensing boilers, unlike conventional heaters, are particularly

efficient from a cold start or under part load, which are common operating

conditions for service water heating. These systems are now required by law in

many European countries and are increasingly popular in the U.S.

Performance Specifications

Small capacity water heaters (75,000 Btu per hour or less for gas water heaters)

are described by an energy factor (EF) - the higher the energy factor, the more

efficient the water heater. The EF indicates a water heater’s overall energy

1 This increased efficiency occurs because the standard calorific value attributed to natural gas includes a downward adjustment equal to the latent heat of the steam.

Figure 3: Schematic of a conventional atmospheric (left) and condensing (right) storage gas water heater. The

condensing heater has a longer heating coil and flue gases exit at the bottom, where condensate is removed before gases

are vented through the induction blower at the top.

Source: Maytag & AO Smith, Respectively

the water vapor contains about 10% as much energy as is obtained from burning

the gas itself, condensing boilers can achieve thermal efficiencies 10-15% higher

than conventional boilers, reaching around 97% efficient in some cases.1

Modern condensing heaters and boilers have a separate chamber where the flue

gases condense, along with other refinements, to reliably prevent acidic condensate

from corroding the metal wall of the flue or combustion chamber (see Figure 3).


efficiency based on the amount of hot water produced per unit of fuel consumed

during typical operation.

Large water heaters, on the other hand, are rated by their Thermal Efficiency (TE).

TE is based on steady state (i.e., full fire) operation. It is not possible to directly

compare EF values with TE values because they are calculated using different test

procedures. However, energy modeling software allows direct comparisons to be

made, in terms of annual Btu consumption, for the purposes of code and program

compliance modeling.

Table R3-44 of the 2008 Residential ACM Manual gives a target (prescriptive) TE

value of 0.8 for systems over 75,000 Btu/hr (almost all central systems), which is

the standard for most commercially-available boilers.

Central DHW Recirculation Loops

If a central DHW system serves just a few apartments, it is possible to quickly

provide hot water upon demand by using a ‘trunk and branch’ system, where a

network of pipes fans out from the storage tank to each shower or faucet.

When the distance to the farthest apartment is more than fifty feet, however, the

wait for hot water becomes excessive and recirculation loop systems are used. In

recirculation loops, hot water is constantly circulated by a pump through a closed

pipe loop to showers and faucets that are connected to the loop by ’runout’ pipes

no more than twelve (12) feet long. This configuration ensures that hot water is

delivered quickly and at a consistent temperature.

When serving more than a handful of apartments, recirculation loops are more

efficient than trunk-and-branch systems because hot water that cools in the pipes

between uses can be recirculated rather than wasted and users receive hot water

almost instantaneously.

However, each recirculation loop requires a pump that uses energy and there are

heat losses from the pipes. Analysis performed in 2002 by the Heschong Mahone

Group, Inc. for the California Energy Commission found that heat loss from the

pipe can be 25-40% of that required for the whole system. Careful design of the

entire system, including controls, is important to reduce energy consumption.


There are many different ways to design a recirculation loop system. All of the

following factors can be chosen to minimize energy use and provide the highest

level of service for tenants.

n Number of heaters or boilers: Larger heaters may be more efficient and

cost-effective than smaller heaters, but having two or more small heaters per

loop provides redundancy so that tenants still receive hot water if a unit fails

or is taken off line for maintenance.

n Routing and insulation: Recirculation loops should be as short as possible

while minimizing runout lengths and routed close to conditioned space to

reduce energy loss to the outside. Pipes laid underground or attached to the

side of the building are highly inefficient, although energy loss can be reduced

somewhat by adding insulation that is completely protected from water ingress.

Horizontal recirculation loops are usually the best choice because they

minimize pressure differentials and the potential for stratification. A single

horizontal loop can serve up to three stories via additional vertical legs.

n Bends: Using gradual bends instead of ‘hard ninety’ elbows allows water

to flow faster and more smoothly, with reduced heat losses and friction.

Consequently, the hot water supply temperature and pump size can be

reduced for increased savings. Gradual bends also reduce the chance of

pinhole leaks from pipe wall erosion.

n Pump sizing: Correct pump sizing is important both for energy

consumption and the quality of service provided to tenants. Generally, large

pumps are required to serve demand-controlled systems, since the whole loop

must be primed with hot water. Smaller pumps are sufficient for timeclock or

temperature modulation controls, which circulate the water continuously.

For demand systems, size the pump to provide hot water to the last tenant

on the loop within a reasonable time frame. To approximately calculate the

time delay, divide the volume of water in the loop by the flow rate through

the pump.


Designers should be careful not to specify a pump that’s more powerful than required, since this will increase the electricity use of the system, increase the rate of crossover flow, and create faster and more turbulent flows that erode the pipe more quickly, leading to leaks

For continuously circulating systems, flow rates should be lower to reduce

erosion corrosion at elbows and bends. However, very low flow rates can allow

debris to settle in the pipe and can increase the required supply temperature.

Flow rates between one and one half (1½) and three and one half (3½) feet

per second provide a reasonable trade-off.

n Runouts: Usually, all hot water in a runout is wasted because it cools before

the next use of the outlet and must be drained before useable hot water is

received. A twelve (12) foot long, three-quarter (¾) inch wide runout contains

over two (2) pints of water that must be heated each time the outlet is used.

Typically, 3/8” pipe is adequate unless appliances are being served.

n Protection of the pump from air pockets and cavitation:

Recirculation pumps can fail if air pockets develop in the recirculation loop.

Section 113(c)5A of 2008 Title 24 includes new requirements that mandate

either installing an air release valve on a riser immediately upstream from the

pump or mounting the pump on a vertical section of pipe. Also, per Section

113(c)5C, a hose bibb must be installed immediately downstream of the pump

to allow the pump to be primed after maintenance and Section 113(c)5D

requires that isolation valves must be provided to allow the pump to be easily

removed. When sizing the pump, ensure that the manufacturer’s requirements

for net positive suction head are met.

n Prevention of crossover flow: Crossover flow is the flow of water between the

hot and cold water pipes, which results in wasted energy and unpredictable

delivery temperatures. It is usually caused by leaking single-lever valves in

showers or faucets or devices such as portable dishwashers that allow free flow

between the hot and cold lines. The pressure differentials set up by the

recirculation pump make this problem especially acute in recirculation systems.


Crossover can be prevented by specifying more durable single-lever valves or

retrofitting leaking valves with new valve seats. Section 113(c)5F of the 2008

standards includes a new mandatory requirement for the installation of a check

valve on the cold water make-up line into the heater to minimize crossover flows.

Recirculation Loop Controls

Recirculation loops constantly consume energy due to heat loss through the pipe

insulation and the electrical power draw of the pump. This section describes

recirculation loop controls that can save energy while maintaining water

temperature at the time and point of use.

Basic Controls

These are the simplest controls permissible by Title 24.

n Timer controls: These devices shut off the recirculation pump during times

when the hot water draw is expected to be minimal. A challenge with these

controls, which is especially prevalent on larger loops, is that tenants may have

to wait for hot water when the pump is switched off. If they complain to the

building manager and he/she overrides the timeclock, all savings are lost.

n Temperature controls: This type of control shuts off the recirculation pump

when the hot water return temperature reaches an upper threshold value and

switches it back on when the temperature falls to a lower threshold.

Figure 4: Swing-type check valve on cold water supply line

Source: Velan Inc.


Advanced Controls

More advanced controls likely offer better service, more robust savings, and

are eligible for credits within 2008 Title 24. Since there is little design guidance

available for these systems, it is advised to consult with the controls manufacturer

to ensure the product is compatible with your system. Some advanced controls

offer continuous monitoring to detect failures and identify savings opportunities.

n Demand Controls: These controls charge the loop with hot water in response

to either demand or a combination of demand and temperature. ‘Demand’

may be signaled by the user manually pressing a switch or activating a motion

sensor or detected by a flow sensor in the loop. The pump turns off when the

demand stops or a required return temperature is reached.

Demand controls can provide better service than timeclock controls by

eliminating the occasional long wait times. Additionally, they are a good

choice for use with shorter recirculation loops for two reasons. First, short

loops reduce the wait time for hot water, which increases the likelihood

tenants will accept the control system. Second, short loops serving only a

few apartments are likely to experience long periods of zero demand, during

which the system saves energy.

n Temperature Modulation Controls: These controls circulate hot water

continuously but save energy by reducing the storage tank temperature at

times of anticipated low demand. Temperature modulation controls can

function on a fixed schedule or adapt their schedule to measured demand

and are, thus, classified as ‘monitoring control systems’ by Title 24.

This control is a good choice for longer recirculation loops that serve many

apartments since demand controls may not provide a fast enough response.

They also allow for a smaller recirculation pump.


Commissioning, Performance Monitoring, Fault Detection and Diagnosis

Each building and its tenants place different demands on the DHW system. To

maximize savings and provide optimum levels of service, each system must be

tailored individually.

This process begins with commissioning, in which the heater or boiler setpoints

are determined, supply and return temperatures are verified, and other functional

checks are carried out. Usually, the control system manufacturer will set up the

control system and log data to verify its proper function.

Some control systems can monitor energy use and the levels of service, detect

and diagnose faults, and give feedback to the maintenance staff. These services allow

the system to improve efficiency over time while consistently delivering sufficient

hot water. Instant alerts of faults can be sent to maintenance staff for early detection

and diagnosis. Monitored data can provide the building owner with a record of the

performance and maintenance of the system over time (see Figure 5).

Figure 5: Example output from DHW performance monitoring, showing hot water supply and return temperatures

Source: Heschong Mahone Group, Inc.


Title 24 Code Sections for DHW: Section 113 – Mandatory requirements for service water-heating systems and equipment (all occupancies)

Section 123 – Mandatory requirements for pipe insulation (high-rise residential)

Section 145 – Prescriptive requirements for service water heating systems (high-rise residential)

Section 150j – Mandatory features and devices (low-rise residential)

Section 151a, 151(f )8 – Performance and prescriptive compliance approaches (low-rise residential)

The full text of the Title 24 standards, along with the manuals and alternative compliance methods, can be found at www.energy.ca.gov/title24

Code Provisions Set Baseline for Energy Performance

The 2005 and 2008 California Building Energy Efficiency Standards (Title 24)

significantly altered the way central DHW systems are treated in compliance

calculations and set revised baseline energy performance criteria for them. This

section explains some key compliance options. It should not, however, serve as

a substitute for reading the standards themselves; Title 24 contains more

requirements than those discussed here.

Mandatory Requirements

The 2005 Title 24 standards incorporated a significant change to the requirements

in Section 113(c) 2 by mandating that recirculation systems have a control capable

of automatically turning off the system.1 Low-cost compliance options include:

n Timer Controls - Must initially be set to operate the pump for no more than

sixteen (16) hours per day

n Temperature Controls - Must have a temperature sensor installed on the

return line that has a ‘deadband’ (the difference between the temperature at

which the pump is switched on and that at which it is switched off ) of less

than 20°F

Additionally, Section 113(c)5 of 2008 Title 24 requires supplementary valves to

prevent crossover flows and the formation of air bubbles.

1 For recirculation loops, ‘the system’ means the pump; for heat trace it means the heating elements.


As detailed in Table123-A of the 2008 code, one (1) inch of insulation must be

provided on pipes up to two (2) inches diameter in recirculation loops, one and

one half (1½) inches on thicker pipes, and one half (½) inch on runouts. It is

important to note that there is little additional energy benefit to installing higher

levels of insulation than those specified.

Prescriptive Requirements

Per Section 151(f ) 8 C-E of 2008 Title 24, central DHW systems are now a

prescriptive compliance option. To comply, water heating equipment must meet

minimum efficiency standards (Title 20), the system must use a recirculation loop,

and the pipes to the kitchen fixtures must be insulated. Additionally, buried pipes

are required to be encased in a waterproof, non-crushable, insulated sleeve.

Performance Requirements

Starting with the 2005 standards, DHW calculations are done hourly, similar

to those of other end uses like heating and cooling, and include the impacts of

Time Dependent Valuation (TDV). Heat losses through recirculation pipes are

specifically assessed through hourly calculations based on pipe location, length of

pipe, and pipe diameter. Based on these results, the code provides incentives for

locating pipes in conditioned or semi-conditioned spaces and disincentives for

large pipes run underground or exposed to outside air.

Perhaps the biggest change has been the evaluation of central DHW systems

in performance calculations. Per Section 151(b) 1, the code now compares a

central DHW system with recirculation loop and controls to an equivalent system

rather than to individual water heaters. This modification reduced the typical

margin of compliance for central DHW systems with recirculation loops in the

performance calculations.

The 2008 code incorporates a 20% credit in the calculation of distribution

system losses for recirculation loop controls that work based on hot water demand

and monitoring control systems that can assist in fault diagnosis. The eligibility

requirements for each type of control are specified in Appendix RA4.4.9.2 of the

Residential Appendices to Title 24.

High-rise vs. low-rise buildings: Alternative calculation methods (ACMs) for single family and low-rise multifamily buildings are set out in the Residential ACM Approval Manual.

Methods for high-rise multifamily (four stories or more) are set out in the Nonresidential ACM Approval Manual.

For service water heating in high-rise residential buildings, the Nonresidential ACM refers to the Residential ACM, so all multifamily buildings use the same calculations, irrespective of the building’s height.

Resources for Designers and Contractors:

n Title 24 Energy Efficiency Standards: www.energy.ca.gov/title24

n DHW System design:

o American Society of Plumbing Engineers Domestic Water Heating

Design Manual - 2nd Edition, ASPE, Chicago, IL

o American Society of Heating, Refrigerating, and Air-Conditioning

Engineers (ASHRAE), ASHRAE Handbook: 2003 HVAC

Applications. ASHRAE, Atlanta, GA

n Energy Efficiency Programs for Multifamily Buildings:

http://h-m-g.com/multifamily

n IRS tax credits for home builders:

http://www.energystar.gov/index.cfm?c=products.pr_tax_credits#s6

Acknowledgements

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