2.9 Domestic Hot Water (DHW)

Water heating energy use is based on the number of dwelling units, number of bedrooms, fuel type, distribution system, water heater type, and conditioned floor area. Detailed calculation information is included in Appendix B.

The water heating system is defined by the heater type (gas, electric resistance, or heat pump), tank type, dwelling-unit distribution type, central system distribution, efficiency (either UEF or recovery efficiency with the standby loss), tank volume, exterior insulation R-value (only for indirect), rated input, and tank location (for electric resistance and heat pump water heater only).

Heat pump water heaters are defined by energy factor, volume, and tank location or, for Northwest Energy Efficiency Alliance (NEEA) rated heat pumps, by selecting the specific heater brand, model, and tank location.

• Consumer storage: ≤ 75,000 Btu/h gas/propane, ≤ 12 kW electric, or ≤ 24 amps heat pump, rated with UEF.

• Consumer instantaneous: ≤ 200,000 Btu/h gas or propane, or ≤ 12 kW electric. An instantaneous water heater is a water heater with an input rating of ≥ 4,000 Btu/h/gallon of stored water, rated with a UEF.

• Residential-duty commercial storage: > 75,000 Btu/h, ≤ 105,000 Btu/h gas/propane, ≤ 12 kW electric, ≤ 24 amps heat pump, and rated storage volume < 120 gallons, rated with a UEF.

• Residential-duty commercial instantaneous: ≤ 200,000 Btu/h gas/propane, ≤ 58.6 kW electric, and rated storage volume ≤ 2 gallons, rated with a UEF.

• Commercial storage: > 75,000 Btu/h gas/propane, >105,000 Btu/h oil, or > 12 kW electric, rated with thermal efficiency and standby loss.

• Commercial instantaneous: >200,000 Btu/h gas/propane, > 12 kW electric. Instantaneous water heater is a water heater with an input rating of ≥ 4,000 Btu/h per gallon of stored water, rated with thermal efficiency.

• Heat pump water heater: ≤ 24 amps Northwest Energy Efficiency Alliance (NEEA) rating or rated with UEF.

• Mini-tank (modeled only in conjunction with an instantaneous gas water heater): a small electric storage buffering tank that may be installed downstream of an instantaneous gas water heater to mitigate delivered water temperatures (e.g., cold water sandwich effect). If the standby loss of this aftermarket tank is not listed in the Energy Commission appliance database, a standby loss of 35 W must be assumed.

• Indirect: a tank with no heating element or combustion device used in combination with a boiler or other device serving as the heating element.

• Boiler: a water boiler that supplies hot water, rated with thermal efficiency or AFUE.

For water-heating systems serving a single dwelling unit, a dwelling unit distribution type must be specified. Dwelling unit distribution system types for systems serving individual dwelling units include:

When a multifamily building has central water heating, both a dwelling unit and a central system distribution type must be specified. Dwelling unit distribution types for this case include:

Multifamily central hot water heating central system distribution types include:

Some distribution systems have an option to increase the amount of credit received if the option for HERS verification is selected. See Appendix B for the amount of credit and Reference Appendices, Residential Appendix Table RA2-1 for a summary of inspection requirements.

Applicable to single dwelling units or multifamily with individual water heater in each dwelling unit. Distribution compactness identifies the proximity between the water heater and use points. The distribution compactness of the water heating system must be specified. The choices include:

For both compact distribution basic credit and expanded credit, the plan view distance from the water heater to the master bedroom, the kitchen, and the furthest fixture must be specified. The software will determine if the distances qualify for the credit.

Drain water heat recovery (DWHR) is a system where the waste heat from shower drains is used to preheat the cold inlet water. The preheat water can be routed to the served shower, water heater, or both.

The user specifies the DHWR device for the water heating system. The rated efficiency of the DWHR device, the number of shower(s) served, and the configuration must be specified. The configuration choices include:

• Equal flow to shower and water heater: The potable-side heat exchanger output feeds both the fixture and the water heater inlet. Potable and drain flow rates are equal, assuming no other simultaneous hot water draws.

• Unequal flow to shower: The potable-side heat exchanger output feeds the inlet(s) of the water heater(s) that are part of the parent DHW system. (The inlet temperature is adjusted to reflect recovered heat.)

• Unequal flow to water heater: The potable-side heat exchanger output feeds only the associated fixture.

2.9.2 Individual Dwelling Units

If the proposed water heater is natural gas or propane, the standard design is a single gas or propane consumer instantaneous water heater for each dwelling unit. The single consumer instantaneous water heater is modeled with an input of 200,000 Btu/h, a tank volume of zero gallons, a high draw pattern, and a UEF meeting the minimum federal standards. The current minimum federal standard for a high-draw-pattern instantaneous water heater is 0.81 UEF or the equivalent of 0.82 energy factor for the standard system.

If the proposed water heater is an electric resistance or a heat pump water heater, the standard design is a single heat pump water heater with a 2.0 UEF. The water heater is installed in an attached garage (if available) or the location specified in Table 24, with compact distribution credit (Table 25), and a drain water heat recovery system in CZ 1 and 16. In Climate Zone 1, the standard design DWHR has an exchanger efficiency of 0.42, serving 100 percent of showers, with an unequal shower configuration. In Climate Zone 16, DWHR has an exchanger efficiency of 0.65, serving 100 percent of showers, with an equal shower configuration.

Table 24: Heat Pump Water Heater Location

Climate Zone	Single-Family	Multifamily
01	Outside	Outside
02	Conditioned	Conditioned
03	Outside	Outside
04	Conditioned	Conditioned
05	Outside	Outside
06	Outside	Conditioned
07	Outside	Conditioned
08	Conditioned	Conditioned
09	Conditioned	Conditioned
10	Conditioned	Conditioned
11	Conditioned	Conditioned
12	Conditioned	Conditioned
13	Conditioned	Conditioned
14	Conditioned	Conditioned
15	Conditioned	Conditioned
16	Conditioned	Conditioned

2.9.3 Multiple Dwelling Units

When the proposed design is a central water heating system, the standard design consists of the water heating devices, a recirculation system, and solar systems as follows:

Water-heating device. The standard design consists of the same number of water-heating devices as the proposed design using the efficiencies required in the Appliance Efficiency Standards. The standard design is natural gas when the proposed device is natural gas. The standard design is propane if the proposed device is propane. Each water-heating device in the proposed system is examined separately. If the proposed water-heating device is gas or propane, the standard design is set to the same type and characteristics as the proposed design.

If the proposed water-heating device is electric resistance or heat pump with no recirculating loops (fewer than eight dwelling units), then the standard design is a heat pump water heater with 2.0 UEF. If the proposed central water-heating device is electric resistance or heat pump with recirculating loops, the standard design is natural gas or propane.

The appropriate efficiencies and standby losses for each standard water-heating device are then assigned to match the minimum federal requirements. The standards for consumer water heaters, as defined by 42 U.S.C 6291(16), are specified in 10 CFR 430.32(d); the standards for commercial water heaters, as defined by 42 U.S.C 6291(16), are specified in 10 CFR 431.110.

Table 25: Electric Water Heating Compactness Factor

Climate Zone	Compactness Factor
01	Enhanced (0.6)
02	Enhanced (0.6)
03	Enhanced (0.6)
04	Enhanced (0.6)
05	Enhanced (0.6)
06	Basic (0.7)
07	Basic (0.7)
08	Basic (0.7)
09	Enhanced (0.6)
10	Enhanced (0.6)
11	Enhanced (0.6)
12	Enhanced (0.6)
13	Enhanced (0.6)
14	Enhanced (0.6)
15	Basic (0.7)
16	Enhanced (0.6)

Recirculating system. The standard design includes a recirculation system with controls that regulate pump operation based on measurement of hot water demand and hot water return temperature, and capable of turning off the system as described in Appendix B4 Hourly Recirculation Distribution Loss for Central Water Heating Systems. The standard design has one recirculation loop.

Solar thermal water-heating system. The standard design has a solar water heating system meeting the installation criteria specified in Reference Residential Appendix RA4 and with a minimum solar savings fraction of 0.20 in Climate Zones 1-9, or 0.35 in Climate Zones 10-16.

All modeled features and the number of devices modeled for the water heating system are reported on the CF1R. Electric resistance and heat pump water heaters indicate the location of the water heater. NEEA-rated heat pumps are identified by the brand and model, which must be verified by the building inspector.

Where water heating system features or distribution systems specify or require HERS verification, those features are listed in the HERS required verification listings on the CF1R.

2.9.4 Solar Thermal Water Heating Credit

When a water heating system has a solar thermal system to provide part of the water heating, the solar fraction (SF) is determined using the Energy Commission Solar Water Heating Calculator, OG-100 calculation method, or the certified OG-300 rating. (Note: The OG-300 rating can be used only for system servings individual dwelling units and not central systems.) The calculation method requires that the user specify the climate zone and conditioned floor area, in addition to published data for the solar thermal water heating system.