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.
Proposed Design
The water heating system is defined by the heater type (gas, electric resistance, or heat pump), tank type, dwelling-unit distribution type, 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).
Unitary 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.
Water heater and tank types include:
•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, 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.
•Unitary heat pump water heater: ≤ 24 amps 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 CEC 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.
Heater element type includes:
•Electric resistance.
•Gas.
•Heat pump.
Dwelling unit distribution system types for systems serving individual dwelling units include:
•Standard (all distribution pipes insulated).
•Point of use.
•Central parallel piping.
•Recirculation with nondemand control (continuous pumping).
•Recirculation with demand control, push button.
•Recirculation with demand control, occupancy/motion sensor.
•HERS-required pipe insulation, all lines.
•HERS-required central parallel piping.
•HERS-required recirculation, demand control, push button.
•HERS-required recirculation with demand control, occupancy/motion sensor.
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:
•Standard (all distribution pipes insulated).
•HERS required pipe insulation, all lines.
Multifamily central hot water heating central system distribution types include:
•No loops or recirculation system pump.
•Recirculation with no control (continuous pumping).
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.
Distribution Compactness
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:
•None.
•Compact distribution basic credit.
•Compact distribution expanded credit (HERS).
Once basic credit or expanded credit is specified, either the plan view fixture distances (to master bathroom, kitchen, and furthest fixture) will need to be input for the DHW system or, if the distances are unknown, allow a user input compactness factor to be used.
If the fixture distances are specified, the compliance software will determine if the distances qualify for the credit.
If the fixture distances are not specified, compliance with the user input compactness factor will be verified on the CF2R where the actual fixture distances for the design will need to be specified.
Drain water heat recovery (DWHR) is a system where the waste heat from shower drains is used to preheat the cold inlet water. The preheated water can serve 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.
Multiple DHWR devices can be used for a water heater system.
Drain water heat recovery is a HERS-verified measure.
If the proposed design uses electricity as the fuel source, the standard design is a single heat pump water heater with a 2.0 UEF. with compact distribution basic credit in Climate Zones 1 and 16, and a drain water heat recovery system in Climate Zone 16.
If the proposed building has an attached garage, then the standard design HPWH location is the garage. If the proposed building does not have an attached garage, then the standard design HPWH location is in the conditioned space with the air inlet and outlet ducted to the outside.
If the proposed design is gas, then 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.
The energy performance of central water heating systems is determined by the primary heating equipment, primary heating storage volume, location, secondary heating equipment, secondary heating storage volume, set point controls, and the way in which the components are plumbed.
Water-heating device.
If the proposed central water heating device uses electricity as the fuel source, the standard design is a central split heat pump water heater system that includes the following:
Primary single-pass, split-system heat pump plumbed to a primary storage volume. The standard design heat pump water heater output capacity and the primary storage tank capacity are automatically sized so that the heat pump and primary storage volume jointly meet the peak water used on the design (coldest) day. The algorithm sizes the primary tank volume to meet the peak water draw period and the heat pump output capacity so that the system runs for approximately sixteen hours on the design days.
The primary single-pass heat pump is a generic heat pump, based on the R-134 refrigerant operating cycle, with minimum output capacity as determined above.
In the standard design, the recirculation loop is decoupled from the primary system. The secondary heater and tank are connected to the primary system in series and both the primary tank outlet and hot water circulation return are connected to the bottom of the secondary tank.
The secondary tank is an electric resistance water heater with output heating capacity calculated as follows:
Output Capacity (watts) = 1.75 * 100 * Number of Dwelling Units
The secondary tank storage volume is determined by the following:
Tank Volume (gallons) = 80 if Number of Dwelling Units < 48
Tank Volume (gallons = 120 if Number of Dwelling Units > 48
Both the primary and secondary storage tanks have insulation R-values of 16 (°F ft2 hr/BTU)
The locations for the standard design storage tanks and heat pumps are the same as the proposed design.
The temperature setpoints are:
Primary single-pass HPWH: 140 °F
Secondary water heater: 136 °F
Thermostatic mixing valve outlet: 125 °F. If the proposed central water heating device uses gas or propane as the fuel source, the standard design uses natural gas-fired or propane commercial packaged boiler. In Climate 1 through 9, if the total installed water heating input capacity is 1 MMBtu/hr or greater, the standard design gas water-heating equipment thermal efficiency is 90 percent.
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.
Recirculating system. If the central water-heating system has recirculation loops, the standard design includes a recirculation system with no controls and one recirculation loop.
Solar thermal water-heating system. If the proposed system uses gas or propane water heater, 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.
Verification and Reporting
All modeled features and the number of devices modeled for the water heating system are reported on the LMCC or NRCC. 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 LMCC.
When a water heating system has a solar thermal system to provide part of the water heating, the user enters information about the Solar Rating and Certification Corporation approved collector (manufacturer, brand, model number), including details of the installation (azimuth, tilt).
Alternatively, the solar fraction (SF) is determined using the CEC Solar Water Heating Calculator, or approved method showing minimum energy equivalency, OG-100 calculation method, or the certified OG-300 rating. 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.