Use this option the appropriate type from the drop-down box. If a space heating boiler is used for hot water (water heater type is Space-heating boiler w/storage tank or Space-heating boiler w/tanklesscoil) then both heating and non-heating season efficiencies must be entered. Type input is also used for Home Energy Ratings.


Select a fuel from the drop down box. Any fuel entered on Fuels/Rates screen may be chosen.

Rated Volume and Rated Input Capacity:

This value may be taken from the water heater nameplate.


Rated volume:

This input is used to calculate jacket loss if the Additional Insulation R-value input is greater than 0 for an existing or proposed water heater. It is also used to calculate annual average efficiency if stand-by loss is entered instead of the energy factor. Rated Input Capacity is used for records only.

Design Supply Water Temperature:

The water heater or mixing valve set point. This input is used to calculate hot water demand and pipe losses.

Additional Insulation R-Value:

This R-value is used to adjust heater jacket loss and overall efficiency.


Select the space where the water heater is located. This input is used to estimate the jacket loss of the water heater. Jacket loss affects heater energy consumption and internal heat gain of the space where the heater is located.

Solar Fraction:

Select the fraction of the hot water that is generated by solar. This default is set to 0.

Number of identical water heaters:

This input is used to calculate total jacket loss.


Entering the year is used for record-keeping purposes only.

Note: TREAT uses two efficiency values for water heaters – Recovery Efficiency and either Energy Factor or Stand-by Loss.

Efficiency data for variety of water heaters is available in TREAT Water Heater library accessible from the Water Heater screen. The brand-specific data is available in the GAMA directory at AHRI.

Recovery Efficiency:

This input allows the user to define ratio of energy delivered to the water to the energy content of the fuel consumed by the water heater.

Energy Factor:

The energy factor is measure of water heater overall efficiency determined by comparing the energy supplied in heated water to the total daily energy consumption of the water heater.

Note: The EF should only be used for equipment that has been rated and has an Energy Guide label.

Standby loss:

The average hourly energy consumption divided by the average hourly heat energy contained in the stored water, expressed as percent per hour.

Note: The following relationships may be used to calculate the Energy Factor using the Energy Guide annual usage estimate:

  • For gas heaters, Energy Factor = 150 / [Energy Guide Therm/year].
  • For electric heaters, Energy Factor = 4396 / [Energy Guide kWh/year].

The stand-by loss from combination space and domestic hot water heating systems may differ significantly between heating and non-heating seasons. TREAT allows entering two efficiency values for such systems. TREAT uses the input to calculate the average seasonal efficiency taking into account the heating season months entered on the Weather/Defaults screen. Heating season length is assumed to be 6 months if the heating season entered on the Weather/Defaults screen spans more then half the year.

Hot Water Piping:

This box allows describing the domestic hot water piping system.

Insulation R-value:

By default this value is set to 1.5. The value represents average insulation of the piping system.

Total Area of Piping:

Here you can enter the surface area of hot water piping. The default value is equal to 0.8% of the total area of conditioned spaces. Edit the defaults based on field measurements. The surface area of pipe can be calculated as the sum of 3.14 × Exterior Diameter × Length for all pipe segments.

Note: This value may not be reasonable for Multifamily Applications, see “Modeling Multifamily Domestic Hot Water Systems” below for more details.

If part of the piping system is located in an unheated space(s) and is poorly insulated and if the losses from the rest of the piping are not significant, you may choose to adjust Total Area of Piping input to represent only piping located in the unheated space(s). You can then specify insulation R-value to reflect the piping insulation in the unheated space(s) only and allocate all of the pipe area to the unheated space(s). Such a model will reflect pipe losses more accurately and provide a better estimate of savings from pipe insulation.

Recirculating System:

Set to Yes if there is a hot water loop to make hot water readily available for remote loads. Such systems are mainly used in multifamily buildings. This input is used to estimate hot water distribution loss.

For recirculating systems you can specify the control strategy and the circulating pump capacity. Recirculate When Water Temperature F Falls Below input allows specifying the aquastat set point. By default the value is equal to design supply water temperature (no aquastat).

Hours per Day Recirculation is on:

This input allows modeling timer-controlled recirculation. By default it is set to 24 hours (no timer control).

Circulating Pump HP:

This section allows the user to account for energy consumed by the circulating pump. The hours of operation of the circulating pump are calculated using the control strategy inputs. Credit is given for both aquastat-controlled and timer-controlled recirculation.

% Of Piping Running Through Each Space:

Here you can allocate a percentage of piping to each space in the project. By default 100% of the piping is assigned to the space where the water heater is located (specified in the Location field). The sum of percentages for all spaces in the table should equal 100%. This information is used by TREAT to calculate distribution losses and overheating.

Hot Water Demand:

Use this area is used to enter or adjust hot water demand.

Usage Adjustment Multiplier:

This section allows scaling the hot water demand calculated by TREAT upward or downward. The resulting demand is shown in the Calculated Hot Water Demand box. Click the Calculate button in the box to recalculate demand after inputs are changed. The usage adjustment multiplier is not applied to appliance hot water usage that is specified on the Appliances screen.

Are Dishes Hand washed: 

If you enter yes in this field, then the usage associated with hand washing of dishes is added to the hot water demand. If there are dishwashers in the building you must enter them on the Appliances screen in order to account for their water usage.

Click the Help button for information on typical hot water demand.

Note: The ASHRAE Handbook of Application gives low, average, and high hot water demand for apartment buildings as 14, 30 and 54 gallons per person per day respectively. It typically varies from a daily average of 42 gal/apartment for buildings with 20 or less apartments to 35gal/apartment for buildings with 200 or more apartments.

Low usage is associated with buildings having such occupant demographics as all occupants working, seniors, middle income, and high population density.

High usage is associated with high percentage of children, low income, public assistance, and no occupants working.

Unfired Storage Tanks:

This button brings up a screen that allows entering hot water storage tank information to account for additional hot water load due to tank stand-by losses.