This section contains a practical guide for modeling Domestic Hot Water Systems in multifamily structures. Please read carefully as these tips can directly impact the accuracy of your model.

Energy Factor (EF):

This input is only to be used for equipment that has been rated and has an Energy Guide label.  Typically, this is only for residential type equipment.  The following relationships may be used to calculate the Energy Factor using the Energy Guide annual usage estimate.  Remember that you may have to first convert the \$/year on the label to units of energy.

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

Domestic Hot Water Piping:

This area shall include all piping throughout the entire system in both conditioned and unconditioned spaces.  Unlike heating and cooling distribution where it is proper to only model the portion of the distribution that is in unconditioned spaces, losses from domestic hot water piping affect the heating/cooling loads of the building.  The user should modify the total distribution piping area and the percent allocation of that piping to the correct spaces.

• TREAT’s default for domestic hot water piping area is 0.008 * Total Conditioned Floor Area.  This is reasonable for residential systems but may be too small for multifamily building, especially high-rise buildings that have larger pipes for the risers and branches.
• Allocate the DHW piping to the correct spaces.  Typically, the majority of the piping area will be in the conditioned space and only a fraction in the boiler/mechanical room(s).

Tank Volume of the boiler/water heater:

• The Tank Volume input is used to calculate jacket losses if the Additional Insulation R-value input is greater than zero for an existing or proposed water heater.  It is also used to calculate jacket losses if stand-by loss is entered instead of energy factor.
• The Tank Volume is for the boiler/water heater regardless if it is the heating boiler with a tankless coil, a summer boiler, a heating boiler that heats a storage tank, etc. The Tank Volume should include all boilers that fire in order to heat the domestic hot water, but should not include the volume of any unfired storage tanks. Those should be entered separately in the Unfired Storage Tank input screen.

Input Capacity:

The instructions in the magenta question mark next to this input should be followed.