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.
The instructions in the magenta question mark next to this input should be followed.
The Usage Adjustment Multiplier:
This multiplier should be used to get a reasonable Estimated Demand of Gal/per day person. The Help button on the DHW Screen takes the user through a few questions to describe the building’s population profile and uses lookup values from the ASHRAE Handbook to give reasonable domestic hot water usages/day/person.
Note: This multiplier input should not be pushed to unreasonable values just to calibrate the DHW energy usage. It likely means that other inputs are incorrect.
These systems should always be modeled in TREAT if they exist in the building as they significantly affect the DHW energy usage and are a great Energy Conservation Measure (ECM) if they are currently not controlled. The default settings in TREAT when the user selects “Yes” for Recirculating System are for 24/7 recirculation with no control. The user can enter the aquastat temperature and/or the maximum recirculation time, if an aquastat or timer exists or to add as an ECM.