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Hot water delivery systems shall meet efficient design requirements

The DOE Zero Energy Ready Home (Revision 07) program (Exhibit 1, Item 3) requires that hot water delivery systems in single family homes and distributed (individual water heater) systems in multifamily buildings meet the following efficiency requirements:

To minimize water wasted while waiting for hot water, the hot water distribution system shall store no more than 0.5 gallons (1.9 liters) of water in any piping/manifold between the hot water source and any hot water fixture. In the case of on-demand recirculation systems, the 0.5 gallon (1.9 liter) storage limit shall be measured from the point where the branch feeding the fixture branches off the recirculation loop to the fixture itself. To verify that the system stores no more than 0.5 gallons (1.9 liters), verifiers shall calculate the stored volume using the piping or tubing inside diameter and the length of the piping/tubing. System options include manifold-fed systems; structured plumbing systems; core plumbing layouts, and on-demand recirculation systems. The following requirements apply to recirculation systems:

  1. Recirculation systems must be based on an occupant-controlled switch or an occupancy sensor installed in each bathroom which is located beyond a 0.5 gallon stored-volume range from the water heater.
  2. Recirculation systems which operate based on “adaptive” scheduling, meaning that they “learn” the hot water demand profile in the home and adapt their operation to anticipate this profile, are permitted at this time and do not require the use of occupant-controlled switches or occupancy sensors.
  3. Recirculation systems that are activated based solely on a timer and/or temperature sensor are not eligible.

No more than 0.6 gallons (2.3 liters) of water shall be collected from the hot water fixture before hot water is delivered. Only the fixture with the greatest stored volume between the fixture and the hot water source (or recirculation loop) needs to be tested. To field verify that the system meets the 0.6 gallon (2.3 liter) limit, verifiers shall first initiate operation of on-demand recirculation systems, if present, and let such systems run for at least 40 seconds. If an Adaptive Scheduling system cannot be “forced” into recirculation mode, contact DOE for further guidance. Next, a bucket or flow measuring bag (pre-marked for 0.6 gallons) shall be placed under the hot water fixture. The hot water shall be turned on completely and a digital temperature sensor used to record the initial temperature of the water flow. Once the water reaches the pre-marked line at 0.6 gallons (approximately 24 seconds for a lavatory faucet), the water shall be turned off and the ending temperature of the water flow (not the collection bucket) shall be recorded. The temperature of the water flow must increase by ≥ 10 °F. Under the DOE Zero Energy Ready Home program, the approved verifier may confirm compliance with these requirements.

Central hot water delivery systems in multifamily buildings must include on-demand recirculation which operates based on both a demand indicator and the loop water temperature. For qualifying central systems, verifiers must confirm that the pump is installed with flow in the correct direction and that the system’s temperature sensors are installed.


  • On-demand central systems in multifamily buildings do not currently have a stored volume limit. DOE encourages partners to design central hot water distribution systems in multifamily buildings to limit the stored volume between the recirculation loop and the furthest fixture to 1.0 gallons.
  • Piping for central system recirculation loops in multifamily buildings should be insulated per the local code requirements. DOE encourages the use of R-4 pipe insulation on recirculation loop piping.
  • DOE encourages that the recirculation pump for central systems be set to operate at a temperature which is at least 5°F less than the water heater set point temperature.

DOE will evaluate the possibility of making these recommendations into requirements in future updates to these specifications.

See the following Solution Center guides for further information: