Air Conditioner return

March 5, 2018
Air conditioner return air

Water Quality > Ground Water > Underground Injection Control > Heat Pump/Air conditioning Return Flow Wells

Heat pump/air conditioning return flow wells are used to return ground water back to the subsurface that has been circulated through an open-loop, heat pump/air conditioning (HAC) system. HAC systems heat or cool buildings by extracting heat energy from ground water or use ground water as a heat sink (heat is absorbed) when cooling.

UIC rule authorization (approval) is granted for wells if the discharge water will not impair beneficial uses of ground water or nearby surface water. Impairment of a beneficial use for water quality purposes is not the same as water rights impairment. The following is required:

  • No chemicals are added to the HAC system.
  • Ecology’s Water Resource Program water right permitting requirements are met. See Water Resources policy paper, Water Use Associated with Heat Pumps at
  • Discharges from wells do not affect the water quality in nearby surface water bodies that are on the 303d list or subject to a Total Maximum Daily Load plan. Visit the Washington State's Water Quality Assessment [303(d)] webpage at Click on the Search Tool icon and type in the township, range and section to find your section.
  • Discharges from wells do not affect groundwater quality by concentrating or redirecting existing contaminant plumes.

A discharge permit from Ecology’s Water Quality Program is required for systems that do not meet the above requirements.

A primary concern for HAC systems is the movement of heated water away from the injection wells. This situation occurs where heated water is discharged to ground water, but due to hydraulic continuity the hot water is finally discharged to a surface water body. Thus, while the discharge is to ground water, and there is no regulatory limit for temperature in ground water, the indirect temperature effect on nearby surface water is regulated. Such discharges to surface water bodies that are already temperature-impaired are not allowed. For non-impaired surface water bodies, either an NPDES discharge permit or a state waste discharge permit would be required. The same concern must be addressed in ground water for an HAC located in an area where cleanup actions are occurring. Drawdown from wells could redirect contaminant plumes or concentrate contaminants.

Return flow wells in hydraulic connection with surface water bodies or wells requiring a water right permit are not automatically rule authorized, and additional information is required for registration.

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