Heat pump & air conditioning return flow UIC wells

Heat pump/air conditioning (HAC) systems return water back into the subsurface groundwater after it has been used to heat or cool a structure by means of an Underground Injection Control (UIC) well — or injection well. 

HAC systems heat or cool buildings by:

  • Extracting heat energy from groundwater.
  • Using groundwater as a heat sink (heat is absorbed) when cooling.

Our Underground Injection Control rule, Chapter 173-218 WAC, authorizes open-loop HAC system wells when a well is registered and the environmental protection requirements of the rule are met.

Required conditions for UIC authorization

The following conditions are required to authorize HAC systems as an injection well:

  • No chemicals are added to the HAC system.
  • HAC system meets water right permitting requirements.
  • Discharges from wells do not affect the water quality in nearby waters on the 303(d) polluted waters list.
  • Discharges from wells do not affect the water in nearby waters with a Total Maximum Daily Load (water quality cleanup) plan.
  • Discharges from wells do not affect groundwater quality by concentrating or redirecting existing contaminant plumes.
  • Groundwater used in the HAC system must be discharged back to the source aquifer.

When UIC authorization requirements are not met

A wastewater discharge permit is required for HAC systems that do not meet the UIC authorization requirements.

Improperly managed wells can heat rivers

A primary concern for HAC systems is the potential for heated water to enter Washington's surface waters. This can occur when heated water is discharged to groundwater — but due to hydraulic continuity — the hot water does not settle in the groundwater zone and is discharged instead to surface water.

While the initial discharge is to groundwater — and there is no regulatory limit for temperature in ground water — the indirect temperature effect on nearby surface water is regulated. Discharges to temperature-impaired surface waters are not allowed.

Improperly managed wells can spread pollution

The same concern must be addressed in groundwater for an HAC located in an area where cleanup actions are occurring. Draw-down from wells can disturb contaminant plumes or concentrate contaminants.

Hydraulic return-flow systems require a permit

HAC return flow systems in hydraulic connection with surface water — or wells requiring a water right permit — are not automatically rule authorized. Additional information is required for registration.