The Reversing Valve is used to change the flow of refrigerant gas in
the system.
For cooling the liquid refrigerant is piped to the indoor coil that acts as an
evaporator for the liquid as the warm air in the home is passed through
the coil. As the liquid is evaporated it absorbs heat from the home.
In the cooling mode the outdoor coil acts as the condenser coil. The
evaporated liquid (gas) is piped to the outdoor coil under pressure which
causes the gas temperature to rise above the outside ambient
temperature. The cooler outside air chills the "Hot Gas" causing it to
condense into a liquid and releasing the heat that was absorbed in the
home.
For Heating operation the "Reversing Valve" changes the direction of
refrigerant flow and the outdoor coil becomes the evaporator coil where it
absorbs heat from the outside air. The indoor coil becomes the
condenser coil where the pressurized gas from the outside coil is
condensed into a liquid and the corresponding release of heat is blown
into the home.