Common Causes

Common Causes of Unsatisfactory Operation of Heat
Pumps on the Heating Cycle
A. Dirty Filters or inadequate air volume through indoor coil.
When the heat pump is on the heating cycle, the indoor
coil is functioning as a condenser; therefore, the filters
must always be clean, and sufficient air volume must
pass through the indoor coil to prevent excessive
discharge pressure and high-pressure cutout.
B. Outside Air into Return Duct: Cold outside air should not
be introduced in the return duct of a heat pump installation
on the heating cycle close enough to the indoor coil to
reduce temperature of the air entering the coil below 65°F.
Air below this temperature will cause low discharge
pressure, thus low suction pressure and excessive defrost
cycling with resultant low heating output. It may also cause
false defrosting.
C. Undercharge: Undercharge on the heating cycle will
cause low discharge pressure resulting in low suction
pressure and frost accumulation on the lower part of the
outdoor coil.
D. Poor "Terminating" Defrost Thermostat contact. Defrost
thermostat must make good thermal contact on return bend,
otherwise it may not terminate the defrost cycle quickly
enough to prevent unit from cutting out on high discharge
pressure during the defrost cycle.