How to Diognose High Heat Load, Fixed Orface or TXV

High evaporator heat load occurs when there is excessive heat entering the refrigerant at the evaporator coil.  This condition can cause the compressor to shut off on it’s internal overload, or trip the electrical circuit breaker.


  • Infiltration of unconditioned air into the return duct
  • New systems started on a very hot day
  • Economizer/fresh air damper open on a hot day
  • Temporary high people loads
  • System undersized for heat gain


  • Systems running with high evaporator loading will run at pressures above factory required levels.  Abnormally high superheat and normal subcooling levels will be present when the evaporator coil is exposed to higher than allowed heat load.  The high heat load condition could cause the evaporator coil temperature to be very warm.  The compressor shell and hot gas temperature will be extremely high.

TXV  A system with a TXV may keep superheat level close to the required range of 10 to 15F.  In exteme cases of high load the superheat may go higher.


  • Measure the temperature of the conditioned space and then check the temperature of the air entering the evaporator coil at the filter rack.  If the air entering the evaporator is warmer that the conditioned space, infiltration of unconditioned air is present.
  • If the system has an economizer or fresh air damper, make sure it is closed.
  • If the system is new and start up is on a hot day, allow time for the indoor temperature to fall within charging chart range.
  • If the system is still overloaded and the outdoor air temperature is within design range for your area, run a heat load to determine if the system is properly sized.