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Tech Tip: Heat Pump High Pressure Switch

When a heat pump system is operating in the heating mode, the indoor coil is the condenser and the outdoor coil is the evaporator.  The indoor coil/unit is dissipating heat from the refrigerant into the building.  In heating, the indoor coil is seeing high pressure.  If you see a high-pressure switch tripped in the winter, the issue is most likely at the indoor portion of the heat pump system.  Many times, a technician will assume that the system is over charged, but most of the time this is not the case.

One of the first things to check is the pressure switch itself.  This pressure switch (see photo of switch) is rated to open at 630psi and reset at 480psi with a +/- range of 25psi.  This high-pressure switch has the opening range of 605psi to 655psi and reset when the pressure drops in the range of 505psi to 455psi.   Many pressure switches have an opening pressure tolerance of +/- 10%.   If there is no label or if the label is illegible, R410a switches are usually in the 620psi range and R22 is in the 440psi range.

To test the pressure switch, with the unit running, reduce or stop the air flow at the indoor coil.  This will cause the discharge pressure to rise.  Note the pressure when the pressure switch trips.  If the switch trips before the rated pressure range, replace the switch.  Note: do not allow pressure to go above the rated trip value of the high-pressure switch.  If the pressure switch does not trip until it is within rated range, you will need to diagnose further.

The most common cause of a refrigerant high-pressure switch tripping in the wintertime is lack of airflow across the indoor coil.  One of the signs of lack of airflow is a high temperature split across the indoor coil.  Inspect the filter, not just if it is dirty but restrictive.  There are many filters that have a high MERV rating but are very restrictive.  Insure all the registers are open.  Many will close registers to rooms or areas that are not being used “to save money”.

Loose insulation can be pulled into or against the blower assembly and block airflow.  This is sometimes hard to find because the insulation will go back in place when the blower shuts off or the panel is removed.

A dirty coil will prevent proper airflow and heat transfer across a coil.  Inspect and clean the indoor coil as needed.  Most of the time when the indoor coil is dirty, the indoor blower wheel is too.  Quiet often the coil will get cleaned but the blower wheel will be neglected.  If the wheel is dirty, it will not move the required air across the coil.  The best way to clean a wheel is to remove it and thoroughly wash it, taking care not to get moisture in the motor.

High indoor temperature can cause the high pressure to trip in the winter.  This higher the temperature in the house, the higher the head pressure will be.

If there is nothing restricting the airflow, is the blower speed set correctly for the indoor motor?  This is more common on newer systems than in systems that have been installed for several years.  Check to see what speed the indoor blower motor is running at.  Many systems are set to a lower speed at the factory to achieve a better SEER rating.  Manufactures will set airflow around 350 CMF per ton at 0.50” total static and some models are even lower.  400 CFM per ton is recommended for heat pump systems.  Never assume that the blower speed is set correctly at the factory.  Every application is different.  Air flow must be set by the technician when unit is installed.  Airflow setting can be found in the installers guide that is supplied with every unit (Blower Chart).

To determine CFM from a blower chart:

  1. Find the model number of the system.
  2. Must know what speed the blower motor is running on.
    1. Do not assume the motor is on high speed.
  3. Read the total static pressure of the duct system.
    1. Static pressure of return + static pressure of supply = Total static
  4. On the blower chart, cross the motor speed and the total static of the system to find CFM.

If airflow has been confirmed, the issue is within the refrigerant system.   If one is seeing a high head pressure and a lower suction pressure with high subcooling and high superheat, the outdoor metering device may be restricted.  A system that is overcharged will see a high head pressure and high suction pressure with high subcooling and lower super heat.