How to Test Heat Pump Low Pressure Switch
Air Conditioning Pressure Switches
It has finally gotten hot and it seems we are fully into air conditioning season (let's hope it stays fo a while). With air conditioning season comes a whole other set of diagnostics that we probably all have forgotten over the long winter. One of those diagnostics is units cycling or locking out on pressure switches.
Pressure switches are safeties installed on units to protect the unit, compressor, and anyone working on the unit. Pressure switch trip points vary from unit to unit and with the type of refrigerant in the unit. Usually, there are 2 types of pressure switches — HIGH PRESSURE switches and LOW PRESSURE switches. It is important to remember that there are multiple causes for safeties tripping. A complete diagnostic of the refrigerant side of the system is necessary to determine the cause if the safety tripping. Pressures are one part of the diagnostic but so is super heat, subcooling, and amp draw of the compressor along with electrical diagnostics.
Let's look at the diagnostics of the HIGH PRESSURE switch. The high pressure switch is there to keep components from failing and releasing the refrigerant either to the atmosphere if the leak occurs outside, or into the conditioned space if the leak occurs inside.
When a unit cycles or locks out on the high pressure switch, what should you look for? What causes the high pressure switch to trip?
- Most common cause is a dirty condenser coil.
- just because the outside looks clean, don't forget that some units have splitcondenser coils and the inner coil could be dirty.
- you need to verify is the unit has a split condenser coil and clean as necessary.
- just because the outside looks clean, don't forget that some units have splitcondenser coils and the inner coil could be dirty.
- The next most common cause is a malfunctioning condenser fan motor.
- Take an amperage reading on the motor and compare to the motor name plate.
- High amperage could mean the motor is possibly going bad.
- Take an amperage reading on the motor and compare to the motor name plate.
- Check the motor run capacitor
- a capacitor out of the microfarad range can cause a motor to draw high amperage or over heat causing it to fail.
- Of course an over chargeof refrigerant can cause the high pressure switch to open
- this is especially true with microchannel coils.
- Non-condensables in the system can cause high head pressure.
- moisture, air, mixed refrigerants.
- a heavy load in the occupied space adds more super heat to the coil and can cause high pressure readings.
- Unit "short cycling" on a thermostat or because of a "loose connection" in the control circuit.
These are the most common causes of a unit cycling or tripping on the high pressure switch. Now, lets look at the low pressure switch and what causes that to trip.
When a unit cycles or locks out on the low pressure switch, what should you look for?
- the most common cause is low air flowacross the evaporator.
- look for dirty filters, dirty evaporator coil, dirty blower wheel.
- Take an external static pressure reading before the coil and after the filter and compare to the unit specification for proper CFM.
- Check the blower motor amperage and compare to the name plate.
- Check the motor capacitor, if it has one, and make sure it is within specification.
- Check the belt, if it has one, and make sure it has proper tension.
- The unit could be undercharged.
- There could be a high side restriction
- check the filter drierfor a restriction
- a 2 degree drop across the drier means it is "plugged" and need to be replaced.
- check the filter drierfor a restriction
- there could be a metering device, either TXV or orifice, that is under-feeding the coil.
- A very light load in the occupied space can cause low pressure.
- An economizer on a commercial unit that is bringing in too much outside air cooling the coil down too much.
Of course, you could have a defective pressure switch in the unit. This can be determined by comparing where the pressure switch is opening and what the actual pressure is in the unit. To that end, I have attached a list of pressure switch open and close points for our various split systems and packaged units. The information in the spread sheet is based on current part numbers and models. It will get you close but, if there is any doubt, you can call with the serial number of the unit and we can get you the set points for that specific year of manufacturer.
Remember, a complete diagnostic of the system should always be performed before replacing any part. There is nothing worse than replacing a part and still having the unit malfunction. Hope this helps!
Pressure Switch settings
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How to Test Heat Pump Low Pressure Switch
Source: https://yorkcentraltechtalk.wordpress.com/2012/05/30/air-conditioning-pressure-switches/