Pressure Switch
Safety switches that prove draft or protect a refrigerant circuit, and how to read what an open switch is telling you.
Safety
What it is
A pressure switch opens or closes an electrical circuit based on a pressure it senses. In HVAC there are two common families: air pressure switches that prove the draft inducer is moving flue gases, and refrigerant pressure switches that protect the compressor from unsafe high or low pressure.
| Switch | Senses | Protects against |
|---|---|---|
| Draft proving switch | Negative pressure created by the inducer | Operating a burner with a blocked vent or failed inducer |
| High pressure switch | Discharge side refrigerant pressure | Condenser failure, blocked airflow, overcharge |
| Low pressure switch | Suction side refrigerant pressure | Running with a severe undercharge or a frozen coil |
| Loss of charge switch | Standing low side pressure | Starting a compressor with essentially no refrigerant |
Why it matters
These switches are safeties, not nuisances
An open pressure switch is information. Treat it as the starting point of the diagnosis rather than the failed part.
How it works
A diaphragm moves against a calibrated spring. When pressure passes the setpoint the diaphragm moves the contacts. Draft switches have a setpoint printed on the body in inches of water column, along with whether they are normally open and close on a fall in pressure.
Refrigerant switches are either automatic reset, which close again once pressure returns to a safe band, or manual reset for higher risk applications.
How to diagnose it
Measure the actual pressure the switch senses with a manometer or manifold
Compare to the printed setpoint. If the pressure is not there, the switch is right and the system is wrong
Inspect the vent, inducer wheel, hoses and condensate trap on a draft switch
Blockage, a cracked hose or a full trap are the usual causes of a switch that will not close
Measure voltage across the switch during the call
Source voltage across it means it is open. Near zero means it closed
On a high pressure trip, check condenser airflow, coil cleanliness and charge
The switch is reporting a heat rejection problem
On a low pressure trip, check for an iced coil, low airflow or a leak
Adding refrigerant without finding the leak is not a repair
Tools involved
- Manometer for draft switches
- Digital manifold for refrigerant switches
- Multimeter
- Inspection mirror and light
Symptoms
Practice & Mastery
- Pressure Switch Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
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