Low Pressure (LP) Switch
What a low pressure switch does
A low pressure switch — usually shortened to LP switch — is a safety contact that watches refrigerant pressure on the low side of the circuit. It is fitted on the suction line between the evaporator and the compressor. When suction pressure falls below its cut-out setting, the switch opens, the compressor contactor drops out and the machine stops.
That trip is doing real work. Running a compressor at low suction pressure means low mass flow, so the motor loses the refrigerant-cooled gas it depends on and the discharge temperature climbs. Sustained low suction can also pull the evaporator below freezing and let liquid return to the compressor once the ice melts. The LP switch stops the machine before any of that becomes mechanical damage.
On many packaged systems the same switch does double duty as a pump-down control: the liquid line solenoid closes, the compressor pulls the evaporator down, and the LP switch shuts the compressor off once suction reaches the cut-out.
Typical LP switch settings by refrigerant
The correct cut-out depends on the refrigerant and on the lowest saturated suction temperature the system is designed to hold, so the same switch is set differently on a comfort cooling circuit and on a process chiller. The bands below are the ranges normally seen in commercial HVAC work — use them to sanity-check a setting, not to replace the equipment manufacturer’s data.
| Refrigerant | Typical LP cut-out | Where it is seen |
|---|---|---|
| R22 | 25 – 35 psi | Legacy comfort cooling and older process chillers still in service. |
| R134a | 25 – 35 psi | Centrifugal and screw chillers, medium-temperature applications. |
| R410A | 50 – 70 psi | Split, VRF/VRV and package units. Higher operating pressures. |
| R32 | 50 – 70 psi | Newer split and VRF equipment; many comfort applications are set nearer 20 – 30 psi. |
For refrigerants such as R404 and R407, and for any low-temperature or process duty, take the cut-out from the manufacturer’s pressure settings for that machine rather than from a general table.
Types of low pressure switch
Fixed (non-adjustable) LP switch
Cut-out and cut-in are set at the factory for one refrigerant and one duty. Common on packaged and split equipment where the circuit never changes. Cheapest to stock, nothing to mis-set on site.
Adjustable LP switch
Cut-out and differential are set with screws against a scale. Used on built-up plant, retrofits and chillers where the design suction pressure is decided by the project, not the equipment maker.
Auto-reset LP switch
Closes again on its own once suction pressure recovers above the cut-in point. Normal for pump-down control and for circuits where a brief low-pressure dip is expected during start-up.
Manual-reset LP switch
Stays locked out until someone resets it. Preferred where a low-pressure trip means a genuine fault — a refrigerant leak or a blocked expansion device — and short-cycling would hide the problem.
Dual (LP/HP) pressure switch
Low and high pressure sections in one body with a common switching contact. Saves a control-panel terminal and a mounting point on smaller condensing units.
Differential pressure switch
Senses the difference between two ports rather than one absolute pressure. Used for oil pressure protection and for proving flow across a filter, coil or pump.
Selecting an LP switch
- Refrigerant and design suction pressure. These set the cut-out. Fix them before anything else.
- Pressure range of the element. The switch has to cover the working range with margin, and its maximum working pressure has to survive standstill pressure on a hot day.
- Reset mode. Auto-reset for pump-down and for expected transient dips; manual-reset where a trip should be investigated before the plant runs again.
- Differential. Wide enough to stop short-cycling, narrow enough that the plant restarts in time.
- Connection. Flare, solder or capillary with a Schrader fitting, matched to the pipework and to whether the switch has to be replaceable without recovering charge.
- Electrical rating and contact form. The contact has to carry the contactor coil, and SPDT is needed where the trip is also reported to a BMS.
- Enclosure. Outdoor and rooftop plant needs an IP-rated body and UV-stable cable entry.
Why an LP switch trips
A low pressure trip is a symptom, not a fault in itself. Work through the causes in this order — replacing the switch first is the most common wasted call-out.
- Refrigerant leak — the most common cause. Suction pressure falls as charge is lost.
- Blocked or throttled expansion device, filter drier or liquid line solenoid starving the evaporator.
- Loss of evaporator load or airflow: dirty filters, iced coil, failed evaporator fan, closed dampers.
- Low chilled water or brine flow on a hydronic evaporator.
- A cut-out setting left too high for the refrigerant and the design suction temperature.
- The switch itself: a fatigued bellows, a corroded capillary, or contacts that no longer make cleanly.
Where a system trips on low pressure repeatedly, we look at the circuit as a whole — leak test, superheat and subcooling, airflow or water flow across the evaporator, and the control settings — rather than resetting and walking away.
LP switch supply and replacement from Compress India
We supply low pressure, high pressure and differential pressure switches as part of our HVAC sensors and switches range, and we fit and commission them on live plant. Typical work includes replacing failed safety switches on chillers and VRF systems, re-setting cut-outs after a refrigerant changeover, and adding manual-reset protection where nuisance cycling has been hiding a leak.
Switch replacement is usually part of a wider job — leak repair, gas charging, or a scheduled service visit under an annual maintenance contract. Tell us the refrigerant, the machine and the fault and we will come back with the right part and setting.
Enquire about pressure switchesFrequently Asked Questions
What is an LP switch in HVAC?▼
LP switch is short for low pressure switch. It is a pressure-operated safety contact fitted to the low (suction) side of a refrigeration circuit. If suction pressure falls below its cut-out setting, the switch opens and the control circuit stops the compressor before the compressor is damaged by low-pressure operation.
Where is the low pressure switch located?▼
On the suction line between the evaporator outlet and the compressor inlet, usually on a Schrader port or a brazed capillary tee near the compressor. It is wired in series with the compressor contactor coil so that opening the switch drops out the contactor.
What is the difference between an LP switch and an HP switch?▼
An LP switch watches suction pressure and protects against loss of charge, starved evaporators and freeze-up. An HP switch watches discharge pressure and protects against condenser fouling, fan failure, non-condensables and blocked discharge. Most circuits carry both; they are often supplied together as a dual pressure switch.
What should the low pressure switch be set at?▼
It depends on the refrigerant and on the lowest saturated suction temperature the system is designed to run at. Typical cut-out bands are 25–35 psi for R22 and R134a and 50–70 psi for R410A and R32. For refrigerants such as R404 and R407, and for any process or low-temperature duty, set the switch from the equipment manufacturer's data rather than from a general table.
Why does the low pressure switch keep tripping?▼
Repeated LP trips almost always mean the evaporator is not getting the refrigerant or the load it needs. Check for a refrigerant leak first, then for a restricted expansion device or filter drier, then for lost evaporator airflow or water flow. Only after that suspect the switch or its setting. A manual-reset switch is useful here because it stops the plant short-cycling and masking the real fault.
Can a low pressure switch be reset?▼
An auto-reset switch closes by itself once suction pressure rises above the cut-in point, which is the cut-out plus the differential. A manual-reset switch has to be reset by hand at the switch or panel, and should only be reset after the cause of the trip has been found and corrected.
What is the differential setting on an LP switch?▼
The differential is the pressure rise above the cut-out that is needed before the switch closes again. Too small a differential makes the compressor short-cycle; too large delays restart and can leave the space or process uncooled. On adjustable switches it is set with a separate screw from the cut-out.
Does Compress India supply and replace LP switches?▼
Yes. We supply low pressure, high pressure, dual and differential pressure switches as part of our HVAC accessories range, and we replace and re-set them on chillers, VRF systems, package units and built-up plant as part of our service and AMC work across India.