The low-pressure (LP) switch sits on the suction line. It opens when suction pressure falls too low. This protects the compressor from running without enough refrigerant and the evaporator from freezing.

Imagine drinking through a straw from an almost empty glass. You suck harder and harder, but little comes. A starved evaporator is the same: the compressor pulls suction pressure down because too little vapour is coming in.

In heating mode the evaporator is the air coil. In cooling mode on a reversible unit, it is the water-side PHE.

Main causes

• Low air flow over the air coil: dirt, ice, failed fan.
• Heavy frosting without proper defrost.
• Very low ambient temperature beyond the unit's operating range.
• Low refrigerant charge (leak).
• Blocked filter-drier or strainer in the liquid line.
• Expansion valve stuck closed or faulty (or EEV driver fault).
• In cooling: low chilled water flow, low set point, frozen PHE.
• Faulty LP switch or sensor.

Symptom → cause → check → fix (low pressure)

Worked example — Check the drier

Liquid line temperature before the filter-drier: 38 °C. After it: 33 °C.

ΔT = 38 − 33 = 5 K. A healthy drier shows almost no drop (typically below about 1–2 K). A 5 K drop means it is blocked and the liquid is flashing. Replace the drier.

Remember

• First ask: which coil is the evaporator right now?
• High superheat plus low subcooling usually means low charge.
• High superheat with normal subcooling points to a restriction.
• Check air flow and ice before refrigerant.