Heat Pump Compressor Problems: Signs, Causes & Repair Costs

What Does the Compressor Do?

The compressor is the most important and most expensive component in a heat pump. It pressurises the refrigerant gas, raising its temperature so it can release heat into your home. A failed compressor means the system cannot heat or cool at all.

Signs of Compressor Problems

• Loud banging, knocking, or grinding noises from the outdoor unit
• High-pitched squealing on startup
• System trips the breaker shortly after starting
• Hard start — the system struggles to start, takes longer than usual
• Hot, hot discharge line but cold suction line — indicates compressor inefficiency
• System runs continuously but achieves no heating
• Error codes relating to high pressure or compressor protection

Common Causes of Compressor Failure

• Age and wear — most compressors last 15–20 years
• Lubrication failure — oil contamination or loss
• Liquid refrigerant entering the compressor (liquid slugging)
• Overheating from poor airflow or blocked filters
• Electrical failure — voltage spikes or winding insulation breakdown
• Running with low refrigerant charge for extended periods

Compressor Replacement Costs

Compressor replacement is the most expensive heat pump repair. Expect to pay:

• Compressor part: £500–£1,500 depending on brand and kW size
• Labour: £300–£500
• Refrigerant recharge: £150–£250
• Total: £950–£2,250

For systems over 12 years old, compare this against the cost of a new heat pump after the £7,500 BUS grant before committing to a compressor replacement.