Heat Pump Low COP — Why Efficiency Is Poor & How to Fix It
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Post a job — we'll find you an engineer →Heat pumps are excellent for efficiency — when they're working properly. But if you've installed one and your energy bills haven't dropped as much as promised, or your system feels like it's working harder than it should, you're not alone. Many UK homeowners discover their heat pump isn't delivering the seasonal COP (typically 2.5–4.0) that was quoted, often due to straightforward installation issues or running settings that can be adjusted. This happens far more often than manufacturers like to admit, and the good news is that most causes are fixable without major expense or disruption. This guide walks through the most common culprits — from flow temperature settings and immersion heater use to outdoor unit cleanliness and undersized radiators — so you can identify what's dragging your efficiency down. If you suspect a refrigerant issue or a fundamental design fault, you'll need to contact a qualified F-Gas engineer or MCS surveyor to investigate properly.
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Most likely cause & what to check
COP (Coefficient of Performance) is the measure of heat pump efficiency — a COP of 3 means the heat pump produces 3kWh of heat for every 1kWh of electricity consumed. A well-installed heat pump should achieve a seasonal COP (SCOP) of 2.5–4.0. If yours seems much lower, there are several common causes.
Flow temperature too high: running at 55°C instead of 45°C can reduce COP by 30% or more. Check what flow temperature your system is set to and ask your installer whether it could be safely lowered. Lowering flow temperature by 5°C typically improves COP by 10–15%.
Immersion heater running too much: a 3kW immersion heater running for 2 hours per day adds 6kWh to your daily consumption at 100% efficiency — much less efficient than the heat pump. Check how often the immersion runs in your controller settings. Daily legionella cycles should be timed, not run-on-demand.
Low refrigerant charge: a heat pump running with insufficient refrigerant will have a poor COP. This requires an F-Gas engineer to check and recharge — it cannot be done without specialist equipment.
Undersized or dirty outdoor unit coil: the outdoor unit extracts heat from air. If the coil fins are caked in dirt, leaves, or cottonwool from nearby trees, airflow is restricted and COP drops. Gently clean the coil with a soft brush or low-pressure water — never a pressure washer.
Poor system design: if radiators are undersized for heat pump temperatures, the system must run at a higher flow temperature to compensate, reducing efficiency. A properly conducted heat loss calculation and radiator sizing check by an MCS engineer is the only way to confirm this.
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Frequently asked questions
What's a normal COP for a heat pump in the UK?
A well-installed system should achieve a seasonal COP of 2.5–4.0 across the year, though this varies with outdoor temperature and how you've set it up. Winter COP will be lower than autumn or spring because the outdoor air is colder and the heat pump has to work harder. If you're consistently below 2.5, something's not right.
Will lowering my flow temperature actually save me money?
Yes — reducing flow temperature from 55°C to 50°C typically improves COP by 10–15%, which translates directly to lower electricity bills. It also means your radiators work better and the system runs more quietly. The catch is that your radiators need to be large enough to deliver comfort at lower temperatures, so check with your installer first.
How often should I clean the outdoor unit myself?
Check it monthly during autumn and spring when leaves and pollen are worst, and after any nearby hedge cutting or tree work. A gentle brush or soft cloth is all you need — never use a pressure washer, as it can damage the delicate aluminium fins. Most UK homeowners only need to clean it 2–3 times a year unless they're surrounded by trees.
Could my immersion heater be the problem?
Almost certainly, if it's running more than once a day on demand. A 3kW immersion uses electricity at 100% efficiency (no multiplication like a heat pump), so frequent use will tank your overall COP and bills. Check your controller and set legionella cycles to run at a fixed time (e.g. 3am), not on-demand.