Heat Pump Not Working? What to Check First
A heat pump that isn’t heating your home the way you expect, or that’s showing an error code, doesn’t always mean something’s wrong. Many issues turn out to be quick, easy fixes, and in some cases what feels like a problem is actually your heat pump working exactly as it should. This guide walks through what to check first, how to tell the difference, and when it’s worth getting an MCS certified engineer involved.
Is It Normal for a Heat Pump to Feel Less Hot Than a Gas Boiler?
Heat pumps are designed to work differently to a gas boiler, and that’s actually part of what makes them so efficient. Rather than blasting out short bursts of very hot water like a boiler, a heat pump warms your home gently and consistently at a lower temperature, typically 35 to 50°C, often running for longer periods to maintain a steady, even warmth throughout the day.
This means your radiators will feel comfortably warm rather than hot to the touch, and many homeowners find this creates a more even, comfortable feel in their home compared to the peaks and dips of a boiler-heated house.
If your home is reaching the temperature you’ve set, your heat pump is working exactly as it should. The checks below are for situations where your home isn’t reaching temperature, or the system has stopped entirely.
Heat Pump Not Working at All? Check These First
1. Check the Power Supply
If the unit is completely silent with no display or lights, check your consumer unit (fuse board) for a tripped breaker. After power cuts or storms, heat pumps can sometimes fail to restart automatically. If you find a tripped breaker, reset it once. If it trips again immediately, do not keep resetting it, this can indicate an electrical fault and you should call a qualified engineer.
2. Check the Thermostat Settings
Make sure the thermostat is set to heating mode and the target temperature is set above the current room temperature. If you have a wireless thermostat (such as a Hive or similar), check the batteries, a thermostat with low batteries can sometimes stop sending signals to the heat pump entirely.
3. Check the Outdoor Unit for Blockages
Heat pumps need a free flow of air around the outdoor unit to work. Check for a build-up of leaves, snow, ice or debris around and underneath the unit, and make sure nothing has been placed on top of it or close enough to restrict airflow. A light build-up of frost during cold weather is normal, this is part of the defrost cycle, but heavy ice build-up that doesn’t clear can indicate a fault.
4. Check the System Pressure
Most heat pump systems display a pressure reading on the control panel or display. As a general guide, this should be around 1 to 1.5 bar. If the pressure reading is low, your installer will have shown you how to top this up using the filling loop when the system was commissioned. If you’re not confident doing this, or the pressure keeps dropping, this can indicate a leak somewhere in the system and is worth getting checked.
Heat Pump Running but House Not Warming Up?
If the unit is running but your home isn’t reaching the temperature you’ve set, this is usually something that can be adjusted or improved rather than a sign of a faulty unit. Common causes include:
- Radiators that were sized for a gas boiler and may need upgrading to work efficiently at a heat pump’s lower flow temperature
- The heating curve being set incorrectly on the controller, this is an engineer adjustment, not something to change yourself
- Air trapped in the radiators or pipework, often fixable by bleeding the radiators
- Zone valves or lever valves that have been knocked partly closed
If you have radiators that never feel more than lukewarm, even when the system has been running for a while, this is often a sign the radiators weren’t sized correctly for a heat pump system during installation, something a proper heat loss survey is designed to prevent.
Unusual Noises, Smells or Error Codes
Buzzing, Rattling or Grinding Noises
These usually point to something loose, worn, or running incorrectly inside the unit. A hissing sound in particular can sometimes indicate a refrigerant leak, if you hear hissing, switch off the unit at the isolator if it’s safe to do so and call an engineer rather than continuing to run the system.
Musty Smell from the Outdoor Unit
A musty smell is usually mould or algae growth on the outdoor coil, this is common and not serious. It can often be gently cleaned with a damp cloth, but if the smell persists or is accompanied by reduced performance, it’s worth having it checked at your next service.
Error Codes on the Display
Most heat pump brands, including Samsung, Vaillant, Daikin and Mitsubishi, use a numbered fault code system. The exact meaning varies by brand and model, so rather than guessing, make a note of the exact code shown and contact your installer or an engineer who can look up what it relates to. Some codes (such as low water pressure) can be simple to resolve, while others need a professional diagnosis.
When Should You Call an Engineer?
Call a qualified engineer if:
- The unit won’t power on after checking the consumer unit
- You hear hissing, grinding or other unusual noises
- The system pressure keeps dropping after topping up
- An error code is displayed that you can’t resolve
- Rooms consistently aren’t reaching temperature despite the system running
As MCS certified engineers covering Samsung, Vaillant, Daikin and Mitsubishi systems. Our team can diagnose and repair heat pumps across London, whether we installed the system or not. You can read more about our heat pump repair and servicing service.
Prevent Issues Before They Start
Many of the issues above can be reduced or avoided entirely with an annual service. A service includes checking system pressure, cleaning filters and the outdoor unit, and verifying the controller settings are correct for your home. If your heat pump hasn’t been serviced in the last 12 months, or you’re noticing any of the signs above, it’s worth booking a check-up before the colder months arrive. You can call us on 020 3837 0778 or email info@electro-heating.co.uk to arrange a service or speak to our team.
Still not sure what’s wrong with your heat pump?
Our MCS certified engineers can diagnose and repair heat pumps across London, including systems we didn’t originally install. Get in touch and we’ll help get your heating back to normal.
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Sources: Industry troubleshooting guidance for UK air source heat pumps, manufacturer fault code documentation, MCS installation standards, June 2026.
By Matthew Fasami