The short answer
- Check the controller is on and set to heat, and that nothing has been switched off at the isolator or consumer unit.
- Steam from the outdoor unit in cold, damp weather is usually a normal defrost, which Mitsubishi Electric says normally clears within a couple of minutes.
- Clear leaves, snow or debris from around the outdoor unit so air can flow freely.
- Write down any error code. If a breaker keeps tripping, you smell burning or water is leaking from pipes or the cylinder, switch off at the isolator and call us.
Safety first
These checks are things you can safely do yourself. Don't remove any covers from the outdoor unit, cylinder or controls: a heat pump contains mains electricity and refrigerant under pressure, and any work inside it must be done by a qualified engineer.
Check the power and controls
- Is the controller or thermostat switched on, set to heating and asking for heat? Check it isn't in holiday or standby mode.
- Has the timer or schedule changed, for example after a power cut?
- Is the isolator switch next to the outdoor unit on?
- Has a circuit breaker tripped in your consumer unit? You can reset it once. If it trips again, leave it off and call us.
Steam, ice and defrosting
In cold weather, frost builds up on the back of the outdoor unit and gradually blocks the airflow, so the heat pump runs a defrost cycle to melt it. During defrost you may see steam-like vapour and hear the unit change sound. Mitsubishi Electric says this is perfectly normal and is usually cleared within a couple of minutes, after which heating resumes.
Near freezing in damp weather, a heat pump may defrost several times an hour. What isn't normal is ice that keeps building up and doesn't clear. Vaillant advises contacting your installer if you notice reduced heating or increased frost build-up.
Is the outdoor unit blocked?
Leaves, snow, garden furniture or overgrown plants around the outdoor unit restrict the airflow and reduce output. Clear the area around the unit, keeping well away from the fan, and make sure defrost water can drain away from the base.
Note any error code
If the controller shows an error or fault code, write it down or take a photo. When you contact us, it helps to have:
- the make and model of the heat pump
- the error code and when the problem started
- who installed it and when it was last serviced
- whether you've lost heating, hot water or both
When to call an engineer
Switch the system off at the isolator, if it's safe to do so, and call us straight away if:
- a breaker keeps tripping
- you can smell burning
- water is leaking from pipes or the cylinder (water dripping from the outdoor unit during defrost is normal)
- the heat pump is making a loud grinding or banging noise
If you simply have no heating or hot water, leave the power on so the heat pump's frost protection keeps working, and call us.
Call 01590 641 146 or book online. Our emergency repairs and fault diagnosis service covers heat pumps, solar PV, batteries and EV chargers. To avoid breakdowns in the first place, read how often a heat pump should be serviced.
Common questions
Why is steam coming out of my heat pump?
In cold, damp weather frost builds up on the outdoor unit, and the heat pump runs a short defrost cycle to melt it. The vapour you see is normal and usually clears within a couple of minutes.
Should I pour hot water on an iced-up heat pump?
No. Let the heat pump defrost itself and keep the area around it clear. If ice keeps building up and doesn't clear, the unit needs looking at by an engineer.
Can you repair a heat pump that someone else installed?
Yes. We diagnose and repair heat pumps from other installers as well as our own. Tell us the make, model and any error code when you get in touch.