A heat pump that quits on a cold night is usually not broken. A fair chunk of what looks like a fault is the unit doing exactly what it was built to do, or something you can sort out in five minutes with a damp cloth and a bit of patience.
Before you open anything up, switch the unit off at the isolation switch. That’s usually the one on or near the outdoor unit.
Heat pump not heating? Five things to check before you call anyone
Check the mode and the set temperature. Make sure it’s on heat, not cool, dry or fan, and set it warmer than the room is right now. It’s easy to bump the mode button on the remote and not notice.
Check the filters. Blocked filters are one of the most common reasons a heat pump struggles. If they’re grey with dust, pull them out and clean them. Heat pump servicing walks through it.
Clear the space around the outdoor unit. It needs 30 to 50 cm of clear air. Move the pot plants, the bins, the firewood stacked against it and anything leaning on the casing.
Shut the doors and windows. A heat pump can’t keep up with an open ranchslider, or a window left ajar in the bedroom down the hall.
Reset it. Press the reset button on the remote, wait a few minutes, then start it again.
Still blowing cold after all five? The likely causes are ice or grime on the outdoor coil, a refrigerant leak, or a part that’s given up: a fan, a capacitor, a compressor. Those need a technician.
Why it stops heating for a few minutes at a time
On a cold morning, this is usually the defrost cycle. Once it drops below about 5.5°C outside, frost starts forming on the outdoor coil, and every heat pump stops heating for short spells so it can melt that frost off. In Hamilton fog or a Canterbury frost, that can happen several times a morning.
Signs it’s defrosting rather than broken:
- a light flashes on the indoor unit, or a defrost symbol shows on the wall controller
- warm air stops coming from the indoor unit for a spell
- water runs off the outdoor unit as the frost melts
Most units won’t check whether they need to defrost until they’ve been running for at least 45 minutes, and a defrost can finish within about 15. If yours stops far more often than that, something else is going on.
It runs and runs and never gets the room warm
Flat out all day and the room still feels cold usually means the unit is too small for the space, or not built for frosty nights. Check the heating output at 2°C on its energy label against what the room actually needs. What size heat pump you need makes sense of the numbers.
→ Related: Heat pump problems and what causes them
Ice on the outdoor unit: what’s normal, what isn’t
A light frost that clears during the defrost cycle is normal. Thick ice that never clears is not. Ice on the coil blocks airflow and acts like insulation, so the unit heats less and less until it stops. Lost refrigerant, a dirty coil and anything blocking the airflow can all cause it. If the ice comes back after the unit has had a proper chance to defrost, switch it off and call a technician.
Why is my heat pump dripping water?
From the outdoor unit: normal in heating mode. It gives off water as it works and as it melts frost, which is why it should be mounted so the water can drain away. Pooling on a path or deck, or freezing into a slippery patch on a frosty Taupō morning? An installer can fit a drain to carry it somewhere better.
From the indoor unit: not normal. That’s usually a blocked drain or a drain tray that needs cleaning. Turn it off, put a towel down, and book a service.
Why does my heat pump smell musty?
Musty smells usually come from dust or mould on the filters, the coil or the drain tray. Smells from the room, cooking or smoke, can get pulled in and blown straight back out at you. Clean the filters first. If it still smells with clean filters in, the coil and drain tray need a professional clean.
A burning or electrical smell is a different animal. Kill the power at the isolation switch and call someone straight away.
Why are the lights on my heat pump flashing?
Flashing lights on the indoor unit, or a code on a wall controller, mean the unit has picked up a fault. Sometimes it’s minor, like a glitch after a power cut.
- Turn it off at the isolation switch for at least five minutes, so it powers down completely.
- Turn it back on.
- If the lights are still flashing, call for service. Use the warranty if it’s covered, or a licensed technician who knows the brand if it isn’t.
Each brand uses its own codes, and the same flashes can mean different things on different models. Before you ring anyone, write down the pattern, which lights are flashing, and the model number.
Does my heat pump need re-gassing?
Be wary of anyone who suggests a regas as routine. Refrigerant circulates in a sealed system and doesn’t get used up, so a correctly installed unit shouldn’t need a top-up, and there’s no such thing as stale gas. If refrigerant has gone missing, there’s a leak, most often at a poorly made or loose pipe joint. Fix the leak before any gas goes back in, or you’ll be paying for the same top-up next winter. Checking refrigerant takes specialist tools and certification, so it’s never a DIY job.
When should I call a heat pump technician?
Pick up the phone when:
- it still won’t heat after you’ve checked the settings, cleaned the filters, cleared the outdoor unit and reset the power
- the error lights keep flashing after a five-minute power-off
- ice keeps building up and won’t clear
- water is coming out of the indoor unit
- there’s a burning smell, or something sparks or trips the power
If the unit is under warranty, go to the installer who fitted it, or the brand, first. The warranty may depend on who does the repair. If it isn’t covered, find a heat pump repairer nearby, or start from your town.
→ Related: How to choose a heat pump installer
Not sure the old unit is worth fixing? Get a quote for a replacement and you’ll have a real number to weigh up.