Heat Pump Not Heating? Causes and What to Do
If your heat pump is running but the room still isn't warming up, five things cause this in most NZ homes, in order of likelihood: a dirty filter or coil, a defrost cycle you're mistaking for a fault, a fan or airflow problem, a refrigerant issue, or a genuine compressor fault. Most of these don't need a technician straight away.
Why a heat pump stops heating: the five causes in order
A dirty filter or coil. This is the most common cause by a wide margin. A blocked filter restricts airflow across the internal coil, and the system can't transfer heat into the room even though it's running. The unit sounds normal and the fan spins, but the air coming out barely changes temperature.
A defrost cycle. On cold Auckland mornings, frost builds on the outdoor unit's coil. The system reverses briefly to melt it before returning to heating. During this the indoor unit can blow cool air or stop for several minutes. This is normal and isn't a fault.
Fan or airflow restriction. If the internal fan is coated in dust or mould, or the outdoor unit is blocked by leaves, debris, or a wall too close behind it, the system struggles to move enough air to heat the room effectively.
Refrigerant issues. If the system has lost refrigerant through a slow leak, it can run continuously without producing meaningful heat. This isn't something a clean fixes, and MiHT doesn't handle refrigerant work directly. It requires a certificated refrigeration technician.
A compressor fault. The least common cause. If the compressor itself has failed, the unit runs but produces no heat exchange at all. This is a hardware failure, not a maintenance issue.
Is it a defrost cycle? What's normal in an NZ winter
Defrost cycles are frequently mistaken for faults. During one, the outdoor fan may stop, steam or water may appear around the outdoor unit, and the indoor unit can blow cool or no air for five to ten minutes before returning to heating on its own.
What isn't normal: a defrost cycle running every few minutes, lasting longer than fifteen minutes, or the system never returning to heating afterward. If that's happening, the outdoor unit likely has restricted airflow or a fault that needs assessment.
What to check before calling anyone
Clean or replace the filter first. It resolves the most common cause on its own in many cases. Check the outdoor unit isn't blocked by plants, furniture, or debris within 500mm on all sides. Confirm the thermostat is actually set to heat mode and not accidentally left on fan-only or dry mode.
If none of that changes anything, the cause sits deeper in the system and a filter clean at home won't reach it.
Mitsubishi, Daikin, Fujitsu, and Panasonic: what differs by brand
Mitsubishi units often display a specific flashing pattern when a fault triggers a shutdown rather than reduced heating. Daikin systems run a mould proof cycle after some shutdowns that can be mistaken for the unit not heating on restart. Fujitsu units show a distinct three-lamp pattern for a genuine fault, separate from the defrost-related single-lamp blink. Panasonic's fault codes, including H98, H99, F95, and F96, are directly tied in the brand's own documentation to dirty filters and coils, not hardware failure.
If your unit's lights are flashing rather than just underperforming, see the Fujitsu flashing lights guide for what the pattern means before assuming the worst.
When the problem needs a professional
Book an inspection if cleaning the filter and clearing the outdoor unit doesn't change anything, if the defrost cycle behaves abnormally, or if the system is more than a year or two past its last service. Most not-heating calls MiHT sees trace back to a system that's never been serviced. A clean, well-maintained heat pump handles a cold NZ winter considerably better than a neglected one.
MiHT now carries out repairs alongside servicing. If a service uncovers a genuine fault, MiHT can complete the repair directly rather than referring it elsewhere.
Completely separate from all of that: if you want to know exactly where your system stands before booking anything, the Home Energy Health Assessment takes three minutes.
Frequently asked questions
Why is my heat pump not heating?
Most often a dirty filter or coil restricting airflow, or a defrost cycle you're mistaking for a fault. Less commonly, a refrigerant leak or compressor fault, which need a professional.
Why does my heat pump run but not heat the room?
The unit is likely operating but not transferring heat effectively. A blocked filter or coil is the most common cause. If the outdoor unit is also obstructed, that compounds the problem.
Is it normal for a heat pump to blow cold air sometimes in winter?
Yes, briefly, during a defrost cycle. This should last five to ten minutes and resolve on its own. If it happens constantly or the unit never returns to heating, that's not normal.
Why is my Mitsubishi, Daikin, or Fujitsu heat pump not heating?
The underlying causes are the same across brands: dirty filter, restricted airflow, defrost cycle, or a genuine fault. Check the brand's remote symbols guide if the lights are flashing rather than just underperforming.
Can a dirty heat pump stop heating completely?
Yes. Severe buildup on the internal coil or fan can restrict airflow enough that the system stops producing meaningful heat even though it's running. A professional service resolves this in most cases.
When should I stop troubleshooting and call someone?
If cleaning the filter and clearing the outdoor unit doesn't help within a day, or if the system has gone more than a year without a service.