A heat pump that's running but not heating properly is one of the most frustrating home comfort problems — especially during a cold New Zealand winter. The good news is that many causes are straightforward to diagnose, and some can be fixed without calling a tradie. This guide walks you through the most common reasons your heat pump isn't performing, what you can safely check yourself, and when it's time to bring in a professional.
Common Causes of Poor Heat Pump Heating
Before diving into diagnosis, it helps to understand what typically goes wrong. Here are the most frequent culprits:
- Dirty or blocked filters — the single most common cause of reduced heating performance
- Incorrect thermostat or mode settings — surprisingly easy to overlook
- Outdoor unit blocked or iced up — especially common in colder South Island or high-altitude locations
- Low refrigerant (gas) charge — caused by a leak in the system
- Undersized unit — the heat pump was never big enough for the space
- Faulty components — compressor, fan motor, or control board issues
- Poor insulation or draughts — the room is losing heat faster than the unit can produce it
Step-by-Step Diagnosis
Work through these checks in order before calling a tradesperson.
1. Check the Settings First
- Make sure the unit is set to Heat mode (not Cool, Fan, or Auto — Auto can sometimes select cooling if the room is warmer than the set temperature).
- Confirm the set temperature is higher than the current room temperature — try setting it to 22–24°C.
- Check the fan speed isn't set to its lowest setting, which limits heat distribution.
- Replace the remote control batteries if the display looks dim or the unit is unresponsive.
2. Inspect and Clean the Filters
This is the most impactful DIY fix. Clogged filters starve the unit of airflow, dramatically reducing heating output.
- Turn the unit off at the remote and wait a few minutes.
- Open the front panel of the indoor unit.
- Slide out the mesh filters.
- Vacuum loose dust, then rinse under warm water.
- Allow to dry completely before reinserting — never run the unit with wet filters.
- Most manufacturers recommend cleaning filters every 4–6 weeks during heavy use.
3. Check the Outdoor Unit
Head outside and look at the outdoor unit (the compressor/condenser):
- Is airflow blocked by overgrown plants, a fence, stored items, or debris? Clear at least 300–500 mm of clearance around the unit.
- Is the unit iced up? A light frost on the coil in cold weather is normal — the unit will run a defrost cycle. But if it's heavily encased in ice and not clearing, that points to a refrigerant or airflow problem.
- Is the outdoor fan spinning when the unit is running? If not, there may be a fan motor fault.
4. Assess the Room Itself
Sometimes the heat pump is working fine — the room is just losing heat too fast.
- Check for obvious draughts around doors, windows, and gaps in the floor or ceiling.
- Is the room well-insulated? BRANZ research consistently shows that underinsulated NZ homes require significantly more heating capacity.
- Is the heat pump sized correctly? A unit that was adequate for a mild Auckland winter may struggle in a Christchurch cold snap.
5. Note Any Error Codes
Most modern heat pumps display a flashing light pattern or an error code on the indoor unit or remote. Note the code and check your owner's manual — this can pinpoint the fault quickly.
What You Can Safely Fix Yourself
| Issue | DIY Fix | Difficulty |
|---|---|---|
| Dirty filters | Clean with water and vacuum | Easy |
| Wrong mode/settings | Adjust via remote | Easy |
| Blocked outdoor unit | Clear debris and vegetation | Easy |
| Dead remote batteries | Replace batteries | Easy |
| Draughty room | Draught-stop tape, door seals | Easy–Moderate |
These tasks are safe for any homeowner. Everything else — refrigerant handling, electrical work, and component replacement — must be done by a qualified professional.
What a Professional Must Do
Refrigerant (gas) work is legally restricted in New Zealand. Handling refrigerants requires a certified technician under the MBIE Building Performance framework and relevant environmental regulations. Never attempt to top up or handle refrigerant yourself.
Electrical work on the heat pump's wiring, control boards, or the dedicated circuit is restricted electrical work under the Electricity Act. Only a registered electrician (check the Electrical Workers Registration Board) may carry this out legally.
A qualified heat pump technician should handle:
- Refrigerant leak detection, repair, and recharging
- Compressor or fan motor replacement
- Control board faults
- Full system performance testing and commissioning
- Assessing whether the unit is correctly sized for your space
⚠️ Safety note: Never remove panels from the outdoor unit or attempt to access internal electrical components. Capacitors inside can hold a dangerous charge even when the unit is switched off.
Preventing the Problem From Recurring
- Clean filters every 4–6 weeks during winter and summer — set a phone reminder.
- Book an annual service with a qualified heat pump installer. They'll check refrigerant levels, clean the coil, inspect electrical connections, and test performance.
- Keep the outdoor unit clear of vegetation year-round. Trim plants back each season.
- Improve your home's insulation and draught-stopping — this reduces the load on your heat pump and can make a dramatic difference. Check Energywise for government insulation support schemes you may be eligible for.
- If your home must meet Healthy Homes Standards (rental properties), heating requirements are set out by Tenancy Services — your heat pump must be capable of heating the main living area to 18°C.
When to Call a Professional
Call a qualified heat pump installer if:
- The unit runs but produces little or no warm air after you've cleaned the filters and checked settings
- You see or hear refrigerant leaking (hissing sound, ice on copper pipes, oily residue near fittings)
- The outdoor unit fan isn't spinning
- Error codes appear that aren't resolved by a reset
- The unit trips your switchboard breaker
- Performance has gradually declined over one or two seasons
- Your heat pump is more than 10–12 years old and struggling — it may be due for replacement
A good installer will carry out a full diagnostic, advise on repair versus replacement, and ensure any work meets the NZ Building Code. You can find qualified, local heat pump installers on tradedirectory.co.nz.
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