Mould inside a heat pump indoor unit is a surprisingly common problem in New Zealand, especially in humid regions or homes with poor ventilation. The good news is that basic cleaning of the filters and accessible surfaces is something most homeowners can tackle safely — but deeper internal mould requires a professional heat pump service technician.
Why Mould Grows Inside Heat Pumps
Heat pump indoor units (also called the wall-mounted cassette or head unit) create the perfect conditions for mould: warm, moist air passes over cool coils, leaving condensation on internal surfaces. In NZ's often damp climate — particularly in Northland, the West Coast, and many older, poorly ventilated homes — mould spores quickly take hold on the evaporator coil, fan blades, drain tray, and plastic housing.
Running your heat pump in heating mode only (never in cooling or dry mode) can make this worse, as the unit rarely gets cold enough to drain condensation properly.
Safety First
Before you start, keep these safety points in mind:
- Switch the unit off at the wall and isolate the power at the switchboard before opening any panels. Never work on a live unit.
- Mould can trigger respiratory issues. Wear an N95 or P2 respirator mask, safety glasses, and disposable gloves.
- Work in a ventilated space — open windows and doors.
- If you or anyone in the household has asthma, allergies, or a compromised immune system, consider calling a professional rather than disturbing mould yourself.
- Check WorkSafe NZ for general guidance on working safely with mould.
What You Can Safely Clean Yourself
Homeowners can safely clean the following accessible parts without specialist tools or training:
1. Filters
Most indoor units have removable mesh filters behind the front panel.
- Lift or unclip the front panel (consult your unit's manual).
- Slide out the filters carefully.
- Vacuum loose dust, then wash filters in warm water with a mild detergent.
- Rinse thoroughly and allow to dry completely before reinserting — damp filters encourage more mould growth.
- Clean filters every 4–6 weeks during heavy use.
2. Front Panel and Louvres
- Wipe down the plastic housing, front panel, and adjustable louvres with a cloth dampened with a mild detergent solution or a diluted white vinegar solution (1 part vinegar to 1 part water).
- Avoid harsh bleach sprays directly on plastic, as these can cause cracking or discolouration over time.
- Dry thoroughly with a clean cloth.
3. Visible Drain Tray
If your unit design allows you to see the drain tray without disassembly, wipe it out with a damp cloth and a small amount of diluted white vinegar to inhibit mould regrowth.
What Requires a Professional
The evaporator coil, fan blower wheel, and internal drain system are where mould most seriously accumulates — and these are not safely accessible to most homeowners without disassembly. Attempting to clean these yourself risks:
- Bending the delicate aluminium coil fins, reducing efficiency.
- Dislodging electrical connections or refrigerant lines (refrigerant work must only be performed by a suitably certified technician — check with the relevant authority (the EWRB covers electrical work; confirm current refrigerant handling certification requirements with MBIE or a qualified technician)).
- Spreading mould spores further into the unit or your home.
A qualified heat pump technician will use a foaming coil cleaner, a purpose-built bib kit (a plastic shroud that catches dirty water), and a pressure rinse to safely clean the coil and blower wheel in place. This is often called a full internal clean or deep service.
As a rough guide, a professional internal clean typically costs somewhere in the range of $150–$350 depending on the unit size, location, and level of contamination — but get quotes from local technicians, as prices vary.
You can find qualified heat pump installers and service technicians in your area at tradedirectory.co.nz.
Preventing Mould Coming Back
Once your unit is clean, a few habits will keep mould at bay:
- Run the fan-only mode for 15–30 minutes after using cooling mode to dry out internal surfaces.
- Use your heat pump's dry mode periodically in humid weather.
- Ensure your home has adequate ventilation — the Healthy Homes standards require rental properties to have adequate ventilation, and the same principles benefit all NZ homes.
- Keep indoor humidity below 65% where possible. A dehumidifier or improved subfloor/ceiling ventilation can help in persistently damp homes — see Energywise for practical tips.
- Schedule a professional service every 1–2 years, or annually in high-humidity environments.
A Note on NZ Building Code and Healthy Homes
If you're a landlord, note that the Healthy Homes Standards require rental properties to have an acceptable heating source. A mould-clogged heat pump that isn't functioning properly may not meet those requirements. Keeping your unit serviced is both a health and a compliance issue.
When to Call a Professional
Call a qualified heat pump technician if:
- Mould is visible on the coil, blower wheel, or deep inside the unit.
- The unit smells musty even after you've cleaned the filters.
- You notice reduced airflow or efficiency.
- There is visible mould growth that returns quickly after cleaning.
- Anyone in the home has respiratory sensitivities.
Refrigerant handling is legally restricted work in New Zealand and must only be carried out by a certified technician. Do not attempt to remove refrigerant lines or access the sealed refrigerant circuit yourself.
ISO 9001
