Mould growing inside your heat pump is a common problem in New Zealand, especially in humid regions or homes with poor ventilation. Left untreated, it can reduce your unit's efficiency, trigger allergies, and spread spores through your living space. The good news is that surface mould on accessible parts — like filters and the front casing — is something most homeowners can tackle safely with the right approach.
Why Mould Grows in Heat Pumps
Heat pumps move warm, moist air across cool coils, creating the perfect conditions for mould and mildew to take hold. In New Zealand's climate — particularly in regions like Northland, the West Coast, and Wellington — high humidity makes this even more likely. Poor indoor ventilation, infrequent filter cleaning, and running the unit in cooling mode for extended periods all contribute to mould build-up.
The Tenancy Services Healthy Homes standards also highlight the importance of ventilation and heating in preventing mould — relevant if you're a landlord or tenant dealing with this issue.
Safety First
Before you start, keep these safety points in mind:
- Turn the unit off at the wall and, ideally, switch off the circuit breaker before opening any panels or touching internal components.
- Wear disposable gloves, a P2 or N95 face mask, and eye protection to avoid inhaling or touching mould spores.
- Work in a well-ventilated area — open windows and doors.
- Never spray liquids directly onto electrical components, wiring, or the circuit board.
- If you see heavy black mould deep inside the unit or on the coils, stop and call a professional — this goes beyond DIY cleaning.
For guidance on working safely around mould, refer to WorkSafe NZ.
What You'll Need
- Soft cloths or microfibre cloths
- A soft-bristle brush or old toothbrush
- A vacuum cleaner with a brush attachment
- Mild detergent or a purpose-made coil/filter cleaner (available at Bunnings or Mitre 10)
- A diluted white vinegar solution (1 part vinegar to 1 part water) or a diluted bleach solution (no stronger than 1 part bleach to 9 parts water)
- A spray bottle
Avoid harsh abrasive cleaners or solvents, which can damage plastic components and coatings.
Step-by-Step: Cleaning Accessible Parts
1. Clean or Replace the Filters
The air filters are the most common place mould accumulates and the easiest to clean. Most indoor (wall-mounted) units have filters that slide out from behind the front panel.
- Remove the filters carefully.
- Vacuum off loose dust and debris.
- Wash in warm water with a small amount of mild detergent, or use your diluted vinegar spray.
- Rinse thoroughly and allow to dry completely in the shade before reinserting — never put damp filters back in.
- Check your unit's manual for how often to clean filters; generally every 4–6 weeks during heavy use is recommended.
2. Wipe Down the Front Casing and Vents
- Use a damp cloth with a mild detergent or diluted vinegar solution to wipe the outer casing, louvres, and air outlet vents.
- Use a soft brush to get into the slats of the louvres where mould often hides.
- Dry thoroughly with a clean cloth.
3. Clean the Drain Tray (If Accessible)
Some units have an accessible condensate drain tray at the base of the indoor unit. If you can reach it safely:
- Wipe out any standing water, slime, or mould with a cloth.
- Clean with diluted bleach or vinegar solution.
- Check that the drain line isn't blocked — a blocked drain can cause water to pool and worsen mould growth.
4. Run the Unit in 'Fan Only' Mode
After reassembling, run the heat pump on fan-only mode for 20–30 minutes with windows open. This helps dry out any remaining moisture inside the unit and flush stale air out of the room.
What NOT to Clean Yourself
Do not attempt to clean the following without professional help:
- Evaporator coils (the aluminium fins inside the unit) — these are delicate and require specialist coil-cleaning spray and technique
- Electrical components or wiring
- The outdoor condenser unit's internal components
- Any area where mould is extensive, deeply embedded, or appears black and widespread
Heat pump servicing — including coil cleaning and refrigerant checks — must be carried out by a qualified technician. Refrigerant handling in particular is restricted work under NZ regulations and must only be performed by a licensed refrigeration/air conditioning technician.
Preventing Mould Coming Back
- Clean filters every 4–6 weeks during periods of regular use.
- Run the heat pump in dry/dehumidify mode periodically in humid weather.
- Ensure your home has adequate ventilation — refer to BRANZ for guidance on NZ ventilation best practice.
- Keep indoor humidity below 65% where possible — a cheap hygrometer from a hardware store can help you monitor this.
- Have your unit professionally serviced once a year, which typically includes a coil clean and drain check.
When to Call a Professional
Call a qualified heat pump technician if:
- Mould is visible on the evaporator coils or deep inside the unit
- There is a persistent musty smell even after cleaning the filters
- The unit is leaking water, which may indicate a blocked drain
- You're unsure about any step of the process
- The unit hasn't been professionally serviced in over 12 months
A licensed heat pump installer or HVAC technician will have the right tools — including purpose-made coil cleaners and pressure equipment — to do a thorough job safely. You can find qualified local heat pump professionals on tradedirectory.co.nz.
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