That stale, musty odour hitting you the moment your heat pump kicks on is one of the most common complaints from NZ homeowners — and it's almost always a sign that mould, mildew, or bacteria have taken up residence somewhere inside your unit. The good news is that in most cases it's preventable and fixable, often without calling a technician. Here's how to track down the source and get your home smelling fresh again.
Why Heat Pumps Develop a Musty Smell
Heat pumps work by drawing air across a cold evaporator coil, which causes moisture from the air to condense — much like a cold drink on a humid day. That condensation creates a persistently damp environment inside the indoor unit, which is the perfect breeding ground for mould, mildew, and bacteria. New Zealand's climate doesn't help: our humid summers and damp winters mean indoor units are often working hard in exactly the conditions these organisms love.
The smell itself is typically caused by microbial volatile organic compounds (MVOCs) — gases released by mould and bacteria colonies as they grow on the coil, drain tray, fan blades, and filters.
Common Causes and Symptoms
- Dirty or blocked air filters — the most common culprit. Dust and debris trap moisture and become a food source for mould.
- Mould on the evaporator coil — visible as dark or grey streaking on the aluminium fins.
- Blocked or dirty condensate drain tray — standing water in the tray quickly turns stagnant and smelly.
- Mould on the fan (cross-flow) blower wheel — hard to see but very effective at spreading spores through your room.
- Mould inside the ductwork — relevant for ducted systems; spores can spread throughout the whole house.
- Infrequent use — units left idle for weeks (e.g. after winter) often smell on first start-up as dormant mould reactivates.
Symptom check: If the smell is strongest in the first few minutes of operation and then fades, the mould is likely on the filter or coil surface. If it persists, the blower wheel or drain tray is more likely involved.
Step-by-Step Diagnosis
1. Check and Clean the Air Filters
Most heat pump filters slide out from the front panel without tools. Hold them up to the light — if you can't see through them, they're overdue for a clean. Wash with warm water and a mild detergent, rinse thoroughly, and allow to dry completely before reinserting. Never run the unit with wet filters.
2. Inspect the Evaporator Coil
With the filters removed, shine a torch onto the coil fins. Look for dark spots, grey fuzz, or a slimy film. Light surface mould can be treated with a heat pump coil cleaner spray (available at Mitre 10 or Bunnings). Follow the product instructions carefully — most are foaming sprays that you apply, leave to dwell, then allow to drain away.
3. Check the Condensate Drain Tray
The drain tray sits beneath the coil. If accessible, wipe it out with a damp cloth and check that the drain outlet isn't blocked with slime or debris. A blocked drain causes water to pool and stagnate. You can flush it with a diluted white vinegar solution to kill bacteria.
4. Look at the Blower Wheel
This is trickier. Peer through the front of the unit (with it switched off at the wall) and look for dark build-up on the fan blades. A heavily soiled blower wheel usually needs a professional clean, as accessing it properly often requires partial disassembly.
5. Ducted Systems — Check Your Vents
For ducted heat pumps, remove a couple of vent grilles and shine a torch inside. Visible mould growth in ductwork is a more serious issue and warrants a professional inspection. BRANZ research highlights that poorly maintained ducted systems can significantly affect indoor air quality.
What You Can Safely Fix Yourself
| Task | DIY Friendly? | Notes |
|---|---|---|
| Clean/replace air filters | ✅ Yes | Do this every 4–6 weeks |
| Light coil cleaning with spray | ✅ Yes | Use a purpose-made heat pump cleaner |
| Flush condensate drain tray | ✅ Yes | Diluted white vinegar works well |
| Wipe accessible surfaces | ✅ Yes | Use a damp cloth, mild detergent |
| Blower wheel deep clean | ⚠️ Caution | Often needs partial disassembly |
| Refrigerant-side work | ❌ No | Restricted — licensed technician only |
| Electrical wiring or components | ❌ No | Restricted work under NZ law |
Safety note: Always switch the heat pump off at the wall (not just the remote) before opening the unit or cleaning any internal components. Never spray water directly onto electrical components.
How to Prevent the Musty Smell Recurring
- Clean filters every 4–6 weeks during heavy use periods. This single habit prevents the majority of mould problems.
- Run the fan-only mode for 15–20 minutes after using cooling mode. This dries out the coil and drain tray before the unit shuts down. Many modern units have an auto-dry function — check your manual.
- Book an annual professional service — a technician will deep-clean the blower wheel, coil, and drain system with specialised equipment and anti-mould treatments.
- Improve ventilation in the room. The Tenancy Services Healthy Homes standards highlight the importance of adequate ventilation in reducing moisture — the same principle applies to owner-occupied homes.
- Use a dehumidifier in particularly damp rooms to reduce the moisture load on your heat pump.
- Don't block the return air grille — restricted airflow reduces efficiency and increases condensation.
When to Call a Professional
Call a qualified heat pump technician if:
- The smell persists after you've cleaned the filters and coil.
- You can see significant mould growth on the blower wheel or inside the unit casing.
- There is visible mould in ducted system ductwork.
- Water is leaking from the indoor unit (a blocked drain may need professional clearing).
- The unit is making unusual noises alongside the smell (could indicate a failing component).
- You're not comfortable opening or accessing internal parts.
Any work involving electrical components, refrigerant, or wiring is restricted work under New Zealand law and must only be carried out by a licensed technician registered with the Electrical Workers Registration Board (EWRB). Attempting this work yourself is illegal and dangerous.
A professional service typically involves a high-pressure coil clean, blower wheel clean, drain flush, and an anti-bacterial/anti-mould treatment — well worth the cost for a unit that's been neglected for a season or two. You can find qualified, local heat pump installers and service technicians on tradedirectory.co.nz.
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