A heat pump dripping or pooling water inside your home is one of the most common calls heat pump technicians receive across New Zealand. The good news is that most causes are straightforward — but left unattended, even a small leak can damage your walls, ceiling, or flooring and encourage mould growth. Here's how to work out what's going on and what to do about it.
Why Heat Pumps Produce Water in the First Place
Your indoor heat pump unit (the wall-mounted head unit) works by pulling warm, humid air across a cold evaporator coil. As the air cools, moisture condenses on the coil — just like water droplets form on a cold drink on a summer's day. Under normal operation, this condensate drips into a drain tray and flows away through a condensate drain pipe to the outside. When any part of that system fails or is blocked, water ends up where it shouldn't: dripping down your wall or pooling on the floor.
Common Causes and Symptoms
| Cause | Typical Symptom |
|---|---|
| Blocked condensate drain | Steady drip or overflow from the unit |
| Dirty or clogged air filter | Ice forming on coil, then dripping when it melts |
| Frozen evaporator coil | Large amounts of water, often intermittent |
| Cracked or misaligned drain tray | Dripping from one side or corner of the unit |
| Incorrect unit installation angle | Water pooling and overflowing the tray |
| Low refrigerant (gas) | Coil freezes repeatedly; restricted work to fix |
| Condensation on pipework | Dripping from the refrigerant pipes or wall penetration |
How to Diagnose the Problem Step by Step
Step 1 — Turn the unit off
Before inspecting anything, switch the heat pump off at the unit and at the wall isolator switch. If water is dripping near any electrical components, do not touch the unit — turn off the circuit breaker and call a technician immediately.
Step 2 — Check the air filter
Open the front panel of the indoor unit and remove the air filter. A heavily clogged filter restricts airflow, causing the evaporator coil to freeze. When the unit cycles off, the ice melts and overwhelms the drain tray. If the filter is grey with dust and debris, that's very likely your culprit. Clean it with warm water, let it dry fully, and reinstall.
Step 3 — Look for ice on the coil
With the front panel open, shine a torch at the evaporator coil (the finned metal section behind the filter). If you can see ice or frost, the coil has been freezing. This is often caused by a dirty filter (see Step 2), but can also indicate low refrigerant — a restricted task that only a licensed refrigeration technician can legally handle in New Zealand.
Step 4 — Inspect the condensate drain tray
The drain tray sits beneath the evaporator coil. Look for standing water, slime, or algae build-up. A tray full of water means the drain is blocked. Some trays can crack over time, especially in older units, causing water to bypass the drain entirely.
Step 5 — Check the condensate drain line
The drain line is usually a small plastic pipe that runs from the indoor unit through the wall to the outside. Go outside and check whether water is dripping from the end of the pipe during or after operation. No drip at all when the unit has been running in a humid environment suggests a blockage somewhere in the line.
Step 6 — Check the unit's tilt
Heat pump head units must be installed with a very slight downward tilt toward the drain outlet so gravity moves condensate in the right direction. If the unit has shifted or was installed incorrectly, water can pool at the wrong end and overflow. Sight along the bottom of the unit — if it looks noticeably off-level, installation angle is likely the issue.
What You Can Safely Fix Yourself
- Clean the air filter. This should be done every four to six weeks during heavy use. Rinse under warm water, never use a dishwasher or harsh chemicals.
- Clear a blocked condensate drain line. If you're comfortable doing so, you can try flushing the drain line with a small amount of warm water poured carefully into the drain tray outlet. Some homeowners use a wet/dry vacuum on the outside end of the pipe to suck out the blockage. Avoid using harsh chemicals that could damage the pipework.
- Clean the drain tray. Wipe out any slime or algae with a damp cloth. A diluted white vinegar solution helps prevent regrowth.
- Check and clear the outdoor drain outlet. Make sure the pipe end outside isn't blocked by dirt, leaves, or a spider's nest.
Safety note: Never attempt to open the sealed refrigerant circuit, adjust electrical wiring, or reposition the unit on its mounting bracket yourself. These tasks carry real risk of injury and, in the case of refrigerant handling, are legally restricted under New Zealand regulations.
What Requires a Licensed Professional
Some causes of heat pump leaks are restricted work under New Zealand law and must only be carried out by a qualified technician:
- Refrigerant top-up or leak repair — Handling refrigerants requires a licence under the Ozone Layer Protection Act. A technician will also need to find and fix the underlying leak, not just top up the gas.
- Electrical faults — Any work inside the electrical components of the unit is restricted electrical work under the Electrical Workers Registration Board (EWRB) rules. Do not attempt this yourself.
- Repositioning or reinstalling the unit — If the unit needs to be relevelled or remounted, this should be done by a Licensed Building Practitioner or the original installer, as it may also involve refrigerant line disconnection.
- Replacing a cracked drain tray — While not legally restricted, this involves partial disassembly of the unit and is best left to a technician to avoid voiding your warranty.
If you're unsure, it's always safer to call a qualified heat pump installer or refrigeration technician. You can find licensed local professionals on tradedirectory.co.nz.
How to Prevent It Recurring
- Clean your filter regularly — every 4–6 weeks during peak use (winter heating, summer cooling).
- Book an annual service — a technician will flush the drain line, check refrigerant levels, clean the coil, and inspect the tray and pipework.
- Keep the area around the outdoor unit clear — good airflow to the outdoor unit reduces strain on the system and helps prevent coil freeze-up.
- Monitor humidity — New Zealand's Healthy Homes standards highlight the importance of managing indoor moisture. Running your heat pump on a dry/fan mode occasionally can help reduce humidity build-up.
- Act quickly — water damage and mould can develop fast, particularly in NZ's wetter regions. Don't ignore even a small drip.
When to Call a Professional
Call a qualified heat pump technician if:
- The leak continues after you've cleaned the filter and cleared the drain.
- You can see ice forming on the coil.
- The unit is making unusual noises alongside the leak.
- Water is dripping near electrical components.
- The unit is under warranty (DIY repairs may void it).
- You suspect a refrigerant issue.
A reputable installer will carry out a full diagnostic, not just a quick fix. For guidance on what to expect from a service visit, BRANZ and MBIE Building Performance both provide useful resources on building systems and moisture management.
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