A light frost or thin layer of ice on your heat pump's outdoor unit during cold weather is completely normal — the unit has a built-in defrost cycle to handle it. However, if your outdoor unit is becoming heavily encased in ice and not clearing, something is likely wrong. Here's what you need to know to keep your heat pump running efficiently through a New Zealand winter.
Why Does the Outdoor Unit Ice Up?
When your heat pump is in heating mode, the outdoor unit extracts heat energy from the outside air — even cold air contains usable heat. As the refrigerant in the outdoor coil drops below the ambient air temperature, moisture in the air can freeze onto the coil fins. This is normal physics, not a fault.
Modern heat pump systems automatically run a defrost cycle every 30–90 minutes (timing varies by model). During defrost, the unit briefly reverses operation to warm the coil and melt the ice. You may notice steam rising from the outdoor unit and the fan stopping temporarily — this is all normal.
Problems arise when ice builds up faster than the defrost cycle can manage, or when the defrost cycle isn't working properly.
Common Causes of Excessive Ice Build-Up
- Restricted airflow — Leaves, grass clippings, garden debris, or overgrown plants blocking the unit prevent warm air from circulating around the coil.
- Dirty air filters — A clogged indoor filter reduces airflow through the whole system, causing the outdoor coil to run colder than it should.
- Low refrigerant (gas charge) — A refrigerant leak causes the coil pressure to drop, leading to abnormally low temperatures and rapid icing. This is one of the most common causes of severe icing.
- Faulty defrost control or sensor — If the defrost cycle isn't triggering correctly, ice will accumulate unchecked.
- Poor installation location — Units installed in shaded, low-lying, or poorly ventilated spots are more prone to icing.
- Very cold temperatures — Most standard heat pumps become less efficient below about 5–7°C. Some NZ regions (Central Otago, Canterbury foothills, Waikato frost hollows) regularly see temperatures where icing is more likely.
What You Can Do Yourself
1. Keep the Area Around the Unit Clear
Maintain at least 200–300 mm of clear space around the outdoor unit (check your manufacturer's manual for exact clearances). Remove leaves, trim back shrubs, and make sure nothing is leaning against the unit. Never cover the unit with a tarpaulin or box — it needs airflow to operate.
2. Clean or Replace the Indoor Filter Regularly
A dirty filter is one of the most common and easily fixed causes of poor performance. Most manufacturers recommend cleaning the indoor unit's filter every 2–4 weeks during heavy use. Rinse it gently under lukewarm water, let it dry fully, and refit it.
3. Check the Unit Isn't Recirculating Its Own Cold Air
If the outdoor unit is installed in a confined space — such as a narrow side passage or under a deck — it may be drawing in the cold air it just expelled. This dramatically worsens icing. Ensure exhaust air can escape freely and isn't looping back to the intake.
4. Don't Manually Defrost With Hot Water
Pouring hot water over an iced-up unit may seem like a quick fix, but it can cause thermal shock to components and may not address the underlying cause. If the unit is heavily iced and not self-defrosting, turn it off and let it thaw naturally, then investigate the cause.
5. Check the Installation Location
If your unit is in a frost-prone spot, consider whether a wind deflector or rain/frost canopy (available from heat pump suppliers) could help. These are designed to shield the unit without restricting airflow. Ask your installer before fitting any accessory.
Normal vs. Abnormal Icing — A Quick Guide
| Situation | Normal? |
|---|---|
| Light frost on coil fins in cold weather | Yes |
| Steam rising from unit during defrost | Yes |
| Fan stopping briefly during defrost cycle | Yes |
| Ice clearing within 10–15 minutes of defrost | Yes |
| Thick ice encasing the entire unit | No |
| Ice not clearing after multiple defrost cycles | No |
| Ice forming in mild weather (above 10°C) | No |
| Reduced heating output alongside icing | Investigate |
Healthy Homes and Heat Pump Performance
If you're a landlord, keeping your heat pump in good working order isn't just good practice — it's a legal requirement under the Healthy Homes Standards. A heat pump that's icing up and underperforming may not meet the heating requirements for your rental property.
When to Call a Professional
Some causes of excessive icing — particularly low refrigerant and faulty defrost controls — require a licensed heat pump technician. Refrigerant handling is restricted work in New Zealand; only certified technicians are legally permitted to handle refrigerants under the Ozone Layer Protection Act. Attempting to top up refrigerant yourself is illegal and dangerous.
Call a qualified heat pump installer or HVAC technician if:
- The unit is heavily iced and not self-defrosting
- You suspect a refrigerant leak (hissing sounds, oily residue near fittings, sudden drop in performance)
- The defrost cycle appears to not be running
- The unit is icing up in mild weather
- Performance has noticeably dropped alongside the icing
You can find qualified local heat pump installers on tradedirectory.co.nz. Regular annual servicing by a professional is the best way to catch issues early and keep your unit running efficiently — BRANZ recommends routine maintenance as part of good building upkeep practice.
For guidance on energy-efficient heating and heat pump use in NZ homes, Energywise has excellent free resources.
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