Is it just defrosting?
In cold weather, ice forms on the outdoor unit's coil and the heat pump has to melt it off. EECA says frosting starts once it's below 5.5 degrees outside, and some heating capacity then goes into defrosting instead of into your room.
Mitsubishi Electric NZ says all heat pumps do this from time to time in winter, and that the unit stops heating for short periods while it does. Consumer NZ puts a defrost at a few minutes. Water dripping off the outdoor unit during or after one is expected.
Damp, frosty mornings bring more of them. As Mitsubishi Electric NZ puts it, the more moisture in the air, the more freezes on the outdoor coil.
A defrost becomes a worry when the pauses keep coming on every cold night, or the outdoor unit stays iced over. BRANZ links regular defrosting to a heat pump that's too small for the room.
Work through these first
Check it's on Heat
Auto can switch modes on its own, and Fan and Dry won't warm the room. Mitsubishi Electric NZ notes the unit isn't heating in Dry mode, and suggests choosing Heat or Cool by season rather than Auto.
Check the set temperature
Once the room reaches the setting, Mitsubishi Electric NZ says the heat pump ramps down but keeps circulating room air, which can feel cool on your skin. It starts heating again as the room cools. Try a setting within its recommended 18 to 22 degree range.
Check the remote and the power
Flat remote batteries are an easy miss. Samsung NZ suggests a power reset as a first step. If the heat pump's circuit breaker has tripped, switch it back on once. If it trips again, leave it off and call a registered electrician.
Clean the filters
Clogged filters choke the airflow. BRANZ says good performance depends partly on a clean indoor unit filter, and with frequent use that means several times a year. Follow your manual, or see heat pump cleaning.
Look at the outdoor unit
Is the fan turning? Is anything leaning on it, growing over it or stacked beside it? Mitsubishi Electric NZ says too little clearance cuts its output, and units squeezed under decks or houses can create their own microclimate. Clear what you can by hand, from the ground.
Write down any code
If a code shows or a light blinks, note it and the model number before you reset anything. Our error codes guide explains the next step.
Leave the ice and the covers alone
Don't chip, scrape or pour water on ice on the outdoor unit, and don't take panels off either unit to look inside. The fins bend easily, and behind the covers are live electrical parts and the refrigerant circuit. Wiring faults are prescribed electrical work for a registered and licensed electrical worker, and the refrigerant side belongs to a trained heat pump technician.
When it's the heat pump, not the settings
If none of that helps, the cause needs a technician to find. One possibility, especially in cold places, is that the unit is simply too small for the room on a frosty night.
Mitsubishi Electric NZ says a heat pump's output falls as the temperature falls, and that an undersized unit will defrost at the minimum intervals and may never catch up to the set temperature. BRANZ describes the same pattern: running continuously, with the outdoor coil defrosting regularly.
Some units cope with frost better than others. EECA says some give less heat at 2 degrees than at 7, and Rinnai NZ says it can't guarantee heating performance below zero. Our sizing guide explains what to look for next time.
Other causes take tools and training to find, like a refrigerant leak or a failed sensor, fan or circuit board. A heat pump repair specialist will test for those. If refrigerant has been lost, it's leaked out somewhere, so the fix is finding and repairing the leak rather than topping it up.
Renting, or still under warranty?
Renting: tell your landlord in writing, with when it happens and what you've checked. Heating installed to meet the healthy homes standards has to be kept in good working order, or replaced within a reasonable time if it can't be.
Under warranty: check the conditions before calling just anyone. Rinnai NZ's warranty excludes damage caused by repair work from anyone other than its accredited installers, service people or service centres, and other brands have rules of their own. Start with the business that installed it.
What you can try, and what you can't
You can
- Switching to Heat mode and checking the set temperature.
- Replacing the remote's batteries.
- Switching a tripped breaker back on, once.
- Cleaning the filters as your manual describes.
- Clearing plants and clutter from around the outdoor unit, from the ground.
- Writing down any error code and the model number.
Leave to a licensed installer
- Diagnosing a unit that never reaches temperature.
- Finding and repairing refrigerant leaks.
- Testing sensors, fans and circuit boards.
- Electrical faults, or a breaker that keeps tripping.
- Working out whether the unit is too small for the room.
What we'd do
What we'd do
On a frosty morning we'd give it a few minutes, because a defrost pause sorts itself out. Otherwise we'd check the mode, the setting, the remote and the filters, in that order. If it still blows cool air after that, or stops to defrost again and again every cold night, we'd stop guessing and get a heat pump repair specialist to measure what it's actually putting out.