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It is a familiar routine as winter arrives: you wake up to a cold house, grab the remote, and crank the heat pump up. But instead of the room warming quickly, the system spends the first part of the morning repeatedly cycling into defrost mode, slowing the heating of the room.
Spending a bit of time getting your heat pump ready for winter means it will work more efficiently when those cold mornings arrive. This article walks through what to check, what you can safely do yourself, and one operating habit that makes a real difference to your comfort and the health of your home.
April and May are the ideal window. Once the main heating season is underway and cold snaps are hitting regularly, the system is already working hard. Getting ahead of that in autumn means the unit is clean and ready before it is needed most. That said, the outdoor unit check, filter inspection and indoor mould check are worth doing at any point during winter. A clean system runs better than a neglected one, and the operating habits covered here apply from the first cold night through to September.
The first point of call is the outdoor unit. The outdoor unit works by pulling heat energy out of the outside air, even on cold days, and transferring it inside to warm your home. It does this through a series of thin metal fins that run around the outside of the unit. When those fins get clogged with dust, pollen or debris, the unit has to work much harder to do the same job, which means longer run times, higher power use and more wear on the system.
Over summer and autumn, NZ outdoor units accumulate contamination from three sources that are relevant right now.
Airborne dust is a growing issue, particularly in suburban areas where infill housing and townhouse developments have increased across NZ's main centres. Fine construction dust bonds to the fin surfaces rather than sitting loosely, so wind and rain do not shift it the way they would with larger debris like leaves or dirt. Once it packs in, it stays.
Pollen and leaf debris accumulate through spring and autumn. Autumn leaf drop can push organic material into outdoor units and build up inside the housing, particularly on properties with trees nearby.
Salt spray is a factor on coastal properties. Each time salt-laden air contacts the fins and dries, it leaves a fine saline residue that bonds to the metal surface. That residue accumulates over time, degrading the fin material and steadily reducing the unit's ability to transfer heat.
Before touching the unit, turn it off at the indoor controller and use the isolating switch mounted near the outdoor unit to cut power completely.
With power off, remove the top cover of the outdoor unit, it is usually attached with a few screws. This gives you a view inside the unit of the fan and the coil fins around the perimeter. You are looking for leaves or debris accumulated inside the housing, visible dust or blockage on the fin surfaces, and vegetation pressing against the sides of the unit.
One important boundary: do not open the side electrical access panel. That compartment contains capacitors that can hold a charge even with the power off, and it is not a homeowner maintenance area. The top cover for debris inspection is a different matter entirely.
While you are at it, check the clearance distance from the unit to nearby walls and fences. The EECA heat pump installation guide specifies a minimum clearance of 150mm between the outdoor unit and the nearest wall. Units installed too close to a wall have restricted airflow, and that affects performance in both heating and cooling mode.
Start by clearing any leaves or debris from inside the housing by hand.
Outdoor units are weatherproof and designed to operate in rain, so a gentle rinse with a garden hose on a low-pressure setting is safe once the debris is cleared.
What you are rinsing is the fin surfaces that run around the outer perimeter of the unit. Start from inside the unit and spray outward through the fins, letting the water push loose dust and pollen out. Then move to the outside of the unit and rinse inward. Two passes from opposite directions shifts more contamination than one pass alone.
Keep the pressure gentle throughout. The fins are thin aluminium and they bend with very little force. Bent fins restrict airflow and are expensive to repair.
Clear any vegetation pressing against the sides or growing from below.
Do not apply coil cleaners or chemical sprays. Different aluminium alloys require different chemistry, and the wrong product causes corrosion. A professional service uses the right cleanser for the coil type and the correct pressure equipment. For DIY prep, clearing debris and a gentle rinse is the right stopping point.
The next thing to check is the filter on your indoor unit. A clogged filter restricts the airflow the heat pump draws through the system, which means the unit has to work harder to heat the room. The pre-winter check matters, but staying on top of it through the heating season is what makes the difference.
Before winter: pull the filter out and hold it up to the light. A filter that just needs a rinse will look dusty but the mesh will be intact and the frame will hold its shape. A filter that needs replacing will have mesh that is frayed or has holes in it, or a frame that has warped out of shape. If the filter cannot sit flush against the housing, unfiltered air bypasses it entirely and passes straight over the internal coil, carrying everything the filter was supposed to catch.
During winter: in a home running the heat pump regularly through the heating season, pull the filter out and clean it every three to four weeks. A restricted filter reduces airflow. The system works harder, and power costs go up.
This check takes about thirty seconds.
Shine the torch on your phone into the opening of the indoor unit where the air comes out. You are looking at the fan (the long curved blade assembly) and at the white housing around it.
You are looking for black spotting on the white housing, which is a likely sign of mould, or heavy dust and grime buildup on the fan blades. If you can see mould, the unit needs a professional clean before you run it through the heating season. A mould-affected system running constantly through winter circulates whatever is growing on those surfaces through the air in your home.
The fan blades are often dark-coloured, which makes buildup harder to see directly. Focus on the white housing.
The typical pattern goes like this. A cold morning means getting up and turning the heat pump on full blast to heat the room. People in the house have showers, make breakfast and then leave for work or school, turning the unit off as they walk out the door. They then come home to a cold house in the evening and start the whole process again. The assumption driving this is that running the heat pump during the day or overnight would cost considerably more in power. For a modern inverter heat pump, that assumption is worth examining.
A modern inverter heat pump is designed to run continuously at variable capacity. Once the room reaches the set temperature, the compressor does not switch off. It slows down, holding the room at setpoint while drawing a fraction of the power needed to get it there. Holding a room at 18 degrees through the night takes far less power than heating it from cold each morning.
A heat pump starting from a cold house has to work at near-maximum capacity to recover the room. Not just the air. The walls, floor and furniture that went cold all have to warm up before the room feels comfortable.
This is particularly relevant early in the morning, when outdoor temperatures are at their lowest. When the system is running hard and the outdoor temperature is low, the outdoor coil frosts up faster. That triggers the defrost cycle, a normal function where the system briefly reverses to clear ice off the outdoor coil. During defrost, the system stops heating the room. If you have ever experienced the frustration of the unit kicking into defrost mode when you are trying to heat a cold room, you now know why.
A clean, well-maintained system defrosts quickly and recovers. A system pushed hard from cold, with any contamination on the outdoor coil, cycles through defrost more often and takes longer each time.
Rather than turning the unit off at night, consider dropping the setpoint to 18 degrees and leaving it running. Running overnight at 18°C will likely use more power than switching the unit off completely. The difference is smaller than most people expect, because heating a cold house from scratch each morning uses considerably more power than holding a warm one steady through the night.
It is worth being clear: this approach is not primarily an energy saving strategy. It is a comfort and health strategy that comes with a modest power cost. Whether that cost is worth it over the winter months is a decision only you can make. The health case below is what we think makes it worth considering.
There are two reasons to consider overnight operation as an investment in your home and health.
The first is comfort. The room stays at a stable temperature. No cold starts, no waiting for the room to recover before you get out of bed.
The second is indoor health. When you heat your home in the evening (cooking, showering, breathing), the air picks up moisture. Warm air holds more moisture than cold air. When you turn the heat pump off and the temperature drops overnight, that moisture has to go somewhere. It comes out of the air and settles on the coldest surfaces: windows, walls, corners and the inside of the indoor unit.
That is condensation. Over a NZ winter, repeated condensation on surfaces is what drives mould growth. It is the same process that produces the black spotting on indoor unit housings that the earlier check was looking for.
BRANZ, New Zealand's building and construction research organisation, found that homes maintained at 18°C experience far fewer periods of high humidity than homes that cool overnight. EECA recommends setting heat pumps to 18°C or above specifically to help combat damp and mould.
There is also a longer term benefit that is easy to overlook. A drier home is easier to heat. Reduce condensation over winter and the house holds warmth more effectively, which means the system does not have to work as hard to keep it comfortable. The investment in overnight running pays back gradually through the season.
Winter power bills are already higher than the rest of the year. Adding overnight running at a low setpoint will add something to that cost. What it protects against is a home that accumulates condensation and mould through the cold months, and a system that starts each morning from a cold house. Whether that trade-off makes sense for your home is worth thinking about. For many NZ households, particularly those with condensation on windows through winter, we think it does.
In a well-insulated home with no condensation issues, turning the heat pump off overnight and using the timer to pre-heat before you wake up is also a reasonable approach, and one EECA explicitly recommends. The key in either case is avoiding a hard cold-start recovery without a timer, which puts the most demand on the system and produces the least efficient run.
April and May are ideal, before the main heating season starts. A clean outdoor unit and a checked filter heading into winter means the system does not have to work as hard when cold snaps arrive. If you have not done it yet, it is still worth doing mid-winter. Use the MIHT Home Energy Health Assessment to get a clear picture of where your system stands.
You can remove the top cover, clear debris from inside the housing, and rinse the exterior fin surfaces with a low-pressure garden hose. Outdoor units are weatherproof, so a gentle rinse is safe. That is the right stopping point for DIY. Do not open the side electrical access panel — that compartment contains components that hold a charge even with the power off. For a chemical coil clean, use a professional. They use the correct product for aluminium fins and the right pressure equipment.
Shine a phone torch into the indoor unit opening where the air comes out. Look at the white housing around the fan. Black spotting on that surface is a likely sign of mould. If you can see it, the unit needs a professional clean before you run it through the heating season. Running a mould-affected system through winter means circulating what is growing on those surfaces through the air in your home. The MIHT Home Energy Health Assessment identifies where your system needs attention.
In a home running the heat pump regularly through winter, every three to four weeks. The filter accumulates enough dust in that time to start restricting airflow. When you put it back, check that it sits snugly with no gaps around the edges. Gaps mean unfiltered air is bypassing the filter and passing straight over the internal coil.
Running your heat pump overnight at 18°C rather than switching it off will likely add something to your power bill. The difference is smaller than most people expect, because heating a cold house from scratch each morning uses more power than holding a warm one steady through the night. What you gain is comfort, a room that is warm when you wake up, and a home that is less prone to condensation and mould. BRANZ research shows homes held at 18°C experience far fewer high-humidity periods through winter. A drier home is also easier to heat over time, so the benefit builds gradually through the season. Whether the trade-off is worth it depends on your home and your priorities. In a well-insulated home with no condensation issues, using a timer to pre-heat before you wake up is also a reasonable option.
The EECA installation guide specifies a minimum clearance of 150mm between the outdoor unit and the nearest wall. Units installed too close to a wall have restricted airflow, which reduces performance in both heating and cooling mode. If your outdoor unit is closer than 150mm to a wall or fence, check the EECA guidance for the full requirements.
The Home Energy Health Assessment takes about three minutes and gives you a clear picture of how your heat pump, ventilation and solar systems are performing. Start the assessment..