Most heat pump owners who notice a musty smell do the same thing. They pull out the filters, give them a wash, and wait to see if the smell goes away.
It doesn't. Or if it does, it's back within a week or two.
That's not a coincidence. The smell comes from components further inside the unit that a filter clean can't reach. This article explains what those components are, why the smell develops there, and what it takes to actually clear it.
What is actually causing the heat pump smell
When mould and bacteria grow inside a heat pump, they release gases as they break down organic material (dust, pollen and skin cells that have accumulated inside the unit). These gases are detectable by the human nose at very low concentrations, which is why a noticeable smell can develop well before any visible growth is apparent.
Those gases circulate through the room every time the unit runs. That's what you're smelling. It's why the smell can be persistent even in a unit that looks relatively clean from the outside.
Where inside the unit the growth occurs
Mould requires two things: moisture and organic material as a food source. Inside a heat pump, both are present under the right conditions.
The internal coil, the component where the heating and cooling actually happen, gets wet during cooling and dehumidification operation. Moisture condenses on its surface and drips into the drain tray below. Dust and organic particles that have passed through the filters settle on the wet coil surface. Over a cooling season, this creates the conditions for mould and bacterial growth directly on the coil.
In heating mode the internal coil is warm, so moisture doesn't condense on it. A heat pump used only for heating will accumulate dust and debris on the coil over time. Mould won't develop there unless the unit also runs in cooling or dehumidification mode. Mould on the coil is a cooling-season problem, which is why units used year-round carry a different risk profile to those used only in winter.
The drain tray is a separate contributor that most people overlook entirely. Moisture from the coil collects there, and in a neglected unit the tray develops a layer of organic growth in standing or slow-draining water. A partial blockage in the drain line makes this worse by keeping water sitting in the tray longer than it should. Units with clean filters can still produce a persistent smell for exactly this reason.
Why the smell gets worse when you switch to heating
The most common moment people notice the smell is when they switch their heat pump from cooling to heating. In Auckland's autumn and spring, that can happen within the same day. Cooling runs during a warm afternoon, then heating switches on when the temperature drops at night.
When cooling stops and heating starts, the internal coil transitions from cold and wet to warm within a short period. During cooling, moisture condenses on the coil surface. Dust, pollen and skin cells settle on it continuously while the unit runs. When heating starts, the coil warms up rapidly and heats that contaminated surface directly. The mould and bacteria sitting on the coil release gases as they're heated, and those gases circulate into the room within minutes of switching modes.
The same mechanism explains why the smell often becomes suddenly and noticeably worse at the start of winter. Over a full cooling season, organic material has been accumulating on the coil for months rather than hours. When the unit switches to heating for the first time in autumn, a season's worth of contamination gets heated at once. What was a mild background odour during summer becomes impossible to ignore almost overnight.
In both cases the cause is the same: a warm coil heating the contamination that has built up on its surface. The difference is scale. A same-day switch produces a noticeable smell because the coil was recently wet and loaded from that afternoon's cooling cycle. A seasonal switch produces a sharp, sudden smell because months of accumulation are being heated for the first time.
The filter misconception
Most advice about musty heat pump smells suggests cleaning the filter. It's not hard to see why. It's the one part of the unit a homeowner can actually reach. But cleaning the filter doesn't fix the problem because the filter isn't where the growth occurs.
Growth occurs on the internal coil and in the drain tray, both of which require disassembling the unit to access. Cleaning the filter addresses the entry point for dust and particles. It has no effect on the mould and organic growth that has already formed further inside the unit. The smell persists because the source hasn't been touched.
This is the Healthy Home Blindspot. Homeowners do the right thing by keeping the filters clean, but the system has a blindspot deep inside where contamination builds without any visible sign. The filter is what you can see. The coil and drain tray are not.
heat pump filter clean vs professional service: what is the difference
The NZ context
A 2025 study found that 65% of NZ households have a heat pump. Of those, 72% use it for cooling. That works out to roughly 47% of all NZ households regularly running their heat pump in cooling mode. That's the population of heat pumps regularly exposed to the conditions that cause internal coil growth.
NZ housing compounds the problem. The indoor climate here is notoriously damp, which means more moisture condenses per hour of cooling operation than in drier climates. Over a full cooling season, that adds up to a wetter coil and a fuller drain tray than the same unit would see in a drier country. In homes that aren't professionally serviced, the risk builds year on year.
A separate 2025 study on fungi found in split-system heat pumps identified that 64% of fungi found on unit filters were capable of triggering allergic reactions. The filter is the last line of defence before air enters the room. What grows on the coil sits upstream of that.
What the health research says
The health concern is sustained, daily exposure from a contaminated system running in a home where people sleep, eat and spend most of their time.
The World Health Organisation concludes that mould and dampness increase the prevalence of respiratory symptoms, allergies and asthma. Research published in peer-reviewed literature links indoor humidity, dampness and mould to cough, phlegm, bronchitis and wheeze. Vulnerable people (children, elderly adults and those with compromised immune systems) carry higher risk from sustained exposure.
The risk accumulates quietly. A dirty heat pump is a slow, invisible contributor to the indoor environment your household breathes every day.
What actually fixes a smelly heat pump
A professional restore addresses the source. At MiHT Home Energy System Care, the process covers four steps. The internal coil gets a low-pressure wash to remove accumulated growth. The internal fan is pressure washed to clear mould buildup. The drain line is flushed and a slow-release tablet inserted to prevent future blockage. The casing and louvres are cleaned and sanitised.
The unit that comes out of that process is clean where the contamination actually lives, not just where it's easy to reach. What does a professional heat pump service include in Auckland covers the full process in detail.
A filter clean handles the entry point. A professional restore handles the source. For a unit that smells, only the second option resolves the problem.
Frequently asked questions
Why does my heat pump smell immediately when I switch from cooling to heating on the same day?
In autumn and spring, Auckland days can be warm enough to run cooling and cold enough at night to need heating. When the unit switches from cooling to heating, the internal coil transitions from cold and wet to warm within a short period. During cooling, moisture condenses on the coil surface and dust, pollen and skin cells settle on it continuously. When heating starts, the coil warms up and heats that contaminated surface directly. The mould and bacteria sitting on the coil release gases as they're heated, and those gases are what you smell within minutes of switching modes. This is a reliable sign that the internal coil carries accumulated contamination and would benefit from a professional restore. A clean coil produces no smell regardless of how quickly the unit switches between modes.
Why does my heat pump smell musty when I first turn it on in winter?
The growth responsible for the smell builds up during summer cooling operation, when moisture condenses on the internal coil. When the unit switches to heating mode in autumn, the warm coil heats that accumulated material and the mould and bacteria release gases sharply. The smell was developing over summer. Winter is when it becomes noticeable. Start the Home Energy Health Assessment to find out how your system is performing
Will cleaning the filter fix the smell?
No. The filter isn't where the growth occurs. Mould and organic buildup form on the internal coil and in the drain tray, both of which require professional access and equipment to clean properly. Cleaning the filter has no effect on contamination that has already formed further inside the unit.
Is a smelly heat pump a health risk?
The research is clear that mould and dampness in the indoor environment increase the risk of respiratory symptoms, allergies and asthma. The concern is sustained daily exposure in a home environment. Vulnerable people carry higher risk. What a professional heat pump service includes
Does my heat pump need a service if it only smells a little?
A mild smell indicates the early stages of the same process that produces a strong smell. The conditions that cause the odour don't stabilise on their own. Left unaddressed, growth on the internal coil and in the drain tray continues. A mild smell treated early is a straightforward restore. A strong smell left for another season is a harder job.
How often should I have my heat pump professionally serviced to prevent the smell returning?
For a heat pump used for both heating and cooling, an annual professional service is appropriate. If the unit is used only for heating and never runs in cooling or dehumidification mode, every two years may be sufficient. Homes with higher humidity, pets or more occupants benefit from annual servicing regardless of usage pattern.
Not sure where your home energy systems stand? The Home Energy Health Assessment takes about three minutes and tells you what to focus on. Start the assessment
MiHT Home Energy System Care provides professional maintenance for heat pump, ventilation, and solar systems across New Zealand. We don't install or repair. Our only focus is care.