Do heat pump hot water systems work in the cold?

Why cold air still has heat in it

A heat pump does not create heat, it moves it. That is why it can be more than 100% efficient in a way an element never can be — and why it keeps working when the air is near freezing. There is still plenty of heat energy in 2°C air; the compressor just has to work harder to extract it.

The ACT government publishes a buyers guide to hot water heat pumps for Canberra, which is about the toughest mainstream Australian climate for this equipment. It states that in summer most systems achieve a CoP of over 4, and that certain models suitable for Canberra's winter can still achieve a CoP of at least 2 in sub-zero temperatures. The same guide says a system can run efficiently in temperatures as low as −10°C.

Put plainly: on the worst mornings of a Canberra winter, a well-chosen unit is still roughly twice as efficient as the electric tank it replaced. In summer it is four times or better. The cold does not break the case for a heat pump — it narrows the margin for error on the model.

The surprise: cold postcodes usually earn a bigger federal rebate

Most people in a cold town assume they are the ones who miss out. The federal scheme works the other way. The Clean Energy Regulator has determined five postcode zones for air source heat pumps and four for solar water heaters, each based on climate and solar radiation levels. Zone 5 is the coldest band. Your postcode sets your zone, and your zone changes how many certificates your system creates.

We checked this directly against the regulator's own register of eligible models on 20 September 2026. Of the 1,148 air source heat pump listings that publish a comparable figure in both zones, 1,140 earn more certificates in zone 5 than in zone 1 — about 99%. Six earn fewer and two are equal.

The logic is straightforward once you see it: certificates track the energy an efficient system displaces compared with a conventional electric heater over the deeming period. A cold climate has more water heating to do across the year, so there is more to displace. Being cold is not a penalty in the federal scheme — it is the reason the number goes up.

The exception worth checking: NA in the zone 5 column

There is a real caveat, and it is smaller than the internet suggests. You will see it claimed that it is “not uncommon” for heat pumps to have no zone 5 rating. On the register as it stands, 30 of the 1,178 listings show NA in the zone 5 column — about 2.5%. So it does happen, but it is the exception, not the norm.

NA means no certificate entitlement is listed for the coldest zone. If you live in one of those postcodes and choose one of those models, the federal discount you were quoted may simply not exist. That is a five-minute check on the register against your exact model number, and it is worth doing before you sign anything in Canberra, Tasmania or the Victorian goldfields.

One related trap on the same register: the regulator states that all systems must be installed exactly as listed, and that if a component is missed or changed during installation the system will not be eligible for certificates. Cold-climate installs are where substitutions creep in, so make sure the tank and outdoor unit on the invoice are the pair that appears on the register.

The booster element: where cold-climate savings actually leak

This is the single biggest difference between a good and a bad cold-climate outcome, and it rarely appears on the front of a brochure. Many systems include an electric booster — a resistance element that switches on to help when the heat pump cannot keep up. The ACT guide puts it bluntly, advising buyers to consider whether the system relies on a booster during winter, because it is an electric heating element that can significantly increase your water heating costs.

A booster is not automatically bad; it is a sensible backstop. The problem is a unit that leans on it for weeks at a time. While the element is running you are paying the same price per litre as the old electric tank you replaced — so a system that boosts through winter can quietly erase a third of the annual saving you bought it for. The question to ask is not “does it have a booster” but “at what temperature does it start using it, and for how long”.

What to check before you sign, in a cold town

On that last point, the ACT government's advice for Canberra is specific: because of the city's extreme winters it suggests selecting from models rated for zone 5 climates with an energy saving of at least 65%. That is a useful bar to borrow anywhere cold, because it forces the conversation onto the figure that matters in your zone rather than the headline number from a mild-climate test.

Run it in the middle of the day — especially in winter

The old habit is to run hot water overnight on off-peak. In a cold climate that is the worst possible choice, and the reasons stack up. Overnight is when the air is coldest, so the system is at its least efficient and most likely to call on the booster. The ACT guide notes that these tanks are well insulated and lose heat very slowly, so a timer can heat during the afternoon to take advantage of warmer air and a higher CoP — and to use your own solar generation if you have panels.

There is a third reason we covered separately: overnight is also the window the NSW and Victorian noise rules restrict for this exact equipment. Three independent arguments, same answer — see our noise and placement guide for the hours that apply in your state.

Placement matters more when it is cold

A heat pump pulls air across an evaporator and blows it back out colder. In winter that exhaust is genuinely cold air, so a unit crammed into a tight, unventilated spot can end up recycling its own chilled output and working against itself. Give it the clearance the manual asks for, keep it out of narrow sunless passages where possible, and remember a spot that catches winter sun is working in your favour.

Frost on the evaporator is normal in cold conditions and systems are designed to shed it. Your installer should be able to tell you where the resulting water drains to — in a Canberra or Hobart winter you do not want it running across a path that ices over.

When to think twice

A heat pump is still the right answer for most cold-climate homes, but not all. If your only available outdoor position is a sunless, enclosed spot right beside a bedroom, if your home is a holiday place that sits empty through winter, or if you are looking at the cheapest unit on the market with no zone 5 figure and a booster that runs from 7°C down, the case is much weaker. Our is it worth it guide covers those cases honestly, and the sizing guide explains why buying a bigger tank is not the fix for a cold winter.

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