"Heat pumps stop working below freezing" — do they really?

I had a bloke in Orange, NSW ring me last winter convinced his new heat pump was "broken" because it wasn't blasting hot air the way his old gas ducted system did on a 2°C morning. It wasn't broken. It was doing exactly what a heat pump does in the cold — working harder for less, and still winning on running cost. This myth that heat pumps "stop working" or "can't handle" freezing temperatures is one of the most persistent pieces of misinformation I deal with, and it's costing people comfortable, cheap heating because they talk themselves out of a system that would suit them fine.

The short version: modern heat pumps operate well below freezing, some rated to -25°C or colder. What actually changes with temperature is capacity and efficiency, not an on/off switch. Understanding that difference is the whole game.

Where the myth actually came from

This isn't a myth invented out of nothing. Heat pumps sold in Australia and the US through the 1990s and 2000s genuinely did struggle in cold climates. Older fixed-speed compressors lost a huge chunk of capacity below about 2-5°C, and many units simply weren't designed or rated for anywhere that saw regular sub-zero mornings. If you lived somewhere like Orange, Canberra, or anywhere in the US Northeast or Upper Midwest, and you tried a heat pump from that era, you probably had a bad experience. That reputation stuck, and it gets repeated by installers, gas fitters and neighbours who are working off twenty-year-old information.

The technology moved on faster than the reputation did. Variable-speed inverter-driven compressors, enhanced vapour injection, and better refrigerant blends changed what's possible. I've written before about what "cold-climate rated" on a spec sheet actually guarantees, and the gap between a bargain-bin split system and a genuine cold-climate unit is enormous. Lumping them together as "heat pumps" and drawing one conclusion is where most of the confusion starts.

What actually happens as the temperature drops

A heat pump moves heat from outside air into your home using the refrigeration cycle, the same physics I've covered in detail in how heat pumps actually work. The colder the outside air, the smaller the temperature difference the system has to work with, and the harder the compressor has to work to extract usable heat. Two things happen as a result.

First, coefficient of performance (COP) drops. A unit that delivers a COP of 4.0 at 7°C outdoor air might only manage 2.0-2.5 at -10°C. Still two to two-and-a-half times more efficient than resistance heating, but noticeably less than its mild-weather number. Second, maximum heating capacity falls off too. Manufacturers publish capacity-versus-temperature curves for exactly this reason, and a unit rated at 10kW at 7°C might only deliver 6-7kW at -15°C. This is why sizing matters so much in cold climates. I've gone through the Manual J sizing mistakes that catch people out, and cold-climate capacity derating is exactly the kind of detail a gut-feel quote skips.

Neither of these is the unit "not working." It's the unit working differently. The confusion happens because homeowners expect flat output regardless of conditions, the way a gas furnace or electric resistance heater behaves. Heat pumps don't work that way, and pretending they do sets people up for disappointment.

Defrost cycles get mistaken for failure

Here's the other bit that trips people up. In cold, humid conditions, frost forms on the outdoor coil. The system periodically reverses briefly to melt it off, a process called a defrost cycle. During defrost, the outdoor fan may stop, you might see steam or vapour rising off the unit, and the indoor supply air can run cool for a few minutes. I've had callers describe this exact sequence assuming their unit had cut out or was blowing cold air by mistake.

It's normal operation, and it's more frequent, not less, as conditions get colder and more humid. A well-designed cold-climate unit manages defrost efficiently, minimising the energy penalty and the comfort dip. A poorly matched or undersized unit spends more time defrosting relative to heating, which is one more reason correct sizing and genuine cold-climate certification actually matter rather than being a marketing tick-box.

The efficiency drop is real, but the comparison still favours heat pumps

I'll be straight about the mildly contrarian bit here: I think some cold-climate heat pump marketing oversells how flat the performance curve stays, and that annoys me because it sets up exactly the kind of disappointment that feeds the myth in the first place. A unit that's "rated to -25°C" is not delivering the same COP at -25°C as it does at 7°C. It's delivering enough capacity to still heat the house, at a COP that's dropped, sometimes to somewhere near 1.5-2.0. That's still efficient heating. It's not the same efficient heating you get in shoulder seasons.

The comparison that actually matters is against the alternative, not against the unit's own summer performance. I went through this arithmetic properly in heat pump vs gas running costs, and even at a derated COP of 2.0 in genuinely cold weather, a heat pump is typically still cheaper to run than gas heating and dramatically cheaper than any form of resistance electric heating. The efficiency drop is real. It rarely flips the economics against the heat pump unless you're somewhere with truly extreme, sustained cold and a cheap gas tariff.

Backup heat is a design choice, not an admission of failure

Plenty of cold-climate installs, especially across the northern US, pair the heat pump with a backup heat source, gas furnace, or electric resistance strip that kicks in during the coldest snaps or when demand spikes past the heat pump's derated capacity. This is often presented, wrongly, as proof the heat pump "can't cope." It's actually just good system design. A dual-fuel or hybrid setup lets you size the heat pump to handle the vast majority of the season efficiently, while a small amount of backup capacity covers the extreme tail. That's an economic and engineering decision, not evidence the primary system failed.

In Australia this comes up less often because our climate rarely produces the sustained deep cold of a Minnesota winter, but it's still relevant in places like the Snowy Mountains region or Tasmania's colder valleys. Anyone specifying a system for genuinely cold conditions should be having this conversation with their installer up front, and it's a fair question to put to whoever quotes the job, alongside the sizing checks in how to vet a heat pump installer.

What actually determines whether cold weather is a problem

Three things matter far more than the general "do heat pumps work in the cold" question. The first is whether the specific unit is genuinely cold-climate rated, with a published capacity curve down to your design temperature, not just a marketing label. The second is correct sizing against your home's actual heat loss at that design temperature, which is where oversizing conversations like the one I wrote up in why oversizing quietly ruins comfort and running costs intersect with cold-climate performance; an oversized unit short-cycles and struggles with defrost management just as much as an undersized one struggles to keep up.

The third is installation quality, specifically refrigerant charge and outdoor unit placement away from prevailing wind and snow drift. A unit installed hard against a wall with poor airflow, or under-charged with refrigerant by a rushed installer, will underperform regardless of what the spec sheet promises. I've covered the licensing side of that in the refrigerant handling licence check most homeowners skip, and it's directly relevant here because a system that's technically correct on paper but poorly commissioned will reinforce the "doesn't work in the cold" myth all on its own.

The US Department of Energy's cold-climate heat pump technology work and the Air-Conditioning, Heating and Refrigeration Institute's certified ratings directory are both useful if you want to check a specific model's real published capacity curve rather than relying on a brochure claim; and the Clean Energy Council's approved product list serves a similar function for anyone shopping in Australia.

What I'd tell that Orange homeowner today

His system wasn't underperforming. It was running at roughly what its capacity curve predicted for that outdoor temperature, and it was still cheaper to run than the gas system it replaced would have been that morning. What he needed wasn't a different heating technology, it was a five-minute look at the manufacturer's performance data so the numbers matched his expectations before winter, not during it.

That's really the whole fix for this myth. Heat pumps in reputable cold-climate installs handle serious cold reliably, across plenty of the US Northeast and increasingly across Australia's colder inland regions. The technology isn't the weak link anymore. Mismatched expectations are.

— Priya Nathan, Heat Pumps & HVAC