The thing that surprises most people about heat pumps: they don't make heat. They move it. That sounds like a magic trick — and it kind of is — but it's also why heat pumps are the most efficient way to heat or cool a home, by a long way.
The basic idea: heat moves from warm to cold
Even cold air contains heat energy. Air at 5°C has way less heat than air at 25°C, but it's not zero — there's still energy in there. Heat naturally flows from warmer to cooler places (think of a hot drink cooling down on the bench). A heat pump uses pressure changes in a refrigerant gas to force heat to flow in the direction it wouldn't naturally — pulling warmth out of cold outside air and depositing it inside your home.
Reverse the cycle and you can move heat the other way too. That's how the same unit cools your house in summer — pulling heat out of indoor air and dumping it outside.
The four-stage cycle (in plain English)
A heat pump moves a refrigerant fluid through a closed loop, switching it between liquid and gas states to absorb and release heat. Here's what happens, with no jargon:
1. Outside: refrigerant absorbs heat
The outdoor unit (the condenser sitting next to your house) contains cold liquid refrigerant. Outside air — even cold winter air — is warmer than the refrigerant. Heat flows from the air into the refrigerant, turning it into a low-pressure gas. The fan on the outdoor unit pulls more air across the coils to keep the cycle going.
2. Compressor: gas gets squeezed and heats up
The compressor (the noisy bit in the outdoor unit) squeezes the gas to high pressure. Compressing a gas makes it hot — same physics as a bike pump getting warm. By the time the gas reaches the indoor unit, it's hotter than your living room.
3. Inside: refrigerant releases heat
The hot gas flows through the indoor unit's coils. Indoor air is cooler than the gas, so heat flows out of the gas and into the room. The fan in the indoor unit blows that warmed air around. The refrigerant, now cooler, condenses back into a liquid.
4. Expansion valve: pressure drops, cycle repeats
The cooled liquid passes through an expansion valve, which drops its pressure dramatically. Lowering pressure on a liquid makes it cold — the reverse of the compressor. Now-cold liquid heads back out to the outdoor unit, ready to absorb heat from outside air again.
Why it's so efficient
An electric heater turns 1 unit of electricity into 1 unit of heat — it's 100% efficient by definition, but it can't do better. A heat pump isn't generating heat from electricity; it's just using electricity to run a compressor and a couple of fans. The actual heat is being moved, not created.
Result: for every 1 kWh of electricity a heat pump uses, it delivers 3–5 kWh of heating or cooling output. The Coefficient of Performance (COP) is how this is measured — modern inverter heat pumps land between COP 3.5 and COP 5.0 in heating mode for typical NZ temperatures.
That's why heating with a heat pump costs about a third of an equivalent electric heater, and a fraction of gas. Same warmth, less power.
Heating, cooling, and dehumidifying — same hardware
Modern heat pumps in NZ are reverse-cycle inverter systems. That means:
- Reverse-cycle: the same unit can run heating in winter and cooling in summer — just press a different mode button on the controller. The refrigerant circuit reverses direction; everything else stays the same.
- Inverter: the compressor varies speed instead of cycling on/off, which makes the unit quieter, more efficient, and longer-lasting than older non-inverter heat pumps.
- Dehumidify mode: running the cooling cycle gently extracts moisture without dropping the temperature much — useful in muggy NZ summers when you want drier air, not necessarily cooler.
Do heat pumps work in NZ winter?
Yes. Modern inverter heat pumps maintain useful efficiency down to about -15°C and most NZ winters bottom out around -5°C even in the colder parts of the South Island. Older units used to lose performance when it got cold, but the current generation handles frosty Waikato mornings without complaint. Defrost cycles automatically clear ice from the outdoor coils every few hours during cold weather, so you don't see the heating drop off.
Frequently asked questions
How does a heat pump heat a house?
It pulls warmth out of the outside air (yes, even cold winter air contains heat energy) and moves it indoors. The refrigerant gas cycle and a clever bit of pressure manipulation let it do this far more efficiently than burning fuel or running an electric heater.
How does a heat pump cool a house?
It runs the same cycle in reverse — pulling heat out of indoor air and dumping it outside. That's all air conditioning is. Modern heat pumps cool and heat using the same hardware; you just press a different mode button.
Why are heat pumps more efficient than electric heaters?
Because heat pumps move heat instead of generating it. An electric heater turns 1 unit of electricity into 1 unit of heat (100% efficient by definition, but no more). A heat pump moves 3–5 units of heat for every 1 unit of electricity — a Coefficient of Performance (COP) of 3–5x. Same electricity, three to five times the warmth.
Do heat pumps work in NZ winter?
Yes — modern inverter heat pumps work down to -15°C and beyond. Even at -10°C the air outside contains plenty of extractable heat energy. NZ winters rarely drop below -5°C, so heat pumps are a safe choice almost everywhere in the country including the South Island.
What is a reverse-cycle heat pump?
A heat pump that can run in either direction — extracting heat from outside and dumping it inside (heating mode), or extracting heat from inside and dumping it outside (cooling mode). Almost every heat pump sold in NZ is reverse-cycle, which is why the same unit handles both heating and air conditioning.
What is an inverter heat pump?
An inverter heat pump runs the compressor at variable speeds rather than just on/off. Older non-inverter units cycle hard on and off — using more power, making more noise, and wearing out faster. Inverter units smoothly modulate output to match demand, which is why modern heat pumps are quieter, more efficient, and longer-lasting than the units sold 15 years ago.
Can a heat pump heat my whole house?
Yes — either with one ducted system serving every room, or with a multi-split system running 2–5 indoor units off one outdoor condenser, or with several stand-alone units. The right answer depends on your home layout and budget. See our ducted vs split guide for the trade-offs.
