What size heat pump do you need?
Enter the room size and how well it holds heat for a rough size. An installer confirms it when they measure up.
About 5 kW
A rule of thumb NZ installers use, for a room with a standard 2.4 m ceiling. High ceilings, big windows and open-plan spaces need more, so an installer should confirm it.
A heat pump that’s the wrong size is a daily irritation. Too small and it runs itself ragged on a frosty night without ever quite getting there. Too big and it blasts, stops, blasts again, burning power and wearing itself out. The number you want sits in the middle, and it comes from your room, not from the price tag.
Four things worth doing before you buy:
- Measure the room, ceiling height included.
- Think about how well it holds heat. Insulation, big windows, which way the room faces.
- Get an installer out to look, and ask how they worked the size out.
- Compare quotes on the 7°C heating figure on the energy label, and check the 2°C figure if you get frosts.
What does the kW number on a heat pump actually mean?
Heat pumps are sized in kilowatts. That’s how much heat the unit can move into a room, not how big the box on the wall is. Heating and cooling capacity are both measured to a set test standard, so a 5 kW unit from one brand and a 5 kW unit from another are meant to compare like for like.
Every new heat pump sold in New Zealand carries the Zoned Energy Rating Label, required on new models since July 2021. Near the top it shows capacity at three outdoor temperatures:
- Cooling at 35°C, in the blue box.
- Heating at 7°C, in the red box. Size on this one if heating is the main job.
- Heating at 2°C, which shows how the unit copes with frost. If your winter nights drop to 2°C or lower, look for a 2°C figure the same as or higher than the 7°C one.
Why two heating numbers? Below about 5.5°C outside, frost forms on the outdoor unit and it has to divert some of its effort into clearing it. Some models put out noticeably less heat at 2°C than at 7°C. Two units with the same headline size can feel quite different on a July morning in Taupō, Alexandra or Ashburton.
How do I work out what size heat pump I need?
Room size is the starting point. It isn’t the whole story.
- Floor area. Multiply the length by the width of each room you want to heat, then add them together. If the lounge flows into the kitchen and hallway, you’re heating all of it.
- Ceiling height. A high-stud villa has far more air to warm than a modern home with standard ceilings.
- Insulation. An older place with little insulation loses heat much faster, so it needs a bigger unit to keep up.
- Windows. Big glass, especially single glazing, lets heat out. A sunny north-facing lounge needs less help than a shady south-facing one.
- Where you live. A Whangārei lounge and an Invercargill lounge of the same size have different needs. Places that regularly drop below zero, like the Mackenzie Basin or Central Otago, need more capacity or a model built for the cold.
- What else heats the room. A log burner you’ll keep using, or a lot of afternoon sun, changes the job the heat pump has to do.
Why a bigger heat pump isn’t better
It’s tempting to buy the biggest unit you can afford, just in case. Don’t. An oversized unit heats the room quickly, then switches off and on more often than it should. That stop-start running wears it faster, wastes power, and can mean it never runs long enough to dry the air properly in a damp house.
Too small is a problem too. An undersized unit runs flat out for longer, costs more to run, and still leaves the room cool on the coldest nights. What you’re after is a unit that reaches your setting on a normal winter night and then settles into a gentle hum.
→ Related: Heat pump problems that come from the wrong size
Is there a heat pump size calculator?
There’s a rule of thumb installers use: watts per square metre. About 120 W per m² in a well-insulated room, 140 in an average one, and 160 in an older, draughty one, all for a standard 2.4 m ceiling. Run your room size through that and you get a rough number to shop with.
There’s no official government calculator that sizes a whole house, and no formula can see your windows or feel your draughts. Treat the answer as a starting point, not a spec sheet.
The better check is a visit. A good installer measures the room, asks about insulation and glazing, and can tell you why they picked that size. If someone quotes a size over the phone without asking any of that, keep looking.
What size heat pump does a rental need?
Landlords have a legal floor to meet. Under the Healthy Homes heating standard, the main living room needs a fixed heater of at least 1.5 kW that meets the room’s required capacity, worked out with the Heating Assessment Tool, the formula in the regulations, or a professional assessment. The Healthy Homes heating standard covers the detail.
How do I compare heat pump quotes on size?
Once two or three quotes are in, line them up on the same numbers:
- Heating capacity at 7°C. Make sure the units are a similar size. A cheaper quote for a smaller unit isn’t a saving if it can’t heat the room.
- Heating capacity at 2°C, if you get frosts.
- Cooling capacity at 35°C, if a hot room is the main reason you’re buying.
- Whether the size covers the right space. One quote might be for the lounge alone, another for the lounge and hallway. Check what each installer measured.
Ask for the model number to be written on the quote, so you can look up its label and check the figures yourself. What a heat pump costs covers the rest of the quote.
What should I ask the installer about sizing?
When someone comes to measure up, these questions tell you whether they’ve done the work or had a guess:
- How did you work out the size? You want to hear about the room, the insulation and the windows, not “this one’s popular”.
- What’s the heating output at 2°C? Especially if you’re inland or in the south.
- Will one unit heat the hallway and bedrooms too, or just this room? If you want the whole house warm, ask about a ducted or multi-room system instead.
- What happens if it can’t keep up? A good installer stands behind their sizing and will come back to check.
→ Related: Ducted vs high-wall heat pumps: which suits your house?
Who should I get to size my heat pump?
Find installers who fit high-wall heat pumps, or start from your town and compare the businesses near you. Heating more than one room? Look at ducted and floor console installers too.