What does heat pump size actually mean?
Size is heating output, measured in kilowatts. Consumer NZ puts it plainly: heating capacity is the most important specification when you're buying a heat pump.
BRANZ says a typical home air-to-air heat pump has between 2 and 16 kW of heating capacity, with cooling capacity generally about 10 to 20% lower. EECA's advice is to size on the heating capacity at 7 degrees for a unit that will mainly heat, and on the cooling capacity at 35 degrees only for one that will mainly cool.
Size isn't the power it draws: a heat pump moves heat rather than making it, so it usually puts out more kilowatts than it uses. Our guide to running costs covers that side.
Working out a starting figure for one room
Measure the room
Multiply the length by the width for the floor area, then by the ceiling height for the volume in cubic metres. Consumer NZ's sizing table assumes a 2.4 metre ceiling and says to adjust up for higher ones.
Count the spaces open to it
An open-plan kitchen, a dining area or a hallway with no door all share the heat. Add them in, because the heat pump will be warming them whether you planned it or not.
Apply BRANZ's rule of thumb
Multiply the volume by 65 watts per cubic metre for an older, uninsulated house, or by 50 to 55 for a newer, well-insulated one. Divide the answer by 1000 to turn watts into kilowatts.
Adjust for your house
BRANZ calls it a rough rule, and it doesn't allow for glazing, orientation or climate. EECA says the size and orientation of windows, insulation and shading all count. Big windows, a room that gets little sun or a frosty town all push the number up.
Have it checked
EECA's own advice is to talk to a professional to find the right size. The size calculator runs the arithmetic above for you, and an installer should confirm it with a proper heat loss check before quoting.
Why frosty nights change the answer
A heat pump draws its heat from the outdoor air, and the colder that air is, the less it can deliver. Mitsubishi Electric NZ says output capacity drops as temperatures drop, and that it's extremely important units aren't undersized for cold conditions.
It gives a worked example. A heat pump rated at 5.6 kW won't deliver 5.6 kW when it's 0 degrees outside: it's likely to manage about 4.76 kW. For a room that needs 5.6 kW, Mitsubishi Electric NZ says a 6.8 kW unit, giving about 5.78 kW at 0 degrees, would be the correct choice.
That's why EECA says that where overnight winter temperatures reach 2 degrees or lower, the heating capacity at 2 degrees on the Zoned Energy Rating Label should be the same as or higher than the figure at 7 degrees. Some units handle frost well and give more heat at 2 degrees. Others give less.
Spec sheets may also show BRANZ's H1, H2 and H3 ratings: heating output at 7 degrees, 2 degrees and minus 7 degrees outside. In a cold part of the country, H2 and H3 tell you more than H1 does.
Too small or too big: what goes wrong
| Too small | Too big | |
|---|---|---|
| What it does | Runs continuously, trying to reach the set temperature (BRANZ) | Cycles on and off (BRANZ) |
| How Rinnai NZ puts it | Ineffective at bringing the room to the temperature you want | Turns off and on frequently, causing wear and tear |
| Knock-on effects | The outdoor coil can get cold enough to defrost regularly (BRANZ) | Lower efficiency and a shorter life (BRANZ) |
Insulation first, then size
BRANZ is blunt that heat pumps don't replace the need for good insulation. In one BRANZ study, half the households averaging under 18 degrees on winter evenings had poor insulation or none at all.
Mitsubishi Electric NZ makes the same point from the other side: the quicker a home loses heat, the bigger the heat pump needs to be. If the ceiling or underfloor insulation is missing, sort that before you size the unit, because the size you need may well come down.
Renting? The legal minimum is a different number
Under the healthy homes standards, a rental's main living room needs fixed heating that can bring it to 18 degrees and hold it there. Tenancy Services' heating assessment tool works out the minimum capacity for that one room in kilowatts, rounded up to one decimal place.
That's a legal floor for the main living room, not a comfort sizing for the house, and it doesn't apply to owner-occupied homes. When you check a heat pump against it, use the nominal heating capacity on the label, tested at 7 degrees, and not the capacity range. The healthy homes heating guide has the rest.
Who should do the sizing, and why it matters
A proper sizing is a heat loss calculation: the room's volume, its walls, windows and floor, the insulation, and how cold it gets where you live. Rinnai NZ says the right choice comes from an assessment and a discussion with an accredited heat pump professional, and Mitsubishi Electric NZ's own sizing tool works from your region's average lowest winter temperature.
It matters after the install too. Black Diamond Technologies, which issues Mitsubishi Electric's NZ warranty, says the warranty covers the equipment only, not design or heat load calculations, and it excludes systems sized or installed incorrectly. A sizing mistake sits with whoever did the sizing, so ask how they reached their number.
For one room, a heat pump installation specialist should measure up and ask about your insulation before quoting. Several rooms off one outdoor unit, or a ducted system, need more careful sums: see multi-room heat pumps and ducted heat pumps.
What you can work out, and what needs an installer
You can
- Measure the room and note the ceiling height.
- List what's open to it, like a hallway or kitchen.
- Find out what insulation is in the ceiling and under the floor.
- Run your measurements through the size calculator.
- Compare the heating output at 2 degrees on every unit you're offered.
Leave to a licensed installer
- A heat loss calculation for the room.
- Choosing a model that holds its output in your winters.
- Checking the outdoor spot won't hold back its cold-weather output.
- Putting the sizing, and how they reached it, in writing.
Questions to ask about size
- How did you work out the kW for this room?
- What does this unit put out at 2 degrees outside, and is that enough for our coldest nights?
- Did you count the open-plan areas and the big windows?
- Would a model that holds its output better in frost suit us?
What we'd do
Our take
We'd measure the room, run BRANZ's rule of thumb, and treat the answer as the least we'd accept. Where frosts are common, we'd choose a unit whose 2 degree output still covers that figure, even if it means going up a size. Then we'd ask the installer to show their sizing in writing before we signed anything.