Last updated September 2026
Most people choose a heat pump the way they choose a TV, by brand and by price. Then they get one that runs constantly and never quite warms the room, or one that blasts, clicks off, and leaves the place cold and clammy twenty minutes later. Both of those are usually the same problem, and it is not the brand.
★View the Best Heat Pump Installers in Nelson→Size is the decision, and it is a calculation
The output of a heat pump is measured in kilowatts, and the right number for your room is worked out rather than guessed. EECA’s guidance puts it plainly: too small and it will struggle to heat the room, too big and you will pay more than you need to.
The reason it has to be calculated is that the answer depends on more than floor area. Two rooms of identical size in the same street can need meaningfully different units.
Too big is a real problem, not a safe bet
The instinct is to round up, on the basis that extra capacity cannot hurt. It can.
An oversized unit reaches the set temperature quickly, shuts down, and starts again a few minutes later. That cycling is hard on the equipment and it is the least efficient way for a heat pump to run, because the efficient part is the long steady middle of a cycle, not the start of one.
It also does a worse job of the thing you actually wanted. Short bursts do not run long enough to pull moisture out of the air, so an oversized unit can leave a room that is technically at temperature and still feels damp and cold. In summer the same effect gives you a chilly, clammy room rather than a comfortable one.
And you pay for the capacity twice, once in the purchase and again in running a unit that never settles.
Too small is the more obvious failure
An undersized unit runs flat out and still does not get there on the coldest days, which are precisely the days you bought it for.
It will look fine in April. The problem shows up in July, by which time it is an expensive thing to discover.
What actually changes the number
Floor area is the starting point, not the answer. The things that move it:
- Insulation, in the ceiling, walls and under the floor. This is the biggest single factor and the one most worth fixing first.
- Windows, how many, how large, and whether they are single or double glazed. Glass is where heat leaves.
- Which way the room faces, and how much sun it gets through the day.
- Ceiling height, because you are heating a volume rather than a floor.
- Whether the space is open plan or closes off, and whether you expect the heat to reach other rooms.
- Draughts, gaps and an unsealed fireplace, all of which quietly undo the work.
This is why Gen Less recommends getting someone to come to the house and look, rather than working it out from a floor plan or over the phone.
The rated output is not what you get on a cold morning
This one catches people out and it is worth understanding before you compare two units on paper.
A heat pump moves heat from outside to inside. The colder it is outside, the less heat there is to move, so output falls as the outdoor temperature drops. The headline capacity on the box is measured under test conditions, not on a frosty morning.
Units also pause periodically to defrost the outdoor coil, which is normal and looks alarming the first time you see steam coming off it.
The practical version: a unit chosen on its rated number can be genuinely undersized on the mornings you care about. Ask what it is rated to deliver at a low outdoor temperature, not just at the test figure.
What a Nelson house does to the calculation
Nelson gets a lot of clear, still weather, and the same conditions that make the days pleasant make the nights cold. Clear skies mean heat radiates away overnight and the mornings are frosty even after a mild day. That is the exact condition where output drops, so it is worth sizing for the frost rather than the average.
The housing stock matters too. Plenty of Nelson homes are older villas and bungalows with high ceilings, large single-glazed windows and limited wall insulation, all of which push the number up.
Summer is a genuine factor here rather than an afterthought. Nelson gets properly warm, so the same unit is doing real work in both directions, and the cooling calculation is not always the same as the heating one.
If you are near the water, whether that is Tāhunanui, the Haven or Ruby Bay, salt in the air is hard on an outdoor unit over time. Coastal-specification units and coatings exist for exactly this, and where you site the unit on the property makes a difference.
One unit, or several
The type follows the layout, not the other way round.
A single high wall unit suits one open living area. It will not push heat down a hallway into bedrooms, whatever anyone tells you. A floor console sits low and suits rooms where warmth needs to sit at floor level, or where a high wall unit has nowhere sensible to go. Ducted systems heat the whole house from a central unit and are far easier to fit during a renovation than after one. Multi-split runs several indoor heads off one outdoor unit, which suits a house with a few separate rooms to cover and only one good spot outside.
Hydronic systems are a different approach again, heating water rather than air.
Working out which shape fits is part of what an in-home assessment is for.
What to ask when someone quotes you
- What kW output are you specifying, and how did you arrive at it?
- What does that unit deliver at a low outdoor temperature, not just its rated figure?
- Is the quote for a single room or for the house, and what will realistically reach the other rooms?
- Where will the outdoor unit sit, and is that a sensible spot for noise and for salt air?
- What is the warranty, and does it require annual servicing to stay valid?
Anyone specifying a unit properly will have answers, because they will have done the calculation rather than picked from a chart.
Where Pete Pumps fits
Pete Pumps is run by Pete Cameron, who started the business in 2017 after years at Nelson Heatpumps and Sturrock and Greenwood. They cover Nelson and Tasman, domestic and commercial, and specialise in Daikin and Mitsubishi.
On the subject of this article, their own approach is the in-home assessment: they visit, discuss what you need and calculate the space to be heated or cooled before recommending anything, and that assessment is free. Their range covers most of the options above, from split and multi-split systems through ducted and central heating to floor consoles, ceiling units, Altherma hydronic heating, air purifiers and app controllers, domestically and for offices, shops and other commercial spaces. The refrigerants in the systems they install have a lower environmental impact than the ones they replaced.
Whoever you get in, the question to hold them to is the first one on that list. If nobody has calculated the number, nobody has chosen the heat pump yet.
