Getting the size of your heat pump right is the single most important decision in the whole job. Get it right and you’ll have a unit that heats your space comfortably and runs efficiently for years. Get it wrong and you’ll either be cold or paying too much. Here’s how sizing actually works, in plain English.
Why size matters so much
Heat pumps are rated by their heating output, measured in kilowatts (kW). It’s tempting to think bigger is always better, but that’s not how it works.
Too small and the heat pump runs flat out on cold days and never quite warms the room. It wears out faster and costs more to run because it’s always working at its limit.
Too big and the heat pump heats the room quickly, then switches off, then comes back on again, over and over. This short-cycling is inefficient, wears the unit, and doesn’t dehumidify or distribute heat as well.
Just right and the heat pump runs steadily in its efficient range, holds a comfortable temperature, and lasts. That’s the goal.
What we look at when sizing
There’s no single magic number, because every room is different. When we come out to quote, here’s what we’re assessing:
Room size. Not just the floor area, but the volume, so the height of your ceilings matters too. A room with a high stud needs more output than the floor area alone suggests.
Insulation. A well-insulated home holds heat, so it needs a smaller unit. An older, draughty home loses heat and needs more grunt to keep up.
Windows and sun. Big windows lose heat at night but gain it during the day. Which way the room faces makes a real difference in the Manawatū.
Layout. Is it a closed-off bedroom or an open-plan lounge, kitchen and dining that flows together? Open-plan spaces need more output to heat the whole area.
What you want from it. A bedroom you want gently warm is different from a living area you want toasty all evening. We size to how you’ll actually use the space.
A rough guide (but only a guide)
As a very rough starting point, people often talk about needing somewhere around 0.1 kW of heating per square metre of floor for an average, reasonably insulated room. So a 40 square metre open-plan living area might land somewhere in the 5 to 6 kW range.
But please treat that as a ballpark, not a recommendation. We’ve seen plenty of homes where the right answer is well above or below that rule of thumb because of the ceiling height, the glazing or the insulation. That’s exactly why we measure properly rather than guessing off the floor plan.
One size rarely fits the whole house
If you’re heating more than one area, the answer often isn’t one big heat pump. A single large unit in the lounge won’t magically warm the bedrooms down the hall. Depending on your home, the right solution might be a couple of well-placed units, or a ducted system that feeds several rooms. We’ll talk you through what makes sense for your place and your budget.
Get it sized properly
The good news is you don’t have to work any of this out yourself. That’s our job. When we quote, we measure your rooms, take in the insulation, windows and layout, and recommend a unit that’s the right size for your home, not just the one that’s on special.
If you’d like a heat pump that’s sized properly the first time, book a free on-site quote and we’ll sort it out. You can read more about our heat pump installation service too.