Sizing is the single most important decision in the whole job, and it is the one most often guessed at. Get it right and you barely think about the system again. Get it wrong and you live with the consequences every summer, in comfort and on your electricity bill. Here is what actually decides the answer for your room.
In this guide
The short answer, and why you will not like it
Nobody can tell you what size unit your room needs without seeing your room. That is not us holding out for a visit. It is that floor area is one of about a dozen things that decide how much heat has to be moved out of a room, and on its own it is not even the most important one.
Two bedrooms of identical size, in identical houses on the same street in Norton, can genuinely need different capacities. One faces north with a small window. One faces south west, has a big window and sits under the roof. Same square metres, different job.
What size actually means
When an installer talks about the size of a unit, they are not talking about the physical box. They mean its capacity, measured in kilowatts, which is how much heat it can move per hour.
That matters, because kilowatts get used for two different things and people mix them up. A unit rated to deliver a certain output of cooling does not draw anything like that in electricity. It draws roughly a third, because it moves heat rather than creating it. Our guide on what air conditioning costs to run covers that.
What a proper calculation looks at
A heat load calculation is not mysterious. It adds up everything that puts heat into the room and everything that lets it escape. These are the inputs that move the number.
- The volume of the room, not just the floor area. High ceilings in an older Middlesbrough terrace hold a lot more air than a modern room with the same footprint.
- Which way it faces. A south or west facing room takes the afternoon and evening sun, which is also when you most want to be in it.
- Glazing. How much glass there is, which way it faces, whether it is double glazed, and whether there is anything shading it.
- Insulation and construction. A solid wall terrace behaves very differently from a modern cavity wall house on an estate in Ingleby Barwick.
- What is above and below. A loft room sits under a hot roof. A room over a garage sits over a cold void. Both change the sum.
- People and equipment. Bodies give off heat. So do computers, televisions, ovens and lighting. A home office with two monitors is not an empty room.
- How the room connects to the rest of the house. An open plan kitchen diner with no door is a much larger space than it looks on a plan.
- How you actually use it. A bedroom you want cool overnight is a different problem from a lounge you want cool at four in the afternoon.
Why the rule of thumb online is wrong
Search this question and you will find a formula. So many watts per square metre, multiply by your floor area, done. It is everywhere, it is easy, and it is the reason a lot of systems disappoint their owners.
The problem is not that the formula is nonsense. It is that it is an average, built by taking a range of rooms and finding a middle. Your room is not the middle of anything, and the rule of thumb has no way of knowing that your whole west wall is glass.
Rules of thumb are also how box shifting works. If someone can quote you over the phone from a floor area, they have not calculated anything. They have picked the unit nearest to a number they looked up, and by the time that matters it is bolted to your wall.
A rule of thumb is an average. Your room is not an average. That gap is where the disappointment lives.
What happens if it is too small
An undersized system is the easier failure to spot. On a mild day it seems fine. On the day you actually need it, it runs flat out and never quite gets there.
- The room never reaches the temperature you set, so you set it lower, which changes nothing.
- The system runs constantly at full output, which is the most expensive way it can possibly run.
- The compressor never gets to throttle back, which is the whole efficiency advantage of an inverter system, so you pay for a feature you never use.
- Everything works harder for its whole life, and things that work harder tend not to last as long.
What happens if it is too big
This is the one people do not expect, because bigger sounds safer. It is not. An oversized system has its own problems, and they are harder to spot because the room does get cold.
- It cools the room fast, hits the target, and shuts down. Then the room warms, and it starts again. That is short cycling, and it is where the energy goes.
- It never runs long enough to dehumidify properly, so the room feels cold and clammy rather than fresh. This is a very common complaint and almost nobody connects it to sizing.
- All that stopping and starting is hard on a compressor. Start ups are the demanding part of the cycle.
- You paid more for the equipment, and you are paying more to run it.
Oversizing is the more common mistake
In our experience it is oversizing we see more often, and it usually comes from good intentions. Somebody was not confident in the calculation and went up a size to be safe.
It is worth understanding that there is no safety margin in going bigger. A correctly sized system is designed to run for long, gentle stretches at part load, and that is precisely where a modern inverter is at its most efficient and most comfortable. Going up a size does not buy you headroom. It buys you a system that spends its life doing the thing it is worst at.
Rooms that need more than the numbers suggest
Some rooms consistently need more capacity than their size implies, and it is worth knowing whether yours is one of them before you start comparing quotes.
- Conservatories and garden rooms. Glass on most sides, often a glass roof, and almost nothing to hold temperature. In a class of their own, and worth a guide of their own.
- Loft conversions. Directly under the roof, with heat rising into them from the whole house. Frequently the warmest room in a Teesside home by a distance.
- Open plan kitchen diners. Cooking is a heat source, glass is usually a feature, and the space rarely has a door on it.
- South and west facing lounges with big windows. The solar gain through the glass can easily exceed everything else in the calculation combined.
- Home offices. Small, often over a garage or in a converted box room, with equipment running all day and the door shut.
Multi room systems change the sum
If you are cooling more than one room from a single outdoor unit, sizing gets more interesting, and this is where a lot of cheap quotes come apart.
The outdoor unit does not need the total of every indoor unit added together, because you will almost never run every room at full output at once. But it cannot be sized for the smallest case either, or your bedrooms will fight each other on the hottest night of the year. That judgement needs a person who has asked how you live in the house.
There is a practical planning point too. If you might want a third room doing in two years, say so now. The outdoor unit is the awkward part to change later, and leaving sensible capacity in it at the start costs far less than replacing it. Our guide on multi-split systems covers the trade offs.
Getting yours sized properly
Our survey is free, it happens at your home rather than over the phone, and sizing is most of what it is for. We measure the room, look at the glazing and the aspect, ask how you use the space, and design from that. Then you get a fixed price in writing.
We are a family run business on North Ormsby Road in Middlesbrough and we cover roughly twenty miles of Teesside, including Stockton on Tees, Thornaby, Billingham, Norton, Yarm, Eaglescliffe, Ingleby Barwick, Redcar, Marske, Saltburn, Guisborough, Marton, Hartlepool, Peterlee and Darlington. If air conditioning is not the right answer for the problem you have, we will tell you that as well. Call us on 01642 680 873.
