The single decision that determines whether your system is quiet, efficient and reliable never appears on the quote. Here’s why it matters.
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When two installers quote you for the same room, the units can look almost identical on paper. Same brand, similar price, comparable warranty. So most people choose on price and move on.
The problem is that the single decision that determines whether your system is quiet, efficient and reliable — or noisy, expensive and short-lived — never appears on the quote. It's the sizing calculation. And in a surprising number of installations across the UK, that calculation is never properly done at all.
At ArcticNord we design every air conditioning and air-to-air heat pump system around a measured cooling and heating load, worked out room by room. It takes longer than reading a capacity off a chart based on floor area, and it's the reason our systems perform in year ten the way they did on day one. This is what that actually means, and why it matters to you.
Every air conditioning system has a rated output, measured in kilowatts (kW). A small wall unit might be 2.5 kW; a larger room might need 5 kW or more. That figure describes how much heat the system can move out of a room in cooling mode — and, because an air-to-air heat pump runs in reverse, how much heat it can move into a room in winter.
Sizing is the process of matching that output to the actual amount of heat your specific room gains on a hot day and loses on a cold one. Get the match right and the system runs gently, holds a steady temperature and sips electricity. Get it wrong in either direction and you pay for it every single day you use the system.
The lazy method — still widely used — is to take the floor area of a room, multiply by a fixed number, and pick the nearest unit. It's quick, it's free, and it ignores almost everything that actually determines how much heat a room gains: the windows, the direction they face, the insulation, the ceiling height, how many people use the room, and what electronics are running in it. Two rooms with identical floor areas can have cooling loads that differ by more than double. A formula based on floor area alone cannot see that difference. A proper survey can.
There's a common assumption that if in doubt, you should fit a bigger unit — better too much power than too little. For air conditioning, that instinct is wrong, and it's worth understanding why.
Modern systems cool a room quickly, then need to keep it at a steady temperature. An oversized unit blasts the room down to the set temperature in a few minutes, then switches off. Minutes later the temperature drifts back up, so it switches on again. This constant stopping and starting is called short cycling, and it causes a cascade of problems:
Oversizing doesn't buy you a safety margin. It buys you a system that's worse at the one job it was installed to do.
The opposite error is easier to understand but equally costly. An undersized system simply can't move enough heat to keep up on the days you most need it.
On the hottest afternoon of the year, an undersized unit runs flat out and still can't reach the temperature you've set. In winter — and this matters enormously for air-to-air heat pumps — it can't deliver enough warmth to heat the space, leaving you reaching for a plug-in heater and wondering why you invested in the system at all.
Running permanently at maximum output has its own consequences: higher energy use, more strain on components, and a system that never gets the chance to settle into the efficient, gentle running it was designed for. You've paid for a comfort solution that leaves you uncomfortable at the extremes.
Here's a subtlety that separates a considered design from a quick quote. An air-to-air heat pump is a 4-in-1 system — it cools, heats, dehumidifies and filters the air. That means it has to be sized for two loads: the cooling load in summer and the heating load in winter.
These are rarely the same number, and heat pumps behave differently in the cold. As the outside temperature drops, a heat pump's heating output falls too — the unit that comfortably delivers its rated output at 7°C outside will deliver noticeably less at -2°C, which is precisely when you need the heat most. A design that only checks the summer cooling figure can leave you short of heat in a January cold snap.
Sizing an air-to-air heat pump properly means calculating the heating demand of the room, checking the manufacturer's output at low outdoor temperatures (not just the headline rating), and confirming the system can hold your target temperature on the coldest realistic day. It's more work. It's also the difference between a heat pump that keeps you warm and one that disappoints you every winter.
You may have read about the government's £7,500 Boiler Upgrade Scheme (BUS) grant for heat pumps. It's worth being straight with you: that grant applies to air-to-water and ground-source heat pumps that heat your radiators and hot water — not to air-to-air systems like the ones we specialise in. Any installer implying otherwise about an air-to-air system isn't giving you the full picture.
We'd rather tell you that up front. Where a whole-home water-based heat pump and BUS grant is genuinely the better route for you, we'll say so — that's exactly the kind of system our sister company Qualis Energy designs. Our job is to recommend the right system for your property, not the one that's easiest to sell.
When we survey a property, we're building a heat gain and heat loss picture of each room, not reading a number off a floor plan. The factors we measure and account for include:
From that we calculate the true cooling load and heating load for each room, then select equipment — single-split for one room, multi-split for several, or ducted where appropriate — that matches those loads and modulates smoothly across them. Modern inverter-driven systems can vary their output rather than simply switching on and off, but only if they've been correctly sized to the load band in the first place. Even the best inverter compressor can't rescue a fundamentally wrong-sized system.
A correctly sized system still has to be installed and set up correctly to deliver its designed performance. This is where a lot of the real-world difference between a good and a poor installation lives, and it's invisible once the covers are on:
Every ArcticNord installation is carried out by qualified engineers and handled under F-Gas regulations, which govern the safe handling of refrigerants. Sizing gets the right equipment onto the wall; commissioning is what makes it perform.
A cheap quote and an expensive quote can describe the same box on the same wall. What you can't see on the page is whether anyone worked out what your room actually needs — and that single calculation determines your comfort, your running costs and how long the system lasts.
We're not the installer who reads a number off a chart. We calculate the cooling and heating load for every room we quote, we're honest about what a system can and can't do, and we commission every install properly so it delivers what we designed it to. That's what precision sizing means, and it's why our systems keep performing long after the sale.
Want to see the difference a proper calculation makes? Get a free on-site assessment and quote — or call us on 0333 090 7546.
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