Most air conditioning advice online tells you to “book a survey” and stops there, without explaining what a survey checks or why any of it matters. We’re F-Gas and Gas Safe registered engineers, and we’ve spent over 8 years installing and repairing air conditioning across London. This guide skips the vague advice. Instead, it walks through the real numbers, standards, and calculations we use on-site, so you can understand your own quote rather than just take it on faith.
Brands tend to dominate the conversation whenever someone starts looking into air conditioning. Bosch, Daikin, Mitsubishi Electric, whichever name came up in a search or a recommendation from a friend. The detail that actually shapes how comfortable the room feels afterward rarely comes up at all: matching the unit’s output to the specific space it’s cooling.
Get that wrong, and the equipment itself becomes almost irrelevant. A room stays stubbornly warm on a genuinely hot afternoon, or it cools fast and then sits cold and slightly damp rather than pleasant. Working out the right output is the very first thing we calculate on any Bosch AC Installation Services job, ahead of any conversation about brand or budget. Here’s the reasoning behind that number.
Measuring the Room Comes First
Grab the length and width of the room in metres and multiply them. A space measuring 5m by 4m comes to 20 square metres.
From there, a workable starting point for an average UK room, standard insulation, ceiling around 2.4m, sits somewhere between 0.1 and 0.15 kilowatts of cooling for every square metre of floor space. Run that maths on a 20 square metre room, and you land in the 2.0 to 3.0 kW bracket before anything else gets factored in.
That’s a fairly loose bracket deliberately. What tightens it up is everything covered below.
A Rough Starting Point by Room
| Room type | Typical floor area | Starting capacity |
| Small bedroom | 10–15 m² | 2.0–2.5 kW |
| Double bedroom | 15–20 m² | 2.5–3.0 kW |
| Standard living room | 20–25 m² | 3.0–3.5 kW |
| Open-plan kitchen/living | 30–40 m² | 5.0–7.0 kW |
| Large open-plan or loft conversion | 40 m²+ | 7.0 kW and above |
Treat this table as a conversation opener rather than a final answer. Every property has quirked a generic table can’t capture, and those quirks are usually what separates a system that performs well from one that never quite feels right.
Five Reasons Your Number Might Climb Higher
A room facing south with plenty of glass pulls in genuine solar heat that a floor-area sum won’t account for on its own. Nudge your estimate up by around a tenth for a south-facing room, and further still for floor-to-ceiling glazing or a roof lantern that floods the space with light.
Ceiling height is another factor that quietly changes the calculation. Standard tables assume 2.4 metres, but plenty of London conversions, particularly Victorian and Edwardian ones, run taller than that. Where the ceiling is noticeably higher, work from room volume rather than floor space alone.
Kitchens run warmer than their floor area suggests once ovens and hobs are factored in, so it’s sensible to add extra capacity on top of the baseline figure there. Rooms that see heavier use, a home office crowded with monitors, or a space that regularly holds more people than most, need a bit more headroom too. And solid brick walls without cavity insulation, common across older London terraces, hold onto heat in a way modern cavity walls don’t.
Three Reasons It Might Drop
A room with limited direct sun, typically north-facing, can often get slightly less than the baseline figure suggests. Flats built more recently, with double or triple glazing throughout, tend to sit toward the lower end of the bracket. And a room with relatively little glazing compared to its floor area simply isn’t absorbing as much solar heat in the first place.
The Mistake That Costs More Than People Expect
There’s a common assumption that a bigger unit is a cautious choice. It usually isn’t. A system that’s too large for the room cools quickly, then switches off before it’s pulled enough moisture out of the air. Rather than feeling comfortably cool, the room ends up cold with a faint clamminess to it. The unit then starts and stops far more often than it should, a pattern called short cycling.
That pattern does more than waste electricity. Each restart adds wear to the compressor, easily the priciest single component to replace if it fails. An oversized system tends to cost more to buy in the first place, run less efficiently day to day, and wear out earlier than a correctly matched one. If a system you already own is showing signs of this, it’s worth booking air conditioning repair services in London before the compressor takes the brunt of it. Between the two extremes, a slightly undersized unit that simply runs a little longer on the hottest days does far less last harm than an oversized one stuck in a constant on-off cycle.
Getting the Size Right Matters More Than the Refrigerant Debate
Once the output figure is settled, the refrigerant inside the unit determines how efficiently it reaches that target. Current Bosch Climate systems, along with most modern Daikin and Mitsubishi Electric ranges, run on R32. Compared with the older R410A gas still found in some ageing installations, R32 carries a much lower Global Warming Potential, roughly 675 against R410A’s approximate 2,088, and transfers heat more efficiently in the process. A correctly sized R32 system reaches the same temperature using noticeably less electricity than an equivalent R410A setup would.
None of that efficiency matters much, though, if the sizing itself is off. A well-matched system on an older refrigerant will often outperform an oversized one running the newest gas available.
What a Proper Quote Should Be Able to Answer
Before signing anything, a homeowner should be able to get straight answers to the following:
- What output has been calculated for this specific room, and what’s the reasoning behind it?
- Has sun exposure, ceiling height, and wall construction been factored in, or is the number based on floor area alone?
- Which refrigerant is the proposed system built around?
- Can the installer show their F-Gas certificate on request, without hesitation?
- Where more than one room is involved, has each been sized on its own rather than average?
A quote for a system size handed over before anyone’s seen the room is a shortcut worth questioning rather than accepting face value.
Get the Exact Figure, not a Table Estimate
A table gets you into the right general range. A proper site survey, one that walks through your room’s glazing, insulation, and layout, gets you the precise figure the system needs to run well. That calculation comes first on every job we quote, whether it’s a single-room Bosch AC Installation Services fit or a full multi-room Air Conditioning Installation Services setup across the property.
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Getting the size right adds nothing to the cost of your quote. Getting it wrong is the single biggest reason a new system falls short of expectations.
Questions We Hear Often
No. Floor area, sun exposure, glazing, and ceiling height differ from room to room, so each space needs its own working-out. A multi-split system should have every indoor unit matched to its own room rather than one blanket figure spread across the property.
Not really. That instinct, rounding up “just to be safe,” is precisely how oversizing happens in practice. A proper room-by-room calculation gets far closer to the right figure than an upward guess ever will.
Most systems on the market now, Bosch Climate units among them, are reversible heat pumps handling both jobs. Cold-weather heating performance is working out separately from cooling capacity, so it’s worth confirming both during a survey instead of assuming one figure covers everything.
It’s one of the more common explanations, alongside a handful of others. Where a room won’t reach setpoint, cycles frequently, or bills have crept up without explanation, a diagnostic visit through our air conditioning repair services in London will pin down whether sizing, refrigerant charge, or something else entirely is behind it.
