Ask a building owner if their air conditioning is too big and you’ll usually get a funny look. Too small, sure, people moan about that every August. But too big? Nobody has ever rung facilities to say the office is suspiciously well cooled.
Which is a shame, because oversizing is one of the more expensive mistakes a business can make with its premises, and almost nobody notices it. There’s no breakdown and no complaint from the third floor. There’s just a bill that’s a bit higher than it should be, every quarter, for as long as the kit stays on the roof.
Right now the bill is what everyone’s looking at. According to ONS figures, 72 per cent of UK businesses are worried about energy prices, and in hospitality it’s 91 per cent. Most of the obvious savings have already been squeezed out by now. Capacity is different. You sort it before the purchase order goes out, so it costs next to nothing.
Why “Bigger To Be Safe” backfires
Nobody sets out to oversize a system. It happens in small steps. The consultant pops a margin on for future growth, then the installer picks the next unit up because the exact size isn’t in the range, and after a hot summer the client asks for “a bit extra, just in case”.
Fair enough, each time. Add those three together, though, and you can easily end up with plant around a third bigger than the building needs on its worst day.
Short cycling eats efficiency
A unit that’s too big reaches temperature fast. Then it switches off. A few minutes later it starts again, and so on all afternoon. Compressors waste the most energy at start-up, and every restart puts a bit more wear on the contactors and valves.
Inverter systems cope with this better than the old fixed-speed units did. Not perfectly, mind. Every inverter has a minimum output, and on a mild day the load can sit below it, at which point the thing cycles anyway.
Humidity gets worse, not better
Here’s the bit that surprises people. To take moisture out of the air, a cooling coil has to run for a while. If the system blasts for six minutes and stops, the room ends up cold and slightly damp, and staff complain it feels stuffy.
So someone turns the thermostat down. Then down again.
The costs nobody itemises
Obviously a bigger unit costs more to buy. The extras are harder to spot. You may need a larger electrical supply, which on a half-hourly meter can mean paying more in capacity charges whether you use it or not. A bigger refrigerant charge can tip a system into more frequent F-gas leak checks.
You won’t find any of that next to the unit price on the quote. It turns up later, spread across other budget lines.
What the evidence says about UK buildings
If this sounds like a niche worry, the research says otherwise. A two-year study by BRE for the Department of Energy and Climate Change put cooling in air-conditioned buildings at possibly a tenth of all UK electricity use.
It also found that a lot of systems keep running in the evenings and at weekends, when the offices are empty. And, more to the point for this article, it reported that installed cooling capacity often sits well above the peak demand those buildings actually see.
Government research into low-carbon heating and cooling in non-domestic buildings found roughly two-thirds of them still heat with gas. As those boilers get swapped for heat pumps, getting the size wrong starts to hurt more, because heat pumps cost a lot more per kilowatt and an oversized one spends much of the year ticking over at low output.
A five-step sizing check before you sign off a quote
You don’t have to be an engineer for any of this. You just need to ask a few awkward questions before you sign.
1. Start from the building, not the old equipment
“Like for like” is how most oversizing gets passed down. The unit being ripped out was sized for a building that might not exist anymore. Maybe the single glazing went years ago. Maybe the fluorescent tubes are now LEDs, which run much cooler. Or the floor that used to seat 120 people now has 60 on a good Friday because of hybrid working.
Daikin and Mitsubishi Electric both have selection software, though it’s really aimed at installers. For a quick sense-check, browser tools such as the HVAC Load Calculator from Dalton Mills let a facilities manager put in floor area, ceiling height, windows, insulation and headcount and get a figure back in a couple of minutes.
The Dalton Mills calculator uses square feet, BTU, and US climate zones, so you’ll be doing some converting and picking a zone that roughly matches London, which sits somewhere between cool and mixed. To be fair to Dalton Mills, the tool itself says the result is a planning estimate and not a full load calculation. I’d treat any free estimator that way.
2. Count heat gains honestly
Everything in a building adds heat. A person sitting at a desk gives off roughly 100 to 130 watts. Laptops produce far less than the old desktop towers. The real hotspots tend to be the comms cupboard, the kitchen, and that glass meeting room facing south.
If the design assumes every desk is full every day, ask if that’s still how the building works. It might not be.
3. Apply one safety margin, once
Pick one margin and write it down. Then make sure nobody adds a second one further along. For comfort cooling, 10 to 15 per cent is fairly typical, and a designer might push for more in an unusual building. That’s fine. What matters is that someone chose it on purpose.
4. Size for part load, not just the hottest afternoon
Most commercial buildings run well below peak for the bulk of the year. A few smaller modules, a VRF system that turns down well, or staged chillers will usually do better over twelve months than one big unit sized for a heatwave.
Controls make a big difference here too. Occupancy-based scheduling and building automation, which Coruzant covers in its look at HVAC and sustainable building design, stop even a well-sized system from cooling empty rooms at 9 pm.
5. Get the calculation in writing
For anything bigger than a small split unit, ask to see the load calculation behind the quote. In the UK that’s normally the CIBSE Guide A method, or for bigger and more complicated buildings, dynamic thermal modelling in something like IES VE. If you get a capacity figure and no workings at all, push back.
Your quick estimate from Dalton Mills or a manufacturer’s app is there to tell you whether a quote is in the right area. It’s not a replacement for the design calc.
What not to do
Please don’t swing too far the other way.
An undersized system is miserable to live with. It runs flat out on hot days, still can’t hold temperature, and tends to fail early. You want a match. Not the smallest unit that might scrape through.
And don’t stop once the size is agreed. A correctly sized system can still waste plenty of energy if it’s poorly commissioned, with airflow out of balance or controls doing things nobody expected. That ties into the wider point made in this guide to preparing business properties for new tenants, where HVAC commissioning comes up as one of the checks worth paying for.
Conclusion
In my experience, oversized HVAC is rarely someone’s deliberate choice. It’s a handful of cautious decisions made by different people at different times, and nobody adds them up. What you’re left with costs more upfront, costs more to run, needs more maintenance, and often makes the building less comfortable into the bargain.
Fixing it is mostly a matter of being a bit stubborn. Look at the building as it is now. Be realistic about heat gains, agree on a margin, think about part load, and don’t accept a number without the calculation behind it. Once the new kit is in, regular energy evaluations give you something to measure it against.
Energy is still one of the biggest overheads most London businesses carry. A few hours spent questioning a quote now could keep paying off, summer and winter, for the next ten years or so.
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