In short
For a combi, size is set mainly by the hot water flow you need, which depends on your bathrooms and your mains supply. For heating, the right output comes from the heat loss of your home, not the number of bedrooms. An oversized boiler costs more and can switch on and off too often.
What does boiler size actually mean?
Boiler size means output, measured in kilowatts (kW). It is not the physical size of the case. A higher output boiler can heat more water more quickly, but only if your home can use it. Fitting more output than you need does not make the house warmer; it just means the boiler spends more time cycling on and off.
How do I size a combi boiler?
A combi heats water as it flows through, so its hot water output decides how many litres per minute of hot water you get. As a rough worked example, heating water by 35°C (from a cold mains around 10°C to a shower around 45°C) takes about 2.4 kW for every litre per minute. That means, before losses:
- about 24 kW gives roughly 10 litres a minute;
- about 30 kW gives roughly 12 litres a minute;
- about 35 kW gives roughly 14 litres a minute.
Check the manufacturer's stated flow rate for the exact model. And remember: a combi can never deliver more water than your mains supplies. If your incoming flow is low, a larger combi will not help, and a system boiler with a cylinder may suit you better.
How do I size a system or regular boiler?
For a boiler that heats a cylinder, the output is driven by your home's heating demand plus a margin for reheating the cylinder. The proper way to work out heating demand is a room-by-room heat loss calculation, which looks at room sizes, walls, windows, insulation and the temperatures you want.
Why is bigger not better?
Approved Document L says a combi boiler should be chosen so it can modulate down to the typical heating load of the home. An oversized boiler cannot turn down far enough on mild days, so it fires, overshoots and stops, over and over. That wastes gas and adds wear. It also costs more to buy.
What should my engineer check before recommending a size?
- Your incoming mains flow rate and pressure, measured at the property.
- How many bathrooms you have and whether they are used at the same time.
- The heat loss of the home, or at least the radiator sizes and any extensions.
- Whether you plan changes, such as a loft conversion or an extra shower room.
- The size of the existing gas pipe, because a larger boiler may need a bigger supply.
What CombiCert would check
We count bathrooms and showers, note the number of radiators, check the existing pipework and flue, and ask how you use hot water. That decides whether a smaller or larger output boiler is the sensible match, rather than guessing from a website chart. The quote is subject to a site inspection.
When to call a Gas Safe engineer
Sizing is part of the survey for any new boiler. If a quote gives you an output without explaining how it was worked out, ask. A Gas Safe registered engineer should also check the gas supply can deliver enough gas for the boiler's maximum rate; an undersized gas pipe is a common reason a bigger boiler cannot simply be swapped in. Our guide to choosing a boiler and new boiler questions cover the next steps.
Need a hand? CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
New boilers from £1,750 inc VAT for a basic like-for-like installation, subject to a site inspection. Most quotes are higher; filters, flushes, thermostats and moving the boiler are paid extras.
See New boiler installation in Telford, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
We cover Telford and nearby, including Wellington, Madeley, Dawley, Oakengates, Shifnal, Newport, Ironbridge and Shrewsbury.
Sources
Frequently asked questions
What size combi boiler do I need for a three-bedroom house?
Bedrooms are a poor guide. What matters for a combi is how many bathrooms you have, whether they are used at once and your mains flow rate. A three-bedroom house with one bathroom may suit a mid-range combi, while two bathrooms used together may suit a system boiler better. Your engineer should measure first.
Can a bigger combi boiler give me a stronger shower?
Only if your mains water supply can deliver more. A combi heats whatever flow comes in, so if the incoming flow is limited, a bigger boiler cannot add to it. Your engineer should measure your flow rate before recommending a larger combi. If it is low, a cylinder system may be the better answer.
Is an oversized boiler a problem?
Yes. An oversized boiler cannot turn down far enough on mild days, so it fires, overshoots and switches off repeatedly. That wastes gas, adds wear and can shorten its life. Approved Document L says a combi should be able to modulate down to the typical heating load of the home.
Do I need a heat loss calculation for a boiler swap?
Not always for a like-for-like replacement, but it is the proper way to size heating output, and Approved Document L expects one for a completely new heating system. For a straight swap, your engineer should at least look at radiator sizes, extensions and insulation rather than simply copying the old boiler's output.
This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.