What size boiler do I need?

How do you work out what size boiler you need?

Boiler size means heat output in kilowatts, not the dimensions of the box on the wall. Approved Document L, Volume 1, the Building Regulations guidance for dwellings in England, says the specification of a space heating system should be based on an appropriate heat loss calculation for the dwelling. It adds a blunt instruction: “Systems should not be significantly oversized.”

That leaves you two jobs before an installer arrives. Count your radiators, bathrooms and showers, so you can describe your demand accurately when you ask for quotes. Then find out what your incoming water main can deliver, because on a combi that caps your hot water whichever boiler you buy.

The rest is the installer's work. A survey turns rooms, windows, insulation and radiators into a heat loss figure in kilowatts, and that number is the specification. It is why two houses that look alike can need different boilers.

The short version

• Boiler size is heat output in kilowatts, not how big the unit is.
• Approved Document L, Volume 1 says the specification should be based on a heat loss calculation, and that systems should not be significantly oversized.
• Radiator and bathroom counts describe your demand. They do not replace the calculation.
• A gas combination boiler should be selected to modulate down to the typical heating load of the dwelling.
• On a combi, your mains flow rate caps hot water however big the boiler is.
• Work on a gas boiler must be done by a competent person, and any business or self-employed engineer doing it must be on the Gas Safe Register.

Why radiator and bathroom counts are only a starting point

Almost every sizing table online runs on two numbers: your radiator count and your bathroom count. They correlate loosely with what matters, which is how much heat the house loses and how much hot water it needs at once. Neither is a measurement.

Bathrooms are the more useful of the two, because they describe how much hot water the house might want at the same time. That is a question about boiler type before it is one about kilowatts, and our combi vs system boiler guide covers the type decision.

Radiator counts are weaker. Two homes with ten radiators each can lose very different amounts of heat depending on insulation, glazing and exposure, and a count sees none of it. Treat any online sizing table as a conversation starter, then ask the installer to show you the calculation behind the output they quoted.

Why an oversized combi is a real problem

Buying bigger is the instinct, and on a combi it usually backfires. A boiler far larger than the home needs cannot turn its burner down far enough on a mild day. It heats the water quickly, hits temperature, shuts off, cools, and fires again. Engineers call that short cycling, and it costs you in fuel and in wear on the ignition components, fan and pump.

The guidance sets a rule for combis. Approved Document L, Volume 1 says: “Where a gas combination boiler is used, the boiler type should be selected to modulate down to the typical heating load of the dwelling.” Modulation is how far a boiler runs below its maximum. The wider that turn-down range, the better it copes when the house asks for very little.

Your mains flow rate caps what any combi can deliver

This is the point most people miss, and it decides whether you like your shower. A combi stores nothing, so every drop of hot water comes off your cold main the moment you open the tap. Flow rate is simply how many litres of hot water reach the tap in a minute.

The limit is not the boiler, it is the pipe in the ground. Our own combi boiler guide puts the mains flow rate to most UK homes at 10 to 15 litres a minute, and a boiler rated at 15 litres a minute sitting on a 12 litre main still only delivers 12. Paying extra for a higher-output combi to fix a weak shower is money in the wrong place.

You can measure it roughly at the kitchen cold tap yourself, and the FAQ below explains how, though a survey will measure flow and pressure properly. If yours is low, the honest options are to improve the supply or take a stored hot water system, where a cylinder feeds several outlets at once.

What decides the size, and who works it out

What the regulations expect when your boiler is replaced

Three rules in Approved Document L, Volume 1 change what should appear on your quote.

Flow temperature. Where a wet heating system is newly installed or fully replaced, all parts should be sized to work “at a maximum flow temperature of 55°C or lower”. Lower flow temperatures need more radiator surface, so this sizes your radiators as well as your boiler.

The extra efficiency measure. A gas-fired combination boiler going into an existing home needs at least one further energy efficiency measure appropriate to the system: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation.

Controls and efficiency. Thermostatic room controls are expected when a boiler is replaced. The minimum efficiency for wet heating in existing homes is 92% as defined in ErP, with a lower SEDBUK figure allowed only in the exceptional circumstances Approved Document L sets out.

None of this is a job you can take on yourself. Under the Gas Safety (Installation and Use) Regulations 1998 nobody may work on a gas fitting unless competent to do so, and any business or self-employed engineer doing that work must belong to a class approved by the Health and Safety Executive, in practice the Gas Safe Register.

Smell gas? Do not touch electrical switches, do not smoke, put out naked flames, open the doors and windows, and call the National Gas Emergency Service on 0800 111 999.

Questions to ask before you accept a quote

1. What heat loss figure did you calculate, and what did you base it on?

2. What is the boiler's minimum output, not just its maximum? A wide turn-down range is what stops a generously sized boiler short cycling.

3. What flow rate did you measure at the kitchen tap? On a combi, that decides your showers.

4. Which of the four extra efficiency measures is included, and why that one?

5. Are the radiators sized for the flow temperature you are designing to?

6. Who is the Gas Safe registered engineer doing the work, and what is the registration number?

One last job while the boiler is new. Register the manufacturer's guarantee straight away, because makers usually require registration within a set window and an annual service to keep it valid. When that guarantee ends, boiler and central heating cover pays parts and labour up to your cover limit: £500 in a 12-month period if the boiler is under 7 years old, or £200 if it is over 7. Manufacturer defects, faults we have already identified on your equipment and damage from negligence, misuse or third party interference are not covered, and a £95 call-out fee applies in the situations the terms set out.

Boiler sizing questions, answered

Working out a new boiler? Talk it through with us.

Tell us about your home and what your boiler is doing, and we will talk you through the options. Or call 0333 772 6247. Once the new boiler is in and its guarantee has run its course, the boiler and central heating module pays parts and labour up to £500 in a 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests, and cover still applies during those 30 days. We aim to get an engineer out to you within two working days, and cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Using a module starts a 12-month agreement period from the first time you use a service, there is a 14-day cooling-off period that ends once a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan is a service plan, not insurance, from UK Boiler Company Ltd, trading since 2014.

Related reading

• Ask us about a new boiler
• Combi vs system boiler: which type do you need?
• What is a combi boiler?
• Thinking about a new boiler?
• Worcester Bosch boilers
• How to check your boiler warranty
• Boiler and central heating cover
• Approved Document L, Volume 1: Dwellings (gov.uk)
• Check an engineer on the Gas Safe Register