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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing).
Published: March 11, 2024. Updated: March 11, 2024

What Size Boiler Do I Need?

As experts in heating and hot water, we can assure you that choosing the right boiler size is crucial for a warm, comfortable home and efficient energy usage. This guide, backed by my knowledge and experience, will help you make an informed decision.

The right-sized boiler is probably smaller than you think. UK boilers are routinely oversized due to a flawed assumption that bigger is better. This contributes to lower efficiencies and shorter lifespans. Unfortunately, some installers have not been trained to set up condensing boilers correctly.

There are two ways Boilerhut can help you calculate your boiler size:

  1. Get a free, instant online quote. Our tool will use your house size and number of radiators and bathrooms to determine the best boiler size for you.
  2. Manually calculate the power you may need for your home using the calculator tables below.

Throughout this guide, remember that we’re here to support you. If you have any doubts or questions about the right size boiler for your home’s heating and hot water needs, don’t hesitate to reach out to us. We’re always ready to assist you.

What size combi boiler do I need?

Combi boilers tend to have a higher power output than system and heat-only boilers. This is because they have to heat water on demand. The most suitable boiler size can be worked out according to the following:

NUMBER OF BEDROOMS NUMBER OF BATHROOMS NUMBER OF RADIATORS RECOMMENDED SIZE
1-2 1 0-10 20-28kW
3-4 2-3 10+ 28-35kW
4+ 3+ 20+ 35-45kW

What size system or regular boiler do I need?

System and regular boilers are designed to store hot water, meaning that they have a smaller power output compared to combi boilers. The table below illustrates the most suitable boiler sizes based on the heating requirements of a home.

NUMBER OF BEDROOMS NUMBER OF BATHROOMS NUMBER OF RADIATORS RECOMMENDED SIZE
1-2 1 0-10 10-20kW
3-4 2-3 10+ 18-28kW
4+ 3+ 20+ 30-50kW
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What does boiler size mean?

Boiler size refers to a boiler’s power output, not the boiler unit’s physical dimensions. This output is measured in kW (Kilowatts).

As a rule of thumb, the bigger your property and the higher your hot water demands, the larger the kW output boiler you will require. No one type or size of boiler is automatically superior to another, but choosing the correct one is still important.

What is boiler modulation?

Boiler modulation refers to the boiler changing its output to match the heating load. For example, if your home hits the desired temperature, a compatible thermostat would be able to turn down the output rather than turn the boiler off. This ensures your home stays warm rather than get hot and then cool in cycles.

Higher modulation range is, generally better. To control modulation, thermostats talk to boilers using OpenTherm. This is a standardised communication language that enables modulation.

Key Factors To Consider

In order to select the right size boiler, there are a few questions you should consider:

  1. Number of radiators
  2. Number of bedrooms
  3. Number of bathrooms

Other considerations:

  • What type of boiler do you currently have
  • Are you looking to change your boiler type
  • How old is your property
  • Your cold water flow rate
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Boiler size by property type

Our recommended boilers for a 1 bedroom property:

BRAND MODEL OUTPUT (kW) PRICE (boiler-only)
Viessmann Vitodens 100-W Combi 26 £1,390
Worcester Bosch Greenstar 4000 Combi 25 £1,320
Vaillant ecoTEC pro Combi 24 £1,460

Our recommended boilers for a 2 bedroom property:

BRAND MODEL OUTPUT (kW) PRICE (BOILER-ONLY)
Ariston Clas ONE Combi Boiler 24 £1,170
Navien LCB700 Blue Flame Oil boiler 28 £2,920
Viessmann Vitodens 050-W Combi 30 £1,110

Our recommended boilers for a 3-4 bedroom property:

BRAND MODEL OUTPUT (kW) PRICE (boiler-only)
Worcester Bosch Greenstar Highflow 440CDi Combi Boiler 29 £2,750
Viessmann Vitodens 050-W Combi 35 £1,270
Ariston Clas ONE Gas Combi Boiler 31 £1,010
Viessmann Vitodens 100-W System Boiler 19 £1,380

Our recommended boilers for a 3-4 bedroom property with multiple bathrooms:

BRAND MODEL OUTPUT (kW) PRICE (boiler-only)
Viessmann Vitodens 111-W Storage Combi Boiler 32 £2,260
Viessmann Vitodens 050-W Combi Boiler 35 £1,270
Viessmann Vitodens 100-W Combi Boiler 35 £1,520
Viessmann Vitodens 100-W System Boiler 19 £1,380

Our recommended boilers for a 4+ bedroom property:

BRAND MODEL OUTPUT (kW) PRICE (boiler-only)
Viessmann Viessmann Vitodens 200-W Combi 35 £2,154
Grant VortexPro Oil boiler 36 £3,360
Ariston Clas ONE Gas Combi Boiler 38 £1,280
Worcester Bosch Greenstar 8000 Style 40 £2,190
Vaillant ecoTEC plus 624 System Boiler 24 £1,600
Viessmann  Vitodens 100-W System Boiler 25 £1,440
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Quote in 30 seconds. No obligation. Next Day installation.
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The issues with choosing the wrong size boiler

If your boiler is too small, it could either never heat your home properly or just take a long time.

If your boiler is too large, it will use a lot of gas to heat your home, then cycle on and off, causing stress on the internal components.

In the past, homeowners were advised to ‘oversize’ the boiler that would be installed in their home. That was to choose a higher kW output boiler to compensate for heat loss. However, this is no longer recommended. In general, we would generally undersize rather than oversize a boiler as this would be more efficient.

Need help choosing the right size boiler?

If you need a new or replacement boiler or are looking at installing a central heating system, get an online quote (it takes less than 1 minute), and you’ll get an instant price. Our online tool will work out the best boiler size for you by asking a couple of simple questions.

But why choose Boilerhut:

  • Next day boiler installation.
  • Instant, no-obligation quote in less than 1 minute.
  • Finance options available for new boilers and central heating installation.
  • 10 to 12-year warranty on boilers.
  • ‘Excellent’ 5* customer satisfaction on Trustpilot.
  • Fixed prices. No hidden extras.
  • Get up to three quotes for different-tier boilers.

Get Your FREE Online Quote Today!
Save up to £590 PER YEAR on your heating bills.
Quote in 30 seconds. No obligation. Next Day installation.
Save up to £590 PER YEAR!

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