Boiler size calculator - Hometree
Boiler size calculator
If you’re reading this, chances are you may be scratching your head over which boiler is the right size for your home. While many make it seem as if it’s dependent on a multitude of factors or won’t even tell you how they determine which boiler is the right one for your home, it’s actually possible to work out which boiler size you need from the following three pieces of information:
- The number of radiators
- The number of bathrooms (with non-electric showers)
- The number of people in the home (basically, the demand for hot water)
Please note that taking the above as a starting point should give you a pretty good idea of what boiler size you need. However, any Gas Safe registered engineer that is worth their salt will take into account other factors such as how well your house is insulated or whether you’re planning to extend your house. By the way, people often refer colloquially to the “size of a boiler”. What they really mean by that is its output, which is measured in kW. Below we have put together a boiler sizing guide that shows you different boiler models based on the number of radiators.
What size boiler do I need?
Find out below:
Gas boilers for properties with up to 10 radiators:
Gas boilers for properties with 10-15 radiators:
Gas boilers for properties with 15-20 radiators:
Best gas boilers for properties with up to 10 radiators
| Manufacturer/Model | Size (kW Output) | Energy Efficiency | Radiators | |
|---|---|---|---|---|
| Baxi EcoBlue System 12 Gas Boiler | 12 | 89.10% | Up to 10 | |
| Baxi Megaflo System 15 Gas Boiler | 15 | 88.60% | Up to 10 | |
| Glow-worm Energy System 12kW Gas Boiler | 12 | 89.30% | Up to 10 | |
| Glow-worm Energy System 15kW Gas Boiler | 15 | 89.30% | Up to 10 | |
| Ideal Logic Max 15kW System Gas Boiler | 15 | 93% | Up to 10 | |
| Ideal Logic System 15kW Gas Boiler | 15 | 89% | Up to 10 | |
| Ideal Vogue GEN2 S15 System Gas Boiler | 15 | 89% | Up to 10 | |
| Ideal Logic Plus System 15kW Gas Boiler | 15 | 89% | Up to 10 | |
| Potterton Ultra System 12kW Gas Boiler | 12 | 89% | Up to 10 | |
| Potterton Assure 15kW System Gas Boiler | 15 | 93% | Up to 10 | |
| Potterton Promax System 15kW Gas Boiler | 15 | 89% | Up to 10 | |
| Potterton Promax System 12kW Gas Boiler | 12 | 89% | Up to 10 | |
| Potterton Ultra System 15kW Gas Boiler | 15 | 89% | Up to 10 | |
| Vaillant ecoFIT pure 612 System Gas boiler | 12 | 89% | Up to 10 | |
| Vaillant Home 15kW System Gas Boiler | 15 | 89% | Up to 10 | |
| Vaillant ecoFIT pure 615 System Gas boiler | 15 | 89% | Up to 10 | |
| Vaillant Home 12kW System Gas Boiler | 12 | 89% | Up to 10 | |
| Worcester Bosch Greenstar 9i System Gas Boiler | 9 | 93% | Up to 10 | |
| Worcester Bosch Greenstar 12i System Gas Boiler | 12 | 94% | Up to 10 | |
| Worcester Bosch Greenstar 15i System Gas Boiler | 15 | 94% | Up to 10 |
Best gas boilers for properties with 10-15 radiators
| Manufacturer/Model | Size (kW Output) | Flow rate (l/m) | Energy Efficiency | Radiators | |
|---|---|---|---|---|---|
| Baxi EcoBlue+ Combi 28 Gas Boiler | 28 | 11.5 | 89.10% | 10-15 | |
| Baxi Duo-tec Combi 28 LPG Gas Boiler Gas Boiler | 28 | 11.5 | 88.60% | 10-15 | |
| Baxi 128 Combi Gas Boiler | 28 | 11.5 | 89.10% | 10-15 | |
| Baxi 228 Combi Gas Boiler | 28 | 11.5 | 89% | 10-15 | |
| Baxi 630 Combi Gas Boiler | 30 | 12.2 | 89% | 10-15 | |
| Baxi EcoBlue Advance Combi 28 Gas Boiler | 28 | 11.5 | 89.10% | 10-15 | |
| Baxi Platinum Combi 28 Gas Boiler | 28 | 11.5 | 89% | 10-15 | |
| Baxi Duo-tec Combi 28 Gas Boiler | 28 | 11.5 | 88.60% | 10-15 | |
| Glow-worm Essential 28kW Combi Gas Boiler | 28 | 11.7 | 89.30% | 10-15 | |
| Glow-worm Easicom3 28kW Combi Gas Boiler | 28 | 11.7 | 89.60% | 10-15 | |
| Glow-worm Energy Combi 30kW Gas Boiler | 30 | 12.5 | 89.20% | 10-15 | |
| Glow-worm betacom4 30kW combi gas boiler | 30 | 12.4 | 89.20% | 10-15 | |
| Glow-worm Ultimate3 30kw Combi Gas Boiler | 30 | 12.4 | 89.60% | 10-15 | |
| Potterton Promax 28kW Combi Gas Boiler | 28 | 11.5 | 89% | 10-15 | |
| Potterton Promax Ultra 28kW Combi Gas Boiler | 28 | 11.5 | 89% | 10-15 | |
| Potterton Titanium 28kW Combi Gas Boiler | 28 | 11.5 | 91% | 10-15 | |
| Potterton Gold Combi 28kW Gas Boiler | 28 | 11.5 | 89% | 10-15 | |
| Potterton Assure 30kW Combi Gas Boiler | 30 | 12.2 | 93% | 10-15 | |
| Vaillant ecoTEC Pro 28 kW Combi Gas Boiler | 28 | 11.1 | 89% | 10-15 | Boiler review |
| Vaillant EcoFIT pure 830 Combi Gas Boiler | 30 | 12.4 | 89% | 10-15 | Boiler review |
| Vaillant EcoFIT sustain 830 Combi Gas Boiler | 30 | 12.4 | 89% | 10-15 | |
| Vaillant ecoTEC Pro 30kW combi Gas boiler | 30 | 12.3 | 89% | 10-15 | Boiler review |
| Vaillant Home 30kW Combi Gas Boiler | 30 | 12.4 | 89% | 10-15 | |
| Worcester Bosch Greenstar 30Si Compact Combi Gas Boiler | 30 | 12.3 | 94% | 10-15 | Boiler review |
| Worcester Bosch Greenstar 30i Combi Gas Boiler | 30 | 12.3 | 94% | 10-15 | Boiler review |
| Worcester Bosch Greenstar 28CDi Compact Combi Gas Boiler | 28 | 11.4 | 92% | 10-15 | Boiler review |
| Worcester Bosch Greenstar 29CDi Classic Combi Gas Boiler | 29 | 11 | 90.1% | 10-15 |
What else do I need to consider when replacing a boiler?
Property type
The layout of your property informs the size and type of boiler required to heat your home. Combination boilers are recommended for small to mid-sized homes with one bathroom as they rely solely on mains water pressure and can’t provide enough hot water for homes with a high demand of hot water. In these cases, a system or regular boiler is more suitable as they have a large water storage tank and hot water cylinder.
Insulation
The level of insulation in your home will also determine the size of boiler required to keep the home warm for longer. A heat loss survey will take into account the materials used in the construction of the house, as well as whether windows have been double glazed, and if there are non-insulated spaces in the home.
Number of radiators
The number of radiators installed in your home will determine which boiler is best to provide enough hot water to heat your entire home.
Please note: For combination boilers, the number of radiators and bathrooms will needed to be factored in. For system and conventional boilers, however, the number of radiators, bathrooms and the size of the cylinder will affect the size of the boiler.
Number of bedrooms
The number of bedrooms indicates the size of the home that needs to be heated. As a general boiler sizing rule of thumb, the more bedrooms you have, the higher the output needed. However, since houses are not identical, we find that taking the number of radiators as a starting point for your boiler calculations is a far more accurate way of sizing boilers.
Number of bathrooms
More than one bathroom in your home? If you have multiple bathrooms, you may need a boiler with a hot water storage cylinder to cope with the higher hot water demand, especially if people tend to take showers at the same time.
Mains water pressure
The water pressure will determine whether your boiler can produce enough water pressure on its own. A conventional boiler is better suited to areas with low mains pressure because it can guarantee high water pressure from a loft-installed tank. A combi boiler is not suited to low water pressure areas.
Current boiler
Your current boiler determines the layout of your central heating system. For this reason, like-for-like boiler replacements tend to be easier to install, which is typically reflected in a lower price, whereas switching boiler types is slightly more complicated, tends to incur higher costs, and is, in fact, not always advisable. For example, switching from a regular boiler to a combi boiler is feasible but wouldn’t work unless the mains water pressure is sufficient.
Boiler brand
In the boiler market, there are several different brands with varying levels of energy efficiency and build-quality. The energy efficiency rating of a boiler is measured on a scale from A to G. A-rated boilers are highly efficient and converts almost 90% of its fuel into heat. G-rated boilers are likely to be older than 15 years and could be costing you an addition £350 a year to run in energy bills. We recommend boilers from both Vaillant and Worcester Bosch.
The type of boiler you would like
You might have a boiler you would like in mind, or you might have been recommended a particular boiler. However, we recommend that you let the boiler calculator determine the correct boiler to suit your home. The calculator considers the significant elements, like the number of rooms in your home and your hot water requirements.
Boiler flue
Boiler flues act as the home’s “exhaust pipe” for the boiler. The type of flue will also determine which boiler is suitable. The needs of a horizontal flue are quite different to that of a vertical flue.
What are the boiler types?
Combination boilers – Combi boilers can control both hot water and central heating. They’re the most popular boiler choice amongst UK households due to their high energy efficiency. A combi boiler heats water directly from the mains supply, which is then fed directly to a hot water outlet the minute its turned on. With combis, you can expect unlimited hot water as and when you need it, without the need for a cylinder or water tank to store water.
System boilers – If your home has more than one bathroom installed with multiple outlets, a system boiler would be your best bet. They require a hot water cylinder which stores enough hot water for several bathrooms and taps. A cold water tank isn’t needed for a system boiler seeing as many of the main components are built into the boiler. As there isn’t any need for a tank, you can expect to save space, as well as peace of mind knowing that you wouldn’t need to worry about frost damage or leaks, which makes them easier to maintain.
Conventional boilers – Conventional boilers, also known as a traditional, regular, open vent or heat only boilers, consist of a cylinder, water storage tank and the boiler itself. They might not be the best central heating system for space-saving, but they’re able to supply large volumes of hot water to multiple bathrooms, making them an excellent choice for larger homes with multiple bedrooms and bathrooms. It’s important to remember that this type of boiler is easy to replace if an existing traditional heating system is in place.
Best gas boiler for 2 – 3 bathroom houses
If your home has 2 or more bathrooms, a combi boiler may not be able to cope with the high hot water demand. We recommend that you install either a system or conventional boiler with a central heating output of 35 – 42kW. A system boiler includes a hot water storage cylinder, whereas a regular boiler includes a cylinder and cold water tank. A hot water cylinder can supply enough hot water for multiple taps, showers and baths without a loss of pressure.
Check your mains flow rate
If you’re replacing an older boiler, your mains pressure would determine whether a combi boiler is suitable for your home. System and regular boilers both take their hot water supply from a cylinder, whereas a combi boiler is completely mains-fed. To ensure you have the correct flow of hot water, the correct flow rate is essential in a combination boiler.
If you’d like to check your flow rate at home, the bucket test is a quick way to get a quick idea. If you run cold water from a tap for 1 minute into a bucket with measurements, you’ll get an idea of the flow rate from your current supply in litres per minute (l/m). Each combi boiler will specify it’s maximum flow rate so you’ll be able to find a good match with this test.
Boiler efficiency tips
Insulation
To prevent the loss of heat through your room, walls, floors or windows, having proper insulation installed is essential. A poorly insulated home will mean that your boiler has to work harder to keep your home heated, although this will waste energy. To reduce the costs of energy bills, you may want to think about installing effective insulation, as well as replacement windows with double glazing.
What radiator size?
As a general rule of thumb, the larger the surface area of the radiator, the quicker it’ll heat a room. Not all radiators are the same size, which tends to be measured in BTUs or British Thermal Units).
Smart controls
Modern boilers are now compatible with most smart thermostats which enable you to control your central heating from your smartphone or tablet, whether you’re at home or away (as long as you have mobile internet or WiFi). Due to advances in home technology, you’re now able to schedule when your heating is switched on, as well as monitor your energy use and find ways to reduce wastage. Some smart thermostats also have features that use the local weather reports to change your temperature settings automatically.
Annual boiler servicing
Boiler servicing will ensure that your boiler keeps running at its best for as long as possible. Servicing will catch any problems with your boiler early-on to prevent more significant problems from developing – causing long-term damage.