We get asked this question all the time, and the answer depends on several different factors, which we will cover in this article. But before we do that, we want to make sure you understand that boiler size refers to the output rating measured in kilowatts (kW) and not the physical dimensions of a boiler.
A kilowatt is a unit used to measure the energy output a boiler can produce in the form of heat, and the more heat and hot water you require, the higher the boiler output will need to be. That’s the general rule anyway.
We’re now going to discuss why boiler size is so important and how to choose the correct size boiler for your home and needs.
Why is a correctly sized boiler so important?
Well, unless you love to feel cold or enjoy wasting money, you are going to want to choose a suitably sized boiler for your home and lifestyle.
If your boiler is undersized for your needs, it will not provide enough heat and hot water to keep you comfortable. And if you end up getting an oversized boiler installed, the initial cost will be higher than necessary, and you will waste energy, which leads to wasted money.
So, now do you understand why a correctly sized boiler is crucial? Great. Time to move on to figuring out the right boiler type for your needs.
Which boiler type is best suited to my home?
Before you can choose the size of your boiler, you need to determine which boiler type will match your heating and hot water requirements. Selecting the right boiler type can be an overwhelming decision, so your best bet is to ask a Gas Safe registered engineer to visit you at home and make a recommendation.
The three main boiler types include combi, system and heat-only, which are available in gas, liquefied petroleum gas (LPG) and oil variants.
Combi boilers
Ideal for homes of all sizes with one or two bathrooms, combi boilers take water directly from the mains and provide hot water and central heating on demand. They have everything they require in one compact unit, removing the need for additional tanks or cylinders.
Because combi boilers supply hot water on demand rather than store it in a cylinder for later use, you never have to wait for hot water. However, a combi boiler may not be the best choice for homes with multiple bathrooms as they can only provide maximum pressure through one tap or shower at a time.
System boilers
Unlike combi boilers, system boilers require a separate cylinder for storing hot water. They take water directly from the mains and do not require a tank of water in your loft, so all you need with a system boiler is the boiler itself and a cylinder (usually kept in an airing cupboard).
System boilers are best suited to homes with a high demand for hot water because they can supply hot water to multiple taps or showers at one time without a drop in water pressure. But the size of the cylinder determines how much hot water is available before it needs to refill. That means you have to wait for the cylinder to fill up again when the hot water runs out before having access to more.
Heat Only boilers
Heat only boilers, also known as regular boilers, conventional or open vent boilers, require a cylinder and a cold water tank in the loft. The boiler relies on the cold water tank to feed water to the hot water storage cylinder, and the boiler then heats the water in the cylinder to supply your taps, showers and radiators with hot water.
If your home has several bathrooms, high demand for hot water and a traditional heating system, a heat-only boiler could be the best option for you because it can supply hot water to multiple taps and showers at once.
What size combi boiler do I need?
Combi boilers have two output ratings. One for central heating (CH) and the other for domestic hot water (DHW). Because system and heat only boilers store hot water in a separate cylinder, they only have an output rating for central heating.
Your heating and hot water requirements will have an impact on which kW combi boiler you need, as will the number of bedrooms, bathrooms and radiators in your home.
Here is a rough guide on a suitably sized combi boiler based on your property size:
| Bedrooms | Bathrooms | Radiators | Recommended Size |
|---|---|---|---|
| 1-3 | 1 | Up to 10 | 24kW to 27kW |
| 2-4 | 1-2 | Up to 15 | 28kW to 34kW |
| 3-5 | 2+ | Up to 20 | 35kW to 42kW |
What size system boiler do I need?
If you need a system boiler for your home, you can get away with a lower output because it uses a separate cylinder for storing hot water.
Like with a combi boiler, you need to consider your heating and hot water requirements and the number of bedrooms, bathrooms and radiators in your home to determine the correct size system boiler.
Here is a rough guide on a suitably sized system boiler based on your property size:
| Bedrooms | Bathrooms | Radiators | Recommended Size |
|---|---|---|---|
| 1-2 | 1 | Up to 10 | 9kW to 18Kw |
| 3-4 | 2-3 | 10+ | 18kW to 26kW |
| 5+ | 4+ | 20+ | 27kW to 40kW |
What size heat only boiler do I need?
The recommended output for a heat only boiler is the same as a system boiler, and the number of bedrooms, bathrooms and radiators you have will help you figure out which size will best match your need.
Here is a rough guide on a suitably sized heat only boiler based on your property size:
| Bedrooms | Bathrooms | Radiators | Recommended Size |
|---|---|---|---|
| 1-2 | 1 | Up to 10 | 9kW to 18Kw |
| 3-4 | 2-3 | 10+ | 18kW to 26kW |
| 5+ | 4+ | 20+ | 27kW to 40kW |
Seek expert advice
With so many boiler options and sizes to consider, we know how confusing and time-consuming it can be to figure out the correct size boiler for your home. We always say you should seek expert advice on sizing a new boiler, so be sure to ask a Gas Safe registered engineer for a more precise recommendation before making a decision.
Also, it pays to shop around and compare prices before purchasing a new boiler. We recommend getting quotes from different boiler installers so that you can find the most competitive price on the boiler and the installation. If you’re ready to compare prices now, complete our online form to get free quotes from qualified heating engineers in your local area.