The size of the boiler you require is one of the first things you will need to decide on before a new boiler installation can take place in your home.
In order to select the right size boiler, there are a few questions you should answer:
How many bedrooms are in your property?
The more bedrooms your property has, the higher your heating demands likely will be
How many radiators are in your property?
This will have an impact on the boiler size you choose as if you only have a few radiators and choose a higher kW output boiler, you could end up with higher energy bills than necessary. Whilst you should count the number of radiators in your property, you should also consider whether you are likely to install additional radiators in the future too
How many bathrooms do you have in your property?
You should also consider the hot water demands of your household and whether multiple showers and hot taps will be being used at the same time. If they will be, this will determine not only the size of your boiler that should be installed but also the type of boiler too. For instance, if you have a large family who are often using multiple showers and hot taps at the same time then a combi boiler might not be the most suitable boiler for your needs
What type of boiler do you currently have?
Is it a combi boiler, conventional boiler, or a system boiler?
Are you looking to change your boiler type?
E.g. by converting a conventional boiler system to a combi boiler system. If you are looking to do this, it is important to make sure your current pipework is good enough
How old is your property?
Was it built pre-1970s or is it a modern construction? If your home is built to older building regulations, it likely won’t be as insulated as modern properties and will have greater levels of heat loss
Check your water flow rate
In addition to the above, it is worth also checking your water flow rate. To do this, you need to:
- Find an unrestricted mains supply such as an outside garden tap
- Get a 5-litre bucket and time how long it takes to fill up
If it fills up in 15 seconds, you’ve got a 20L/minute flow rate. This is a suitable flow rate for system and combi boilers.
What does boiler size mean?
Boiler size refers to the output of a boiler not the physical dimensions of the boiler unit. This output is measured in kW (Kilowatts) and plays a key role in determining which boiler is right for the needs of your property and household. kW is used to measure how much energy the boiler outputs as heat. 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 type or size of boiler is automatically superior to another, but choosing the correct one is still important. For all modern condensing boilers, the boiler size is ‘modulated’ in KW depending on the load, allowing superior efficiency.
What does boiler modulation mean?
Boiler modulation refers to the boiler’s process of changing its output to match the heating load. For example, if your demand for hot water ever changes (say you’re a landlord with a new tenant moving in) or you have an additional radiator installed, your boiler is capable of altering its output to match the new heating load. In doing this, it can help to improve the lifespan and overall quality of its own internal components.
Choosing the right size boiler for your property
Combi boiler sizing
The table below gives you an initial overview of where to start when it comes to sizing a combi boiler.
| NUMBER OF BATHROOMS | NUMBER OF RADIATORS | RECOMMENDED BOILER SIZE |
| 1 | Up to 10 | Up to 29kW |
| 1-2 | Up to 15 | Up to 35kW |
| 2+ | Up to 20 | 35kW+ |
What size boiler do I need for a 1 bedroom property?
On average, a 1 bedroom house or flat will have 6 radiators and 1 bathroom. For a property of this size, we recommend a 24-26kW combi boiler due to the low heating and hot water requirements that the property will likely have.
Our recommended boilers for a 1 bedroom property
| BRAND | MODEL | OUTPUT (kW) | FUEL |
| Viessmann | Vitodens 100-W Combi | 26 | Gas/LPG |
| Worcester Bosch | Greenstar 25i Combi | 24 | Gas/LPG |
| Vaillant | ecoTEC pro Combi | 24 | Gas/LPG |
What size boiler do I need for a 2 bedroom property?
On average, a 2 bedroom house or flat will have 7 radiators and 1 bathroom. For a 2 bedroom property, we recommend a 24-29kW combi boiler. If you have a 2 bedroom house with a conventional boiler or a system boiler, it may be worth converting this to a combi boiler as not only will this save you space in your property, it will also save you money on your energy bills. This is because combi boilers only heat water when it is needed unlike boiler systems with a separate hot water cylinder.
Our recommended boilers for a 2 bedroom property
| BRAND | MODEL | OUTPUT (kW) | FUEL |
| Ariston | Clas ONE Gas Combi Boiler | 24 | Gas |
| Navien | LCB700 Blue Flame Oil | 28 | Oil |
| Viessmann | Vitodens 050-W Combi | 29 | Gas/LPG |
What size boiler do I need for a 3-4 bedroom property?
A typical 3 bedroom property will have 8 radiators if it only has 1 bathroom. For a 3 bedroom property with 8 radiators and 1 bathroom, we recommend a 29-31kW boiler.
Our recommended boilers for a 3-4 bedroom property
| BRAND | MODEL | OUTPUT (kW) | FUEL |
| Worcester Bosch | Greenstar Highflow 440CDi Combi Boiler | 29 | Gas/LPG |
| Viessmann | Vitodens 050-W Combi | 29 | Gas/LPG |
| Ariston | Clas ONE Gas Combi Boiler | 31 | Gas |
| Viessmann | Vitodens 100-W System Boiler | 19 | Gas/LPG |
What size boiler do I need for a 3-4 bedroom property with multiple bathrooms?
If your 3 bedroom property has multiple bathrooms, additional rooms such as a utility room, and more than 8 radiators, we recommend a higher capacity boiler with up to a 39kW output, in order to achieve the sufficient flow rates that homes with higher hot water demands require.
If you need something with higher flow rates, it will be worth looking at storage combi options.
Our recommended boilers for a 3-4 bedroom property with multiple bathrooms
| BRAND | MODEL | OUTPUT (kW) | FUEL |
| Viessmann | Vitodens 111-W Storage Combi Boiler | 32 | Gas/LPG |
| Viessmann | Vitodens 050-W Combi Boiler | 35 | Gas/LPG |
| Viessmann | Vitodens 100-W Combi Boiler | 35 | Gas/LPG |
| Vaillant | ecoTEC plus 938 Storage Combi Boiler | 38.7 | Gas/LPG |
| Viessmann | Vitodens 100-W System Boiler | 19 | Gas/LPG |
What size boiler do I need for a 4+ bedroom property?
If you have a large house with as many as 20 radiators, we recommend that you select a boiler with an output of 35-42kW, if choosing a combi boiler. However, if you have a property of this size, you are likely to have higher heating and hot water demands, for this reason, we would recommend choosing a system boiler instead.
Our recommended boilers for a 4+ bedroom property
| BRAND | MODEL | OUTPUT (kW) | FUEL |
| Viessmann | Viessmann Vitodens 200-W | 35 | Gas/LPG |
| Grant | VortexBlue | 36 | Oil |
| Ariston | Clas ONE Gas Combi Boiler | 38 | Gas |
| Worcester Bosch | Greenstar 8000 | 40 | Gas |
| Vaillant | ecoTEC plus 624 System Boiler | 24 | Gas |
| Viessmann | Vitodens 100-W System Boiler | 25 | Gas/LPG |
