What size boiler do I need? is a question every new buyer needs to ask themselves. Many of us that aren’t industry savvy don’t realise that you’ll need a specific boiler size and type that’s determined by bedroom/bathroom count, boiler location and property size etc. Even though combi boilers are incredibly popular, in this blog post, we’re also going to be talking about open vent and system boilers.
The boiler that you choose also depends on your living situation. Are you part of a large family or do you live by yourself? How many people will be using the bathrooms at any one time? Ask yourself these questions so you know you’re getting the correct performance and highest efficiency from the boiler that you choose.
Don’t worry if you’re feeling overwhelmed – we’re here to help break it down so you no longer need to ask ‘What size boiler do I need?’
If you’re new to the world of Best Buy Combi Boilers in particular, we’ve published easy-to-understand guides on our blog which may help:
- Best Combi Boilers in the UK
- How a Combi Boiler Works
- What is a Combi Boiler?
- Condensing Combi Boilers
The size of a boiler is known as the ‘output’. To correctly work this out, you’ll need to establish details of your current property and what you’ll require from your heating system. Ask yourself the following questions:
- What type of boiler do you already have? Open vent, combi or system?
- Are you looking to change your boiler type entirely? E.g. Converting from open vent to combi?
- Is it an old house or a more modern construction property?
- How many bathrooms are there?
- What’s the demand for DHW (Domestic Hot Water)
If you size a boiler incorrectly, it may take longer to heat up your home and your hot water cylinder if it’s been undersized. If it’s been oversized, the general problem is that the gas and energy expenditure will be more. As long as it isn’t too high, you’ll get a much better flow rate.
If you’re asking Google ‘What size boiler do I need?’, then you need to find that balance.
What size and what type of boiler do I need?
No type or size of boiler is automatically superior to another, but choosing the right one is very important. When asking ‘what size boiler do I need?’, it’ll depend on how big your house is, the number of bathrooms, the hot water demand and the output for all the radiators that you may have.
An advantage with all modern condensing boilers is that this output is modulated between kw depending on the load, to allow superior efficiency.
What exactly is modulation?
Modulation improves the lifespan of internal components, but what does it mean? When demand for hot water changes for different times of the year, boiler output changes within it’s range to match the heating load. This is called modulation.
Lets use an example like the one pictured above. Say you lived in a modern 3 bedroom property with 8 radiators. These radiators will have an output of around 1.5kw which if multiplied by 8, equates to an overall output of 12kw. For this scenario, we’d recommend the 29kw Viessmann Combi Boiler, which has a modulation range of 1 to 6. Because of this, it can operate within a range of 4.8kw and 29kw.
Manufacturers design boilers to modulate. For a premium brand such as Viessmann for example, this is no exception. When asking ‘What size boiler do I need?’ you’ll need to be aware of how modulation plays its own role.
Let’s break it down even more and talk about the different types of boilers that are most suitable for your property:
I need a new boiler – what are my options?
Sealed System Boilers
The heating and hot water components are built into the boiler. It needs a pressurized unvented cylinder for storing hot water. Unlike a conventional boiler, you don’t need a cold water tank in your loft or attic.
Viessmann’s domestic range of system boiler sizes include:
Ideal boiler size
Once again we’ll use the same example – a modern 3 bedroom property with 8 radiators. If we’re asking ‘What size boiler do I need’ about system boilers, we would recommend a 19kw Viessmann system boiler. As the smallest available from Viessmann, it’s modulation range is 1-6. This means that it can cover between 3.1kw to 19kw, with plenty of unused energy to go towards the hot water cylinder.
These require a cold water tank in the loft, and a hot water cylinder elsewhere in the house.
Viessmann’s domestic range of open vent boiler sizes include:
Ideal boiler size
For our usual example, we’ll throw in another bathroom. We’ve already established that the heating circuit is 12kw, so for open vent boilers we would recommend the Viessmann Vitodens 100-W Open Vent 16kw. These only modulate 1-3 so it’s important to get closer to the size of the heating circuit and allow around 3kw for your hot water cylinder. In your search for ‘What size boiler do I need?’, always make sure that you’ve gone for something that allows extra energy for the cylinder!
*All modulations ranges in this article apply only to Viessmann. Check other manufacturers for modulation ranges.
*Example is based on a modern house at 1.5kw per radiator. Older houses tend to have larger radiators so you may be looking at 2-2.5kw per radiator across the board.