Which Size Boiler Does Your House Need?

When we talk about boiler sizes, we don’t mean their physical size – what we’re actually talking about is their output, or the power they consume. Sure, some boilers are bigger than others and this can indicate that it’s a more powerful boiler but that isn’t always the case. As boilers become more efficient, a 20kw boiler from 5 years ago is likely to be a lot bigger than a 20kw boiler that’s new. 


Boiler Size or Boiler Power?

So really what we need to ask is how much power do you need a boiler to be cranking out to heat your home and keep your taps hot?

There’s two reasons why choosing the correct sized boiler is important. First, you obviously don’t want to get a boiler that’s too small for your needs. Nobody enjoys being interrupted during a warm shower by the feeling of someone throwing an Ice Blast at you. 

Secondly, if you go overboard with a massive boiler, sure you’ll never run out of hot water but you’ll absolutely be paying the price for it by burning gas that isn’t needed. The only reason you should ever buy a bigger boiler than you need is when you’ve got a house extension on the cards and you’ll be adding another shower or bath room. 

So, you don’t want a boiler that’s too small or your shower singalong sessions will be interrupted, and you don’t want an oversized boiler because it’ll make baby polar bears cry. What you need is a boiler that’s “juuust right” – the goldilocks equivalent in boilers.  

Cold radiator spots - solution - bleeding the radiator

Let’s talk about what a Combi Boiler actually is and how they work.

The Combi Boiler (Or the Combination Boiler if we’re talking like gentlemen), provides you with both central heating and hot water at the same time. They work by heating up cold water internally, when you need the hot water. You can think of them like electric showers – cold water comes in, gets warm, then leaves. With a combi boiler it either leaves through the central heating piping, the hot water (sinks and baths) piping, or both. 

This means that there isn’t a massive tank upstairs that gradually gets warm and there’s no more complaints that you’ve used up all the hot water! 

With a combi boiler, the water is fed from the mains so the water pressure is generally greater. This boiler type is one of the more high efficiency boiler systems available and at a price that fits most budgets when it comes to the right boiler cost, installation costs and overall energy bills moving forward. And when it doesn’t fit in your price range, we supply and fit them for free! Check if you’re entitled to a free boiler here

Back to boiler sizes

Combi boilers typically fall into 3 ranges for houses:

  • 24kw – 27kw
  • 28kw – 34kw
  • 35kw to 42kw

If you have a small house, or an apartment with one or two bedrooms then you fall into the first category, one bathroom is closer to the 25kw mark or if you have two then go more towards the 27kw range or the low end of the middle range (28ish kw).

If you have a 3 to 4 bedroom house with one bathroom then you’re firmly going to be around the 29kw range, or if you have two bathrooms then slide on over to the 34kw range. 

If you’re rocking a 5 bedroom house or more with a single bathroom, first, how do you manage with just one if there’s 5 bedrooms? Secondly though you’ll be looking for 35kw from your boiler, or thereabouts. If you’ve got bathrooms o’plenty then you’ll be pushing more in the 42kw range. 

Calculating your Boiler requirements with Radiators instead

Let’s say it’s a habit for you not to have baths and you take showers instead, or shower somewhere else like the gym instead, you might not need that power – however we all get cold, and your radiators definitely need enough scope to get hot. We’ll work it out a different way instead. 

Count your radiators. We’re going to add 3kw to your totals just in case there’s ever a moment in time where you need central heating and a bath to warm you up in the winter, or if you want to wash the dishes too (gyms were closed for a long time in 2020, don’t rely on showering there). Each radiator is 1.5kw in our sums.

  • 6 radiators in a small home: That’s 6 radiators multiplied by 1.5kW, plus the 3kW extra. 12kW in total for your boiler to put out.
  • 8 Radiators in a small home multiplied by 1.5kW, plus the 3kW. 15kW in total for the boiler to put out. 
  • 12 Radiators in a medium home comes out at 19kW, plus the 3kW extra is 22kW for your boiler to put out. 
  • 15 radiators in a medium home comes out to 23kW, plus the extra is 26kW for your boiler. 
  • 20 Radiators comes out to 30kW, plus 3kW extra is 33kW for your boiler.

Hopefully, you now have an idea or a rough estimate of what kW range your new boiler should be! If you’re struggling with the costs of a new boiler and you own your home, it’s worth reaching out to us because we might be able to supply and fit a new boiler for you, for free! You can find out if your eligible here

Are you entitled to a Free Replacement Boiler?

At Borthwick Heating we work with the UK Government to install free boilers to eligible UK Homeowners. To check if you qualify click the button below.

Borthwick Heating Inuslation UK is a trading style of Borthwick Group (Energy) Limited Registered in England & Wales| Company No. 06753929 

Borthwick Heating & Insulation are regulated by the Financial Conduct Authority. We are a Credit Broker and not a lender. We offer Credit Facilities from a panel of lenders. Borthwick Group (Energy) Limited is the full legal and registered trading name of all activities that advertise themselves as Borthwick Heating & Insulation and online as www.borthwickheating.co.uk


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