Do you want to know the truth about extensions and boilers?

Maybe you’re so busy planning what your extension is going to look like (I don’t blame you!), with colour schemes and units and new flooring. It’s an exciting time. So exciting that not many people really think about their boiler situation until later.

Well, the truth is, it’s a lot more complicated than a lot of people first imagine. For instance, if you think you can simply switch your old boiler out with a new one of the same size, think again. And that pipe? It might need replaced with a new one. Not to mention upheaval in places you hadn’t thought about!

Surely that’s something your builder can take care of, right? Wrong again.

Sorry, now I’ve probably put your head in a spin. As an experienced engineer, I’m going to explain everything I just said – and more – below.

1. You’ll need a bigger boiler

Not nearly quite as exciting as the Jaws “you’ll need a bigger boat”. But, still true nonetheless. An extension to your home, at the very least, means more space to heat. And if you’re having an extra bathroom in that extension (as many do) then you’ll also need a boiler than can deal with the extra hot water demand.

Say you have a 24kW boiler at the moment. You may want to size up to something like a 28kW or 30kW boiler to suit your needs. And if you were to keep your old boiler, it could struggle to heat your home at its new capacity. No-one wants to end up feeling chilly during the colder months.

2. Don’t let a builder install it

When I say ‘builder’, I mean the contractor a builder usually calls out to get the work done. This is always a Gas Safe registered engineer, but you’re better calling back the engineer who installed it in the first place. They’ll likely show more care and want you to keep your guarantee.

Builders usually get a contractor out, and as soon as the contractor is gone, they’ll take the reigns and start building around your boiler. This often leads to it being boxed in and not adhering to Gas Safe regulations and manufacturer specifications. The result? Well, it will void your guarantee. Always ask for a certificate of compliance to avoid this.

TIP: Check out my blog explaining more about this. It’s called ‘Why You Should Never Get a Builder to Install Your Boiler.’

3. You might need a bigger diameter pipe

It doesn’t matter if you’re moving or replacing your boiler for this one, either. If you’re installing a new boiler a lot further away from the gas meter, it’s not just a matter of taking the gas pipe from the old position to the new. You’re also going to have to upgrade the diameter of the pipe coming from the meter.

This is because every twist and bend can reduce the flow of gas. The result? You’re left with the wrong pressure reaching your boiler. I’ve seen this many times and we’re always called out to fix it.

4. You might have to reroute pipes

When relocating the position of your boiler, you could just take five water pipes from the boiler to their new position. However, when you think about it, hot water would go halfway round the house before it arrived at your taps. Just taking and joining the pipes might not be the best solution.

The better thing to do, in this case, would be to re-route the hot and cold supplies up to the bathroom. Take the shortest route to get hot and cold water to your taps. So, if an engineer tries to explain this to you, don’t worry – they’re just trying to do the best job and make your system more efficient. 

5. It can be more upheaval than expected

When you get an extension and relocate your boiler, it may result in having to lift flooring and carpets in places you hadn’t even considered. This can be a bit overwhelming for some homeowners. You may even have to get the occasional wall opened up.

Hopefully, any good engineer should be able to help manage your expectations about this from the beginning.

Advice from an experienced engineer

Always speak to 2 – 3 engineers, and get a detailed fixed price quote. Ask them about the upheaval, and about getting a good quality boiler with a good guarantee. Ensure they – and your builder – will follow the manufacturer specifications. Your builder should be happy to issue you a certificate of compliance regarding the work.

Choose a good, powerful boiler with a flow rate that will definitely match your home’s water demand – and a little more. You’ll find if your boiler isn’t as powerful as it should be, it can get temperamental. You’re often better spending an extra £500 to get it right, rather than run into costlier problems later on down the road.

I find my customers are usually happy to spend a bit more if it means avoiding headaches and expense later on. They also appreciate honesty.


Hopefully this blog has given you a better idea of all the main considerations regarding extensions and boilers. There’s quite a bit to take in, whether you’re getting a new boiler (recommended, as you’ll most likely need to size up) or moving your old one.

Always speak to your engineer about your options, and get 2 – 3 fixed price quotes. Your builder should also be happy to issue you a certificate of compliance.

Do you have any questions about extensions and boilers? If so, drop me a comment below. I’d love to hear from you!