Size of Gas Boiler with Underfloor Heating

[WilfZwolf]

Good afternoon.

We are building our own house and will be installing underfloor heating. Our supplier says we need a gas boiler to provide 18.7kW for the underfloor heating. I understand the importance of not "over-boilering" and have completed may calculations on the internet to determine what size of boiler to get quotes for. In all cases, we apparently will need less for the whole house than the underfloor heating people say we need just for that!

My question is, therefore, what size should I be looking for. To allay the fears of professionals here, we shall not be installing the boiler ourselves; but I shall be sourcing it and I would like to add the right figure to my budget. The house has a ground floor area of 140sq m; first floor (dormer bungalow) 85sq m. We have three bathrooms, four bedrooms. The house is being built with good U values (floor and roof 0.12 and walls .14).

Thank you for any guidance you can provide.

 

[muggles]

Two schools of thought on this.

One is to say that the UFH guys are wrong and your calculations are correct, in which case purchase the boiler of the size you think you need and hope for the best. If it's not sufficient, you'll need to buy another boiler.

The other is to say that, as the UFH suppliers will almost certainly invalidate the warranty with regards to the effective heating of your property with their system if the boiler isn't as big as they've asked for, you should buy one that is at least 18.7kW plus whatever you need for the rest of the house.

Your call.

As for sourcing your own boiler, that's fine but there are a couple of things to bear in mind. Firstly, your installer might well be able to offer you a better price as they have access to trade discounts that you might not have access to. Secondly, if you supply the boiler yourself it will be you rather than the installer who is responsible for dealing with any issues with it in the future. That is to say, if there was a minor issue with it that was easily fixed without involving the manufacturer's warranty service, your installer would most probably fix it free of charge if he'd supplied the boiler, but if you've supplied it you can be expected to be charged for his time.

 

[WilfZwolf]

Many thanks Muggles.

I definitely would not question the underfloor heating people and their experience/software/knowledge. I am wondering if it is possible to ascertain what extra kW I need for the hot water for rest of the house - three bathrooms and the kitchen and utility.

If it transpires the engineer who is fitting the boiler is more economical than us buying our own, that will be marvellous and he will get the order to supply and fit. But if I don't know what we should be fitting, I cannot provide myself with a budget to compare him against. Every penny counts!

Thank you again for your guidance.

 

[Richardthe3rd]

Where's the Heatloss calcs?? Regardless of heat emitters, the building losses will determined the size of the boiler.

3KW for hot water is the norm but.

UFH is ideal for a condensing boiler, but by the very nature of operating UFH it does require a bit of boiler cycling. What I mean by that; with UFH you build up the heat & then let it tick-over.

You've made a wonderful choice, it's a very pleasant type of heating & comfy to live with.
HTH

 

[WilfZwolf]

Thank you Richard the Third

This was my dilemma - as heat loss calculations indicated a boiler capable of 15kW was all that was required. Whereas, the underfloor heating chaps said their system required 18.7kW

I had expected to follow the instructions for heat loss calculations and discover a higher number than they had given me and thus know what boiler to price up. Unfortunately this did not prove to be the case. I used US and UK online calculators (accurately using my window sizes rather than a provided value as the windows are rather large) but the most I could calculate was 49000 BTU (on the US calculator) and 15kW (on the UK one).

We had underfloor heating back in 2000 and it is indeed marvellous - as you say a delightful to live with. Hopefully the upgrades now available (in comparison to then) will prove similarly comfortable. We are also taking the opportunity to increase the insulation in the house this time and hope to have a very cost effective house, warmth wise.

It is quite a large house of a 1.5 storey design with 3 bathrooms and 4 bedrooms. If we go for 24kW or 27kW would this be sensible in that it should do the hot water and won't be unnecessarily large?

Many thanks for your assistance

 

[Dan Robinson]

Unvented cylinder?

I would be looking at 20 - 24 kw based on your info.

 

[Richardthe3rd]

I'll not give you a size without doing a HL calc for myself.