Boiler Size - what are the consequences of going too big?

frank_olaf

Hi,

We are planning on replacing our boiler this spring/summer and I'm in the process of doing research for it.

My main concern is getting the boiler size (central heating output) correct as I've read that the wrong size can cause issues. Too small and it won't sufficiently heat my home and too big and it will cost us extra and also be bad for the longetivty of the boiler, because it isn't running as efficiently as it should be (https://www.theheatinghub.co.uk/what-size-boiler-do-I-need).

Is there any truth in that, especially what is written on theheatinghub.co.uk? We're hoping this boiler will last us for a long time so don't want to get the wrong one and need to replce it sooner.

We've done a bunch of boiler quotes online (boxt, iheat, LS1, etc) and get different results in terms of the boilrer size they recommend. Some recoomend 24 kw, some recommend 30 kw. I'm not sure if this is just those site pushing certain products because they want to sell them or because they actually think it is best for us.

Our current boiler is a glow worm betacom 30c with a central heating output of 27.6 kw. I have a 3 bed house with 9 radiators and one bath. Based on some formulas i've seen, 24 kw would be enough but I don't really want to go smaller than I currently have (mainly because if I 'feel' the house isn't has hot/water takes longer to heat up, I will be annoyed that I made the wrong decision). 

We will get some specialists to come around and give us a proper quote and hopefully good advice, but my question here is, what is the real impact of having a boiler that is too big. Will it increase my bills by a lot and will it harm the boiler, making it break sooner?

Thanks.

bris

24 is fine for a 1 or small 2 bed property but for 2 or 3 bed properties I would go fir a 30.

daveyjp

A 24kw boiler is more than adequate for the heating side, however with combis hot water flow is generally more important and noticeable for you as a user.

Size the combi on your cold water flow rate.  If you have a 30kw boiler now it suggests a good low rate, at least 12l per minute. If that is the case don't go smaller.  If the flow rate is 15l +  you may be able to go larger if hot water flow is important.

nofoollikeold

1. Older boilers worked on the "on or off" principle.  The central heating room thermostat "calls" for heat.  The boiler turns on, pumps hot water round the system. After a while the room thermostat detects the rooms are warm enough, and it turns the  boiler off.  The property starts to cool, the thermostat calls for more heat, and the cycle repeats.
2. Modern boilers "modulate".  That is they run at a heat output between the maximum they can do (e.g. 30kW), and some minimum.  With a clever thermostat system (e.g. Opentherm), the thermostat turns the boiler on, but, as the rooms come up to temperature, it lowers the heat output of the boiler, to balance the heat being lost to outdoors with the heat required to keep the property warm.  
3. Provided the minimum heat required to keep your property at the required temperature is greater than the minimum heat output of the boiler, it will modulate.  If the minimum heat required is less than the boiler's minimum heat output, the boiler will switch off, and will then cycle on / off.  This puts greater strain on the boilers internals than a constant, more general, heat output.

FreeBear

frank_olaf said: Our current boiler is a glow worm betacom 30c with a central heating output of 27.6 kw. I have a 3 bed house with 9 radiators and one bath. Based on some formulas i've seen, 24 kw would be enough but I don't really want to go smaller than I currently have (mainly because if I 'feel' the house isn't has hot/water takes longer to heat up, I will be annoyed that I made the wrong decision).

Fitting a larger boiler on its own will not make the house warm up quicker. To do that, you'd need larger radiators along with a boiler. A "smart" control system that allows you to set different temperatures throughout the day along with some form of zoning will help to cut your gas consumption.

frank_olaf

Thanks for all the comments. They are really helpful.