What size of gas boiler

matelodave

Have a look on the web and there are heat loss calculators that will give you a fair idea of what size boiler you'd need.


A proper tool might give you a higher level of precision but for that you'd really need accurate information on all the materials that your house is made of. However to ensure that you are in the right area the on-line ones will do. I'd go for one that does it on a room by room basis and takes into account window size, wall types etc. rather than a whole house guesstimate (although they aren't that far out).


If you are using existing radiators then you will be limited by the amount of heat that they will dissipate anyway so even a boiler twice the size won't give you any more heat.


You might just as well work out the dissipation of each rad and add it all together


You don't need to oversize your boiler because it won't be running efficiently, likewise an undersized one won't produce enough heat and you'll be cold.

Robwiz

From what I've seen of SAP and RdSAP there's no guarantee that putting in more data points will give a more accurate estimate because there are so many assumptions. For example, solid walls don't lose as much heat as their U value suggests they should. These government sponsored tools are always created by a committee so there are inherent compromises.

Also, do you really need to size the boiler for a lowest outside temperature of -5º C? That will almost certainly be hit in the middle of the night when the heating won't be running. And then only perhaps once in the lifetime of a condensing boiler (10 years).

For sizing a boiler, a few kW either way won't make much difference. It's probably better to err on undersizing (i.e. sized for a minimum external temperature of 0º) to ensure the boiler operates in condensing mode as intended.

I actually went through the full detailed calculations, using actual values and the worksheet suggested 13 kW (50/50 between fabric and ventilation losses). I've posted previously that our existing gas CH boiler is 32 kW/80% boiler.

Since then we have replaced single glazing with low U triple glazing and installed wall and upgraded loft insulation, yet when I got quotes for a new gas boiler, two installers suggested 40 kW! Sheer stupidity – a like for like replacement should be 10% smaller to reflect the boiler's higher efficiency, before considering the effects of significantly improved insulation and lower heat losses.

captainhindsight_2

Robwiz wrote: »

From what I've seen of SAP and RdSAP there's no guarantee that putting in more data points will give a more accurate estimate because there are so many assumptions. For example, solid walls don't lose as much heat as their U value suggests they should. These government sponsored tools are always created by a committee so there are inherent compromises.

Also, do you really need to size the boiler for a lowest outside temperature of -5º C? That will almost certainly be hit in the middle of the night when the heating won't be running. And then only perhaps once in the lifetime of a condensing boiler (10 years).

For sizing a boiler, a few kW either way won't make much difference. It's probably better to err on undersizing (i.e. sized for a minimum external temperature of 0º) to ensure the boiler operates in condensing mode as intended.

I actually went through the full detailed calculations, using actual values and the worksheet suggested 13 kW (50/50 between fabric and ventilation losses). I've posted previously that our existing gas CH boiler is 32 kW/80% boiler.

Since then we have replaced single glazing with low U triple glazing and installed wall and upgraded loft insulation, yet when I got quotes for a new gas boiler, two installers suggested 40 kW! Sheer stupidity – a like for like replacement should be 10% smaller to reflect the boiler's higher efficiency, before considering the effects of significantly improved insulation and lower heat losses.

Of course assumptions are made, but the factual data INputted the less assumptions that are made thus making it a more accurate figure. Also the assumptions that are made are being updated all the time so over time these assessments are becoming more and more accurate.

These assessments are a mandatory requirement for sizing renewables and imo it is worth carrying them out all all technologies. As if the wrong boiler is fitted from a guesstimate the customer is going to be annoyed. So at least this puts on paper whatt should be installed and can back up a decision. I think it is better to be safe that sorry.

Seronera

Boiler size depends on whether you are going combi or non combi. With a combi the high power is there just to get the hot water output up...hence the oversizing. For heating a modern boiler will modulate down to as low as 6-8kw once the heating achieves target temperature, and in the settings an RGI can often adjust output anyway to suit an installation...or just turn the boiler stat down a bit if the boiler does not have menu settings.

My own system in an 1885 solid stone wall house with 12 radiators is run by an 18kw heat only boiler with a hot water cylinder on a Y plan for those that know (Its a condensing boiler that is now 23 years old. We put it in in 1992) and we might have got away with the 12kW version, but that would have been pushing it in the very coldest weather.

I keep wanting to replace the boiler, but apart from a fan and a condensate siphon kit and a case seal, it has stubbornly refused to break down, which is one reason I stick to heat only.....i.e.simplicity.

Being a fuddy duddy I still use a kind of circular slide rule called a Mears Calculator to work out my heat losses. Whatever their foibles all these heat loss calculators get you into the right kind of ball park for sizing purposes. The rest involves using common sense.