Is my boiler oversized? If so, what can I do?

[btjtaylor]

That can't be right!
If it's a 4 bed detached of the size you say that it is, I haven't seen it.
If you came back with 18Kw, I'd think we were on the same page.

Not for a new build, surely? (Assuming they've actually built it properly, but I can only work out heat loss on the assumption they have put all the proper cavity insulation in etc).

Unless I have totally misunderstood something here

 

[Last Plumber]

Not for a new build, surely? (Assuming they've actually built it properly, but I can only work out heat loss on the assumption they have put all the proper cavity insulation in etc).

Unless I have totally misunderstood something here

Heat loss per room? Is that how you are doing it?

 

[ShaunCorbs]

Also double check you’ve actually got cavity insulation

 

[Last Plumber]

(Assuming they've actually built it properly, but I can only work out heat loss on the assumption they have put all the proper cavity insulation in etc).

We all presume things like that are done.

 

[Last Plumber]

I suspect you've calculated heat loss from the building as if it were an empty box?
It doesn't work like that. Calculate every room and add them together.

 

[Chuck]

Around 7kW is a fairly typical minimum output for a run of the mill domestic combi. Lower minimum outputs are available, e.g. Viessmann, if you hunt around but not all are opentherm-friendly.

If you really do only need a few kW to heat the place even in cold weather, think carefully about the cost-benefit of complicated control systems. If you're only using a couple of hundred quid of gas a year on heating it doesn't make much sense to spend a couple of grand to save, maybe, 10%, of this.

 

[btjtaylor]

Thanks for your help/suggestions so far all

I've double checked and corrected a few things and i'm still getting "only" 7.25KW heat loss

I used this spreadsheet linked above by Desktop987456 to calculate as to the best of my available info (for example - I know for a fact this house has "block on beam" (therefore suspended)? floor but I dont know if it has any insulation

I added internal/external wall lengths together for the spreadsheet just to avoid having to enter a huge number of individual walls but hopefully that hasn't affected the calculations much

My version:

 

[btjtaylor]

Maybe this spreadsheet is a load of the proverbial though so if someone can suggest an alternative I'll give it a go!

 

[Chuck]

I'm not sure that all this effort is being concentrated on the right question. If you are staying with a standard single combi boiler its spec needs to be:

Maximum power: enough to meet the DHW peak demand
Minimum power: as low possible

If you aren't happy with the boiler cycling once demand falls below its minimum, which is normal behaviour for domestic systems, then you'll need to use a some other heat source for the CH (e.g. electricity, second small boiler, heat pump) or install a buffer tank.

As I said before, there's no point in spending several thousand to save a couple of hundred. Don't forget that second heat sources will add to servicing costs and may not end up saving any money in practice.

 

[btjtaylor]

I'm not sure that all this effort is being concentrated on the right question. If you are staying with a standard single combi boiler its spec needs to be:

Maximum power: enough to meet the DHW peak demand
Minimum power: as low possible

If you aren't happy with the boiler cycling once demand falls below its minimum, which is normal behaviour for domestic systems, then you'll need to use a some other heat source for the CH (e.g. electricity, second small boiler, heat pump) or install a buffer tank.

As I said before, there's no point in spending several thousand to save a couple of hundred. Don't forget that second heat sources will add to servicing costs and may not end up saving any money in practice.

Thanks for that - that's more what I was looking for really with the second part of my question - that helps, thanks. You are right - those kind of measures I'd likely never recoup the cost.

Another angle to this is that I wanted to check that the housebuilder had met all their obligations under the latest "boiler plus" regs as there were amendments made in June 2022 which try to deter this poor system design/oversizing problem - as there was a grace period up until June this year, they may have just slipped through. I just don't like the way that building companies that make ludicrous profits can get away with just blatantly failing to meet regs, ultimately potentially costing the homeowner money in premature boiler failure (due to excessive cycling) and wasted gas.