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

[btjtaylor]

Hi all

I've been reading about the Drayton Wiser system and along the way I began to go down a rabbit hole learning about heating system optimisation and efficiency and i've come to the conclusion that the boiler in my property is way oversized for the requirements,

The property is a 4 bed, detached new build property (completed Sept 2022) so should have fairly low heat loss. From the EPC report I have the floor space is apparently 115m2, so even at the "worst case" estimated heat loss I saw online of 40w/m2 the requirement is around 4.6kw. There are 14 radiators, a bath tub and two thermostatic showers (but we wouldn't expect to be able to use them all at the same time). The boiler can generate an enormous amount of DHW (15l/min) but surely the mains supply would struggle to keep up with that?

The boiler that has been fitted is a 36kw Baxi Assure combi boiler, which, if I am reading the specs correctly, has a minimum CH output of 6.7kw, so even on the lowest modulation, it is way too powerful for the installation. To add insult to injury, the builder installed an outdoor weather sensor, but didn't bother to connect it to the boiler (which is against building regs as the installation is not Boiler Plus compliant), but at this point I am not too bothered because I am looking to fit smart boiler controls anyway.

Specs taken from Baxi's website:

My questions:
Is the boiler oversized?
If so, is there anything I can do about it?

I intend to fit the drayton wiser system, connected to the boiler over opentherm and fit smart TRVs on all rads (except the towel rads in 2x bathrooms) - I know this probably won't increase the efficiency too much but it should provide at least better control of the system from a comfort perspective

Thanks in advance

 

[ShaunCorbs]

I wouldn’t go off the epc reports only way to fully know is to do a Heatloss

 

[Last Plumber]

You need to do heat loss calculations for the property as Shaun says.
The high performance on the boiler is there to give the domestic hot water performance but if the mains flow is not up to the job, it is pointless in a lot of situations.

Start with heat loss per room to be most accurate. Again, as above, I personally wouldn't pay much mind to EPC's.

 

[SimonG]

Full heat loss calculations, then go from there. Combi sized for hot water production. Should have been heat only and unvented cylinder if pressures and flow allow.

 

[Desktop987456]

BRE - Calculation tool for design of low temperature domestic heating systems - V1.2

 

[btjtaylor]

Thanks all, when I get a few hours free I'll use the linked template to work this out and come back. Fortunately, on the plans of the property it lists all the room, window, door, and radiator dimensions already so I shouldn't need to do much, if any measuring.

 

[gmartine]

Assuming it's way oversized as you suggest, I'd question the installation of a complete smart control system you propose because all you're doing is further restricting demand and even more efficiency. I'd get the weather comp up and running and let it do it's job with some basic boiler controls and see how that goes, anything more is just lipstick on a pig.

 

[btjtaylor]

I did the heat loss calc and it comes out as 6.73kW which is a bit more than I expected - although the "Design External Temperature" is -5.4c so I am right in thinking this is worst case scenario? It's eerily close to the boiler minimum but I think this is just pure coincidence.

As its detached and probably has a an above average large number of windows and external doors I think this is probably about right.

 

[btjtaylor]

Based on my council tax valuation reduced covered area of 135m2 that works out as 49kw/m2 so a little on the high side but not crazy?

 

[Last Plumber]

I did the heat loss calc and it comes out as 6.73kW which is a bit more than I expected - although the "Design External Temperature" is -5.4c so I am right in thinking this is worst case scenario? It's eerily close to the boiler minimum but I think this is just pure coincidence.

As its detached and probably has a an above average large number of windows and external doors I think this is probably about right.

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.