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.

 

[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.

 

[gmartine]

The boiler provided is under spec'ed rather than oversized as even the lesser powered versions of the same boiler as shown by the data sheet in this thread don't efficiently comply with all your building's heat load requirements.

Independent Engineers report to prove your case, give the house builders a chance to make ammends and when they don't have a Viessmann or other hi mod boiler fitted and sue for costs in small claims. You might win.

 

[btjtaylor]

Cheers, currently trying to decide if investing in an indepedent report is the way to go

I have gone round and measured the rads and compared to the specs on the myson website, they barely meet the heat loss requirements of the rooms which tells me that a) my heat loss calc is fairly accurate and b) the system design is almost certainly not up to scratch with the latest efficiency standards

Whilst it's not the biggest problem in the world it's more principal of the thing, i'm not a litigious person by nature but I resent being treated unfairly when regs are there for a reason, regardless of what you think of them. Worst case scenario I can see is that a very diligent surveyor comes along when we are trying to sell and we end up in an argument with a prospective buyer over giving a discount to upgrade rads etc. etc. or at least buying some kind of pointless indemnity policy (!). Or maybe we want/need to fit an ASHP in the future and 14 rads end up in a skip.

I spoke to local building control but apparently they didn't "submit the developer's application" and I should speak to NHBC (groan)... will see how I get on

 

[gmartine]

Did you actually run the heating over winter?

 

[btjtaylor]

Did you actually run the heating over winter?

Yes, it was "ok". The room temperature wasn't really an issue although was a bit uneven across the house despite adjusting TRVs, but I think that is par for the course with just a single thermostat. I wasn't well informed about all of this topic at the time so I didn't really have a handle on what was going on but in retrospect the boiler was cycling a LOT (makes sense) and the gas consumption (n KWH, obviously in £ it was high due to high prices) seemed to be a lot higher than expected.

 

[gmartine]

System might just need balancing and I'd get onto them about connecting the weather comp. That would ensure it's Boiler Plus compliant.