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.

 

[magician2905]

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.

We had a Heat Loss Calc carried out by Octopus Energy for an ASHP they came back with 4kw ASHP

The current radiator sizing would suggest a HLC of 8.5kw. It's currently heated by an Ideal Logic H15+ Heat Only Boiler & 180L Hot Water Cylinder

We worked out that its likely to be around 6kw HLC.

The difference, between 6 and 4, appears to be the number of Air Changes per Hour (ACH) that they use. Apparently they use an ACH of 2.5?

That's for a 116 square metre, 4 bed detached new build (3.5years ago)

This shows the currently installed radiators, their heat output from Stelrad by size @ Delta 50'C, the Heat Loss Calc per room, the proposed new radiators needed (due to reduced operational temperature).

	Current Radiator	Current Radiator	Current Radiator	Octopus Heat Loss Calculation	Proposed Radiator	Proposed Radiator	Proposed Radiator
Room Description	Description	Type	Stelrad Radiator Watt Rating	Room Heat Loss (Watts)	Type	Description	Output at Design (Watts)	% Coverage
Bedroom 1	H450 x L700	K1/11	538	282	P+/21	H450 x L800	350	124
Bedroom 2	H450 x L600	K1/11	461	217	K1/11	H450 x L800	244	112
Bedroom 3	H450 x L600	K1/11	461	278	K2/22	H450 x L600	336	121
Bedroom 4	H450 x L500	K1/11	384	258	K2/22	H450 x L500	278	108
En-suite	H600 x L500	K1/11	500	185	K1/11	H600 x L600	197	106
Entrance Hall	H600 x L500	K1/11	500	296	K2/22	H600 x L500	348	118
Family Bathroom 1	H600 x L600	K1/11	600	206	P+/21	H600 x L600	260	126
Kitchen/Diner	H300 x L1500	K2/22	1518	707	K2/22	H300 x L2000	702	99
Kitchen/Diner	Plinth Heater		1100	N/A	K1/11	H600 x L400	-	142
Landing	H600 x L400	K1/11	400	117	K1/11	H600 x L400	166	142
Living Room	H600 x L600	K1/11	600	591	P+/21	H600 x L600	276	47
Living Room	H600 x L600	K1/11	600	591	K2/22	H600 x L600	355	60
Office	H300 x L1000	K1/11	517	227	K2/22	H300 x L800	276	122
Utility	H450 x L500	K1/11	378	76	K1/11	H450 x L500	159	209
		Total	8557	4031			3947

 

[ShaunCorbs]

What’s there design temp and heating circuit dt eg 40 dc flow at dt of 5 ?

 

[John.g]

Looking at one rad above, a k22 450X600, at a 50deg rating = ~ 750W, required is 336W which is a "27deg" rad with flow/return temps of 49C/45c to give a dT of 4C. at a room temp of 20C.

 

[gpbeck]

If you’re worried about boiler cycling (experimentally only an efficiency hit of ~3%) it would be best to lower flow temps to the minimum compatible with your heat loss. If your rads are double what would be needed at delta50 then that would suggest a design delta of 30 or so, maybe flow 55 return 45 so close to new recommendations. This will condense pretty well and with simple weather compensation lowering temps further in milder weather overall efficiency should be fine.

So just sort weather compensation, nothing else needed.

 

[Last Plumber]

We had a Heat Loss Calc carried out by Octopus Energy for an ASHP they came back with 4kw ASHP

The current radiator sizing would suggest a HLC of 8.5kw. It's currently heated by an Ideal Logic H15+ Heat Only Boiler & 180L Hot Water Cylinder

We worked out that its likely to be around 6kw HLC.

The difference, between 6 and 4, appears to be the number of Air Changes per Hour (ACH) that they use. Apparently they use an ACH of 2.5?

That's for a 116 square metre, 4 bed detached new build (3.5years ago)

This shows the currently installed radiators, their heat output from Stelrad by size @ Delta 50'C, the Heat Loss Calc per room, the proposed new radiators needed (due to reduced operational temperature).

	Current Radiator	Current Radiator	Current Radiator	Octopus Heat Loss Calculation	Proposed Radiator	Proposed Radiator	Proposed Radiator
Room Description	Description	Type	Stelrad Radiator Watt Rating	Room Heat Loss (Watts)	Type	Description	Output at Design (Watts)	% Coverage
Bedroom 1	H450 x L700	K1/11	538	282	P+/21	H450 x L800	350	124
Bedroom 2	H450 x L600	K1/11	461	217	K1/11	H450 x L800	244	112
Bedroom 3	H450 x L600	K1/11	461	278	K2/22	H450 x L600	336	121
Bedroom 4	H450 x L500	K1/11	384	258	K2/22	H450 x L500	278	108
En-suite	H600 x L500	K1/11	500	185	K1/11	H600 x L600	197	106
Entrance Hall	H600 x L500	K1/11	500	296	K2/22	H600 x L500	348	118
Family Bathroom 1	H600 x L600	K1/11	600	206	P+/21	H600 x L600	260	126
Kitchen/Diner	H300 x L1500	K2/22	1518	707	K2/22	H300 x L2000	702	99
Kitchen/Diner	Plinth Heater		1100	N/A	K1/11	H600 x L400	-	142
Landing	H600 x L400	K1/11	400	117	K1/11	H600 x L400	166	142
Living Room	H600 x L600	K1/11	600	591	P+/21	H600 x L600	276	47
Living Room	H600 x L600	K1/11	600	591	K2/22	H600 x L600	355	60
Office	H300 x L1000	K1/11	517	227	K2/22	H300 x L800	276	122
Utility	H450 x L500	K1/11	378	76	K1/11	H450 x L500	159	209
		Total	8557	4031			3947

I have only glanced at this so please bear with me.
The total heat loss from your house according to the Octopus heat loss calculation is 13753 Btu/hr? (4.031kw). Is that what you are saying? Is the 116 m2, the footprint of the house or the total floor area up and down added together? I'm asking because I personally don't think of houses in meters squared.

How big is bedroom 1?
Length breadth and height and how many outside walls? is it upstairs on a suspended wooden floor?
Just so I can gauge the calculations you have been given. I presume you're wanting 65F (18 C) or thereabouts for a bedroom?

What is the CoP of the heat pump they suggested at 4kw?

Does the current 15Kw boiler do its job well when it is below freezing outside?
How do you find the rooms of the house in those conditions?
What temperature is your boiler set to in those conditions?
If the 116 M2 is up and down together, the 15KW boiler is what I would expect if not the next size up.

Just trying to understand a bit more. my gut feeling is that something is out with the calcs but I could be wrong, I haven't seen the job.

 

[John.g]

If you’re worried about boiler cycling (experimentally only an efficiency hit of ~3%) it would be best to lower flow temps to the minimum compatible with your heat loss. If your rads are double what would be needed at delta50 then that would suggest a design delta of 30 or so, maybe flow 55 return 45 so close to new recommendations. This will condense pretty well and with simple weather compensation lowering temps further in milder weather overall efficiency should be fine.

So just sort weather compensation, nothing else needed.

Efficiency loss due to cycling is very little, around 1.5% if firing time is 3 minutes or so.

 

[magician2905]

I have only glanced at this so please bear with me.
The total heat loss from your house according to the Octopus heat loss calculation is 13753 Btu/hr? (4.031kw). Is that what you are saying? Is the 116 m2, the footprint of the house or the total floor area up and down added together? I'm asking because I personally don't think of houses in meters squared.

How big is bedroom 1?
Length breadth and height and how many outside walls? is it upstairs on a suspended wooden floor?
Just so I can gauge the calculations you have been given. I presume you're wanting 65F (18 C) or thereabouts for a bedroom?

What is the CoP of the heat pump they suggested at 4kw?

Does the current 15Kw boiler do its job well when it is below freezing outside?
How do you find the rooms of the house in those conditions?
What temperature is your boiler set to in those conditions?
If the 116 M2 is up and down together, the 15KW boiler is what I would expect if not the next size up.

Just trying to understand a bit more. my gut feeling is that something is out with the calcs but I could be wrong, I haven't seen the job.

Not attempting to hijack this thread was just providing some detailed information for the OP in respect of his own HLC.

116 square metres is the Total Floor Area (from the EPC)

Attached are all the dimensions and a HLC produced using the Stelrad Basic HLC available here originally - BTU Calculator | Heat Loss Calculator For Your Home - https://www.stelrad.com/advice-hub/heat-loss-programmes/

We had a bit of a go at the developer about this for a number of reasons.

1. The Lounge is the only room that feels cold during winter. The 2 radiators are installed against internal walls as the room has a large DG PVCU French Door. As already mentioned these are 600 x 600 Type 11 (K1) and would appear to be undersized for the room.
2. The upstairs heating has only been turned on 2 or 3 times in the last 2 years.
3. The developer claimed that the floor construction is "Jetfloor insulated beam & block" floors (giving a U value of 0.15W/m²K)
4. The developer also claims that 400mm of quilt insulation is installed in the loft zones (giving a U value of 0.11W/m²K).
   NB. this is beyond the building regulations in effect at the time and is actually around 270-300mm (two bats of earth wool rolled in opposite directions)
5. The First floor construction is unknown but would be whatever was required in 2018.
6. The system was designed by Ideal/Stelrad for Taylor Wimpey with a design temperature for the heating system
   80'C Flow
   60'C Return
   (there is/was a loophole in the building regulations for HEAT ONLY boilers that permitted installation at design temps outside of the condensing parameters of the boiler. This may or may not have been closed with the recent regulation changes).
7. One thing that will 100% impact any future ASHP would be the 10mm Hep2O plastic pipe that has been used in the drops to all radiators.

Octopus quoted the following

• ASHP: Daikin EDLA04E2V3
• Flow Temperature 46'C
• MCS SCOP Heating 3.65
• MCS SCOP Hot Water 2.6944999999999997
• HP System Electricity Consumption Heating: 2740 kWh (Current using Boiler: 10,000 kWh)
• HP System Electricity Consumption Hot Water: 894 kWh (Current using Boiler: 2,000 kWh)
• HP System Electricity Consumption Total: 3634 kWh (Current using Boiler: 12,000 kWh)

Our Actual energy consumption for Gas, August 2022 to August 2023, was 6500kWh.

Even using our actual consumption and adjusting the figures

• Heating: 4400 kWh @ 3.65 SCOP = 1204 kWh
• Hot Water: 2100 kWh @ 2.695 = 779 kWh
  • Plus 52 x Anti Legionella Cycles (3kWh Immersion x 250L x 10'C) = 152 kWh
  • Gives a Hot Water Total: 931 kWh
  • Total Annual Electricity = 2135 kWh

We didn't go ahead for a few reasons:

1. We were not particularly confident in the HLC carried out by them
2. The SCOP for heating looks kind of low but the 10mm Hep2O plastic pipe is likely a factor
3. The SCOP for Hot Water is terrible.
   I suspect that a better designed system, paired to a quality or custom cylinder, should improve that to 4 or more
4. Based on our consumption and those SCOPS it would cost us more per year to run an electric ASHP than it currently does using Gas in addition to the capital outlay for converting.
5. I am certain that we can improve our gas consumption by
   • Increasing the size of the radiators (so we can reduce the flow temp)
   • Adding Weather Compensation
     This has an extra benefit of decoupling our Heating and Hot Water temperatures and allowing the hot water temp to be determined by the cylinder stat. Currently both are determined by the boiler temperature setting of 64'C (cannot go lower as the hot water becomes tepid rather than hot)

It would seem that we are going to have to wait on improvements in tech or the decoupling of Gas & Electricity prices before moving to an ASHP.

I have seen Vaillant have a new, internal, Heat Pump Cylinder (fluoStore?) that was shown at the German ISH 2023 Show that has a SCOP of 5.

 

[John.g]

The reason for the poor HW SCOP is possibly because the HW is heated to 60C, some systems heat it to 40C and boost it up once/week to 60C for legionella protection.

 

[magician2905]

The reason for the poor HW SCOP is possibly because the HW is heated to 60C, some systems heat it to 40C and boost it up once/week to 60C for legionella protection.

40'C isn't hot enough for anything really (that's only 2-3 degrees above body temp)

No, the Hot Water temperature target was 50'C and heated to 60'C once per week for Anti Legionella (which is why I added the energy required to raise 250litres by 10'C once per week using a 3kw immersion element).

I expect the very low HW SCOP is mostly due to using a crappy HW Cylinder.

Whilst a "Heat Pump" HW Cylinder generally has a larger coil internally it can always be better. Using a dual coil cylinder would improve the Hot Water SCOP as bigger surface area = better heat transfer from medium to medium. This could be plumbed in a couple of different ways - in series or in parallel - not much difference in either method.

Perhaps you are thinking of the heating circuit - that is designed to run at 46'C. I guess "potentially" with larger radiators this could be lowered but I think the major issue here is the 10mm microbore.

Also they never gave me a design so I didn't know for certain whether they would have to use a "volumiser cylinder" to prevent cycling of the Heat Pump again due to the 10mm micro bore.

 

[ShaunCorbs]

Or just plate load the cylinder