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

[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.