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

Last Plumber said:

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.

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magician2905 said:

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

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gpbeck said:

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.

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Last Plumber said:

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.

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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.

• 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)

• 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

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)

John.g said:

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.

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magician2905 said:

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.

Click to expand...

magician2905 said:

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

Click to expand...