Posted by: @rob-of-york

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In sizing an ASHP, can the heat loss from those rooms be omitted from the total, and still mean that I am eligible for the BUS grant, with the ASHP sized to meet the heat loss for the remainder of the house?

No.

The regulations are here  Regulation 9(2) b is the key one

Posted by: @rob-of-york

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In practice, I want the ASHP to be able to cover the space heating and hot water requirements for the whole house under normal winter conditions, for which 6kW should suffice; however, if we get extreme weather, below the design temperature of -3C, then I have no issue with having to turn on the air-to-air to supplement the ASHP.

My preference would be for a correct sized ASHP, rather than an oversize one, with all the extra costs and issues that can entail. Being able to specify a 6kW or 7kW ASHP would give me a lot of choice.

As your robustly assessed heat loss is about 6kW, with the right installer (you may need to shop around) a 6kW or certainly a 7kW should be doable.  The requirement is interpreted that the heat pump covers the loss at the design temperature, not in extreme conditions.

You need to find an installer that will take notice of the measurements!  When I did my final round of seeking quotes (by which time I had got the measure of the industry) I was quite specific about what I wanted including the capacity, no buffer, no external controls etc.  My heat loss calculations were included as an appendix to the enquiry.  Of course some installers tried arrogantly to tell me that I didn't know what I was talking about, and were politely shown the door.  In the end you are the customer.

We have electric underfloor heating in our reasonably large kitchen.

Initially our surveyor calculated the heat loss of the kitchen and used the specifications of the electric underfloor heating to determine if it met the heat loss, which it did.

However, once the design team took a look they immediately rejected it due to the bus scheme requirements.

They pushed this back saying that I need a radiator installed in the kitchen to cover the heat loss. They couldn't/wouldn't include the electric underfloor heating as part of the heating requirements.

This nearly caused a complete abandonment of our install, as fitting a rather large radiator or 2 in the kitchen was completely impractical.

In the end I asked them to increase the design temperature from 40c to 50c and then make an allowance for heat transfer from the hallway radiator.

This thankfully just passed the heat loss requirement of the whole house. They accepted that in practice I would obviously be using the underfloor heating in the kitchen, but this covered their backs with the regulations, I even had to sign a waiver that I accepted this workaround.

Our install was designed in December 25, so things could have changed.

Posted by: @jamespa 

The regulations are here  Regulation 9(2) b is the key one

Thanks for the pointer to the regulations, in particular:

9(2)b "it is capable of meeting the full space heating and hot water heating demands of that property"

That's clear, the heat pump has to meet the full heating demands, regardless of any other heating systems present. So as you say, it's then about finding a suitably enlightened installer who is willing to take some notice of measurement data and put the work in to get an accurate model of the heat loss taking into account how the house actually is rather than using defaults for that age of property etc.

It's easier when you are simply the customer and can specify exactly what you want, somewhat less so when there are hoops to be jumped through to access the BUS grant. Nevertheless, nobody is going to get the work without me being completely happy with the design. Giving them a big hint by including heat loss calculations in the inquiry is an excellent suggestion. 

Posted by: @bash 

They pushed this back saying that I need a radiator installed in the kitchen to cover the heat loss. They couldn't/wouldn't include the electric underfloor heating as part of the heating requirements.

This nearly caused a complete abandonment of our install, as fitting a rather large radiator or 2 in the kitchen was completely impractical.

In the end I asked them to increase the design temperature from 40c to 50c and then make an allowance for heat transfer from the hallway radiator.

The new radiators that I have planned should easily meet the room-by-room heat loss at a 42.5C flow temperature corresponding to 37.5C radiator temperature (DT10) with the exception of the hallway which is at about 90% of what is required. I think there is an easy argument there that the extra heat comes from the adjacent rooms, as happens in practice now. Upping the flow temperature to 45C fixes this completely. The only other room of interest is the kitchen-breakfast room. This was originally two rooms, but now they are connected by an open archway. The kitchen has no radiators at present and no space for any. However, the breakfast room will have enough radiator capacity for both. I expect this is non-issue as it will be treated as a single room.

Is there a specific design temperature e.g. 40C that is supposed to be used? (My planned radiators would theoretically work at 40C flow temperature, if the system could run at DT5, but I am very doubtful about that due to the pipework. Designing around DT10 would likely work fine, with a small compromise on efficiency). 

Posted by: @rob-of-york

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Is there a specific design temperature e.g. 40C that is supposed to be used? (My planned radiators would theoretically work at 40C flow temperature, if the system could run at DT5, but I am very doubtful about that due to the pipework. Designing around DT10 would likely work fine, with a small compromise on efficiency).

No, but the heat pump must be capable of delivering a SCOP of at least 2.8, which for most means max 55C.  You are well within that by the sound of it so not a problem.  Just a case of finding a suitable installer. 

I found that things had improved considerably from when I first first did a request for quotations (early in 2023 I think), to when I did the final, successful request, mid 2024.  I get the impression that things have got better still, for example some  installers now offer sensible cost/performance trade offs when comes eg to replacing radiators/cylinder, whereas previously 'everything has to go come what may' seemed to be the mantra.  There are still cowboys and chancers out there, too many it seems judging by what we see here, but at the same time its clear that the better ones have matured their understanding.  

Of course even the good ones still have to do the MCS calculations, its just that they have ways of adjusting the assumptions to fit reality.

We currently have a gas boiler used for central heating and hot water, two A2A units capable of heating and cooling, and a separate gas fire in the living room.

The legislation for the BUS grant here says in Section 9(2)(c) about the heat pump where

(c) it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—
(i) supplementary electric heater, including any immersion heater,
(ii) circulation pump, or
(iii) solar thermal collector.

My question is what constitutes the "original heating system installed in that property"? In particular, would that be interpreted to mean just the central heating system, including gas boiler, pipework, circulation pump, hot water tank and radiators, or would it also include the separate gas fire?

The gas fire is typically used for no more about 10 minutes each year to check that it still functions OK and to burn off any accumulated dust. However, I don't want to get rid of it because when we have a long power cut it saves the day, since it will light and run without any electricity supply. It's enough to keep us nice and warm in the living room while it is freezing outside and the rest of the house drops in temperature.

We have an EPS on our Solar PV, which can run the ground floor lighting circuit and one 13A socket in the event of a power cut. While very useful, the EPS doesn't suffice for any meaningful heating.

Posted by: @rob-of-york

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We currently have a gas boiler used for central heating and hot water, two A2A units capable of heating and cooling, and a separate gas fire in the living room.

The legislation for the BUS grant here says in Section 9(2)(c) about the heat pump where

(c) it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—
(i) supplementary electric heater, including any immersion heater,
(ii) circulation pump, or
(iii) solar thermal collector.

My question is what constitutes the "original heating system installed in that property"? In particular, would that be interpreted to mean just the central heating system, including gas boiler, pipework, circulation pump, hot water tank and radiators, or would it also include the separate gas fire?

The gas fire is typically used for no more about 10 minutes each year to check that it still functions OK and to burn off any accumulated dust. However, I don't want to get rid of it because when we have a long power cut it saves the day, since it will light and run without any electricity supply. It's enough to keep us nice and warm in the living room while it is freezing outside and the rest of the house drops in temperature.

We have an EPS on our Solar PV, which can run the ground floor lighting circuit and one 13A socket in the event of a power cut. While very useful, the EPS doesn't suffice for any meaningful heating.

Its a good question. Th definition of 'original heating system' is given in section 2:

original heating system” has the meaning given in regulation 5(1)(c)(ii);

and 5(1) 

5.—(1) For the purposes of these Regulations, a property is an eligible property where—

(a)it is a building,

(b)it is not social housing,

(c)if it was, or will be, first occupied or used before the date on which the relevant eligible plant is commissioned—

(i)it meets the requirements in regulation 6 (insulation requirements), and

(ii)the system providing space heating or water heating, or both, of the property prior to the installation and commissioning of the eligible plant (the “original heating system”) is—

(aa)fuelled by fossil fuel(1), or

(bb)an electric heating system, and

(d)if it will be first occupied or used on or after the date on which the relevant eligible plant is commissioned, it meets the requirements in regulation 7 (requirements for new build properties).

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Which is almost entirely unhelpful.

So the question becomes, is the word 'system' significant?  Specifically does a system mean a set of interconnected cooperating components (which I would suggest is something like the usual definition), or 'any component performing the function'.

The legislation doesn't offer any insight into that so it would be down to a court to determine, I would suggest.  In coming to a conclusion the court might look at relevant case law, whether the component was essential to the heating of the house or the room (which you say it isnt), or only incidental, but also perhaps the intent pf the legislation which is to replace fossil fuels.  A similar question would probably apply to a standalone coal fired stove.  

I personally wouldn't want to guess how a judge might be persuaded on this matter.  In practice it probably depends on what your installer thinks!

Thanks @jamespa

The gas fire is definitely not an essential part of the heating system, since the gas central heating manages to cover the full heat loss of the house comfortably. Neither could it be referred to as original, since it has not been there since the house was built. It's not part of the gas central "heating system" in the usual sense of the word system. And it was categorised as supplementary heating when teh EPC was done. That said, the regulations don't appear to offer any clarity on this.

Does anyone have first hand experience of this situation, i.e. keeping a gas fire and receiving the BUS grant for gas boiler replacement?  

@rob-of-york Yes! We had the gas boiler based CH system removed and received the £5K BUS Grant. We kept a gas fire as a fallback but later, when we had the gas pipe capped and the meter removed, we also regained some space where the gas fire had been. No mention of the need to get rid of the gas fire was made by our surveyors, nor by the installers we finally chose; the BUS grant went through no problems at all. We also have a MyEnergy Eddi that feeds power to the Sunamp Thermino DHW heat battery (no ASHP power used for this) from solar and / or grid power; this did not cause any ructions in the MCS world either. Regards, Toodles.

Posted by: @rob-of-york

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Does anyone have first hand experience of this situation, i.e. keeping a gas fire and receiving the BUS grant for gas boiler replacement?  

We kept ours and Heat Geek would have been in touch if the BUS for the install that they did a couple of months back had any glitches.