Hello @Simonwhiteley and welcome to the forum! The MCS standard does not prohibit using accurate assumptions to produce an accurate heat loss. There’s more flexibility than some claim.
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Yes- I have another potential installer that seems to be taking that line. But I read the legislation and the Ofgem guidelines and it puts the onus on the installer to satisfy themselves that the installation meets the condition, at risk of having to repay the BUS if they are challenged. I can see why some installers are cautious.
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I think it should be remembered that the internal temperatures used for the BUS grant are to ensure that the property is adequately heatable, not just for the current owners, but for future owners.
It is a grant for the heating installation in the property rather than a gift to the homeowner.
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Agreed about the onus being put on the installer to justify and it’s totally understandable that some will be cautious. The MCS standard should spell out the most accurate assumptions to use with allowance for deviation of course based on installer judgment.
Looking at the limited Electrification of Heat results and from a ground source perspective, there are certain manufacturers that offer fixed speed oversized compressors, because its the only size, matching overestimated heat loads, with poor granular controls and less enthusiastic end users that produce some alarmingly low SCOP’s, far from manufacturers promises. But designers will protect PI by matching CIBSE design guides. MCS assessors will follow MCS guidelines and the poor end user ends up with an honest complaint . I have had my 12kW S1145 Nibe for over 10 years and its been faultless, its oversized because we still haven’t completed the extension, but I put a big buffer in and run the house as we want at 10-19c with a log burner and looking forward to seeing the results that I can access using the open energy monitor and not just MyUplink
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Agreed, and my installer did substitute my real air-change data into my survey after I spoke with MCS who said they will accept real data as well as spreadsheet assumptions.
But the point remains that qualification for the BUS grant is conditional on the HP being ANY amount larger than 100% of the heat loss.
Given the results of the EoH trials and OEM’s other analysis that oversizing is widespread and a significant drag on efficiency, MCS may review their BUS guidelines. They could, for example, add a second condition saying the BUS grant will be tapered away for HPs that exceed, say, 140% of the actual heat loss.
That’s my view. Given that both heat loss and heat pump output estimates are subject to significant uncertainties (that are probably not resolvable by better standards etc- they are just inherent) it seems odd that an installer must be able to produce an estimate that says that fuzzy number 1 must be bigger than fuzzy number 2, or lose £7,500. With significant financial and accreditation risk if their assessment is determined to be incorrect.
Changing the BUS regulations to say that heat pump size must be approximately equal to heat etc demand would go quite a long way to resolving the issue. Or make the grant scaleable - eg 80% for 80-100% of demand, 0% below that.
Real best being enemy of the good here imv.
I get that.
It is a bit raw for me: we are one of the difficult cases that the industry needs to crack to get high uptake: solid wall terrace house.
With great difficulty I have found a house location and HP that meets planning requirements, without major works, and think I have a workable plan to upgrade the radiators. And some potential installers who seem to know what they are doing. But it has taken a lot of effort.
There is one HP on the market that would meet a larger output requirement and be quiet enough, but would require adjusting the plans for our new extension. So I have to navigate between the architect, structural engineer and installer to fit it in. And persuading my wife that it will all work fine.
Having to exclude installers not just on competence, technical quality of proposal or price of the install, but also on how they interpret some fuzzy rules, is a complication I don’t need. Which is why I don’t feel much obligation to future owners (as we are making adjustments to make the house right for the next 25 years, a remote prospect anyway).
And to fit the larger HP, we will need to wait for the new PD rules on size of the HP, or run the risk of seeking specific planning permission.
Quite close to chucking it all in and sticking to gas.
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@Simonwhiteley would you be happy to start another thread to discuss your calculated heat loss and proposed heat pump size and system design? We might be able to help with our experience here…
I understand Simon.
But think about it, you could have your heat pump installed, exactly as you want it, and then move out 3 months later.
You will have had a grant for £7,500 for a heating system that doesn’t meet what the grant provider considers to be a reasonable specification for the average person.
There has to be some level of operation attached to the grant, it can’t be just a gift because you have installed a heat pump, it has to work too.
Of course, if you or anybody else wants to design to their own specifications they can and just do it without the grant.
I think some people do and just self install.
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Thank you. I already have had some useful input from a post I made over on the other place (ahem), so I am good for the moment. I might add my defrost losses estimate to the thread on output estimates in due course.
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Ooo, were is that? 
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… but if that happens it still leaves a potential new owner with a non-standard system and unable to apply for BUS.
So, that’s the same with any house purchase, you buy what is there. It won’t have been partly funded by tax payers in the same way that an avocado coloured bathroom suite that the new owner hates won’t have been funded by tax payers. The new owner will take it into account in their offer to purchase.
If tax payers are funding the heating system then it needs to be suited to any potential beneficiaries, not just one person.
The point is that it’s a grant for the property rather than just the current owner.
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Going back to the heat loss calculation process. As we’ve discussed re the grant and trying to match fuzzy number 1 the heat loss and fuzzy number 2 the heat output capacity of the unit. I think thinking of the first as a particularly fuzzy number is a useful way of looking at this.
Here’s the process I try to follow:
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Calculate a more likely to be reflective of real world design temperature heat loss first. Just use an air change rate of 0.5-0.6 ACH for this for all rooms in the dwelling (assuming basic double glazing has been fitted sometime in the last 20 years). If terraced or semi-detached, assume neighbours are heated to 18C if you do have neighbours occupying the properties next door. This comes out at 3.3 kW for our solid stone mid terrace. This is the heat loss that is most likely to correspond the maximum heat demand on the coldest day in reality, in kWh this is 3.3 x 24 = 80 kWh of heat.
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Use any information that you have on previous heating, ideally daily gas smart meter readings if you have them to cross check this figure. Alternatively with annual gas consumption e.g 11,000 kWh of gas consumed (we didn’t have gas previously so this is more an example) divide by 2.9 (re michael podesta conversion factor) = 3.8 kW. Reasonable agreement, we’re just checking here for multi kW differences.
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Applying a 30-40% margin on top of 3.3 kW gives 4.3 - 4.6 kW for the heat pump unit size. In reality that will be a 5 kW unit but the real world max output capacity during defrosts will likely be in the 4.3-5.0 kW range depending on the model and conditions.
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Work out what the margin between 3.3 kW and say 4.3 kW gives you in terms of the capacity to deal with more unusual events such as a combination of a strong wind storm with sub zero conditions. In my case my heat loss would rise from 3.3 kW to 4.5 kW if the neighbors were unheated and the air change rate increased to 0.8 ACH. I would push beyond the capacity of the heat pump if the air change rate was 1.24 ACH and both neighbors were unheated = 5.1 kW (perhaps consider here what other sources of heat you have available e.g a 3kW fan heater removes any concern about a missing few 100W of heat pump capacity, and electric may be cheap and green during strong wind storms, assuming of course that there isn’t a power cut!)
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Finally: Use the most likely heat loss (e.g 3.3 kW in my case) to work out the design temperature of the system and starting point for the weather compensation curve. If you are designing a new system use this heat loss to spec a radiator system that can run at 35-40C flow temperatures at design temp if you can. You can always bump up the curve if needed.
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a more nuanced question is at what maximum accurate heat loss should you consider the 9-16kW daikin over the 8 kW daikin? how close do you sail to the wind there?
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imo keep away from the 9 kw plus unless you have a heat loss of over 12 kw
i had a wrongly calculated heat loss of 11kw ( -2.7 @ 50 degree c) flow
eventually got octopus to install a 9kw heat pump partly based on my own calcs had a estimated heat loss of 4.8 kw and three years of data
unfortunately the 9kw still over heated the house at on its minimum flow and electricity input
seems i got lucky and octopus then fitted a 6kw heat pump just before the start of november and everything now is perfectly acceptable and my estimated heat loss is not far of reality
https://emoncms.org/app/view?name=My%20HeatPump&readkey=1da9e37af3d5ac9669792233ea85675c
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If your heat loss is close to `needing’ a 9kW or 11kW Daikin I would suggest looking elsewhere.
They are not a real size of heat pump.
Who needs 4.5kW of heat as a minimum and 7-8kW when it’s freezing?
On the other hand, if you don’t care too much about efficiency, the 9kW Daikin is really nice, I miss mine if I am honest 
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Maybe they used some software as the EPC provider.