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Author Topic: RHI and the SPF - Misleading information??  (Read 4027 times)
andyrob
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« on: November 14, 2014, 10:59:39 AM »


Hallo all

I've been trying to calculate the impact of the SPF on my RHI return so that I can choose between a GSHP and pellet boiler. You'll know that the impact of the SPF on a heat pump is significant in terms of the RHI so this is important when making the choice.

On the website of an MCS product certified manufacturer the following is written.  (If the text below is copied/pasted into  Google you may well find them but I'd prefer not to name them here, just yet anyway.)

If you scroll down to immediately below the 'star chart' on their relevant page you see this:

There are a couple of ways in which this data can then be interpreted:
You could take the lowest room SPF and use this as the overall property SPF
Combine all the room SPFs and then divide it by the number of rooms to get an average SPF


On reading more about this calculation elsewhere, in the MCS SPF calculator for example, it was neither clear nor obvious as to whether the second of the above calculations can be used. Indeed, when I looked for this information again last night I could find no other source of an 'average SPF' method.

The benefit of being able to use the room-average method is that if you have one 'bad' room then it is not the SPF of that room which is used as your whole property's SPF. This means your RHI payments are higher which makes the heat pump a more viable financial proposition vis-a-vis the wood pellet boiler which, at present using the 'worst room SPF' for the heat pump, is the better bet.

I have two such bad rooms - both upstairs in a big house which I deliberately want to keep as cold rooms as I use them as exercise rooms. They're not big enough to use as bedrooms although I suppose you might just get a single bed, chair and cupboard in them. Even if we sold up, new owners would never want to use them as bedrooms and I seem to remember the EPC survey treated them as utility rooms or something similar.


Anyway, the main question is this:

Is their any official evidence anywhere that a room-average SPF can be used?

Thanks, Andy


PS as a curve-ball, I understand that upstairs rooms in a domestic property have to be either bedrooms or bathrooms - how does this apply to an 'upside-down' house I wonder?

« Last Edit: November 16, 2014, 10:13:08 AM by andyrob » Logged

12 x 327w Sunpower (4kw) PV, 3 x 10kw WBS and an all-glass conservatory with hot air extraction and circulation.
BruceB
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« Reply #1 on: November 14, 2014, 12:13:45 PM »

From memory you have to use the worst room spf figure. It is in the rhi rules.

Edit:
Go to this ofgem page:
https://www.ofgem.gov.uk/publications-and-updates/heat-pump-spf-calculation-template
Follow the link to the spreadsheet
Go to the second tab
And it states worst performing room...
That is for recalculating spf for a legacy installation, but that principle is what is applied for all

Edit again:
Picking up your last point, there is no rule that says an upstairs room has to be a bedroom, bathroom (or toilet).
I can well imagine some might interpret it that way though.
All BS EN 12831 requires is that an appropriate room design temperature is chosen.
Examples are given in the national annex and reproduced in MIS3005 v4, page 13.
Personally I would have no problem in principle with a design temperature of say 16 degrees for an exercise room.
However, whereas that would be easy to agree with a client in a large architect designed house, it begins to look dodgy if a client is claiming it for a badly insulated ex-bedroom in a 4 bed semi.
Where you lie on that spectrum is a matter for you and your MCS installer, but you both must be willing to argue the case if challenged in an RHI audit or MCS assessment.


« Last Edit: November 14, 2014, 02:26:27 PM by BruceB » Logged
Ivan
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« Reply #2 on: November 16, 2014, 12:39:31 AM »

Bruce confirmed this to me previously, and sent me the link, which I read, though I cannot remember exactly where in the MCS standard it was. It is based on the worst room in the house, though obviously from an end-user's and installer's point of view it would be much nicer if it were based on the average.
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andyrob
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« Reply #3 on: November 16, 2014, 12:01:40 PM »

Thanks for the replies so far.

I want to use RHI funding for a GSHP but the requirement to be able to heat all rooms with this source only to temperatures that are excessive for my needs (because of 3 standalone wood stoves and Aga) is frustrating because of the cost of additional emitters that I must have.

But rules are rules so what can I do to maximise my overall SPF in addition to fitting bigger radiators and insulating walls?

Are different rooms allocated different default temperatures - eg living rooms 21c, bedrooms 18c, hallways 17c  or whatever, and can my 'bad' rooms be designated as something like unheated/unused rooms so that neither is counted as the worst performing room in the SPF calculation?


  
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Ivan
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« Reply #4 on: December 15, 2014, 12:27:15 AM »

I don't think you'll get away with lowering the requirements for 'unheated rooms' - because MCS wants to make sure that the house is fit for purpose when you sell it to someone else who may want to use those rooms (they'd then start questioning the validity of MCS itself, which is the last thing they want!).

Insulate, insulate, insulate! That will not only bring down your heat requirement and therefore your radiator sizes, but it will also save you money every year. If you have solid walls, you can get 4k towards the cost of insulating them, though I think they delayed this until february(?). There's also 1k for other insulation works (eg underfloor, insulated doors etc). Even with no funding, insulation is still worth doing in the long run. We insulated underfloors, heavily insulated walls (120mm phenolic board) and almost 1/2metre of insulation in the loft (PUR insulation). This has helped massively in terms of cutting heat loss. The existing radiators in the house were good for about 40kW, but we had several rooms where the radiators weren't quite achieving the 4.3x oversize that we were looking for, so we ended up spending around 700 on radiators to get every room up to 4.3x oversize or higher. In my view unnecessary, but we did it for MCS. According to the MCS emitter guide, our system runs at 40C (based on the worst room oversize factor), given the heat input and radiators we have in the system. In practice we're seeing circulation temperatures of around 32C, as some of the rooms are rather more than 4.3x oversize - which gives performance better than the assumed underfloor heat delivery temperature of 35C.
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