So I've been discussing with the local company a bit more.

Both their MCS sheert and Aira heat losses agree on the same heating per year expressed in KWh: c 13,000Kwh (which I think is higher than actual as thats more than my total gas for hot water and heating). Given the same space heating KWh, for some reason that same space heating in KWh in the MCS sheet is 11.8KW heat losses, and in the Aira heat loss calcs it is 6.2kw. I have seen the full MCS sheet (V1.6), all the assumptions are similar and all looks in order (Same U values, dimenions etc). So the installer isn't making it up, he's following the MCS sheet which just seems to suggest a very different heat loss value. I can't check the formulas as it's locked.

Anyone seen this before? It surely cannot be that MCS have some form a massive error in their formulas or approach? Or likewise Aira?

Any idea what could be going on??

@pie_eater 

"The Heat Geek cheat sheet estimates your heat loss to be between 4.4 and 8.8 kW

Michael Podesta's quick and dirty method gives it as 4.7 kW (-3C OT to 21C IT) using your annual gas figure (assuming that is all heating and not hot water, it would be less if you use gas to heat you hot water)."

The estimates above from your gas usage will not be far out, plus you have aria and octopus both giving the same range. The local installer is way out, either he has made an assumption the others havent or his spreadsheet is corrupted. When you have one outlier on a set of data points which do you tend to believe?

@pie_eater I'm just offering my 2 cents to say I agree with the comments - your installer sounds a wee bit er... flakey.

MCS guidelines are exactly that 'guidelines' and there's 3 definitions: 'must'; 'shall'; and ' should'  to cover levels of obligation for the installer.  They are not quite as rigid as some installers (including my own) like to suggest. MCS does say that assessments have to consider the specifics of your house and how you live in it. 

The 8kw Daikin is probably one of their best and sounds more than adequate for your house - if you manage to get all your radiators upgraded you are getting a great deal that should run at low temperatures efficiently. If you go too big with the pump it will be a problem for the bulk of the year and cost more. 

I too would say go back to Octopus - they're not perfect but they have a pretty good reputation, the best and cheapest maintenance package, a record of good after care and being willing to rectify mistakes - at least 2 times that I know of (at Heat Pump monitor) they've been fine with going back and swapping out over-sized heat pumps for smaller ones. There's a few Octopus installs on YouTube you could check. Or try British Gas? 

Now I'm off to try to figure out my own radiators - I'm a novice and swimming in delta Ts and trying to make sense of it with the help of some of the very knowledgeable people here. 🙈😁

@pie eater the difference you can’t see will be air leakage usually. There are many assumptions about how many air changes per hour your house has due to leakage or deliberate ventilation. On our house it added 50% more, from7kW to 10+kW. Fortunately we have our own measurements of the house air leakage so we could steer potential installers well.

Your actual heating bill reflects reality including your air leakage so should take priority and act as a sanity check on the other calculations.

Hi all- thanks for all the excellent advice so far.

Quick question, if octopus is correct about my heat losses (6.4kw) will  the 8kw daikin big enough during a cold snap ? I ask because at a low temperature that machine loses a lot of power (only 3kw at -7 or so)? 

@pie_eater If it is of any help, OE carried out a survey and my HL is 6.5 kW and they specified a Daikin 8 kW ASHP. This is what we had fitted though not by OE. When we had minus 7 or minus 8 last winter, the system coped admirably - and we have the temp. set to 22.5 degrees C; the system is on 24/7 with a slight setback at night. We use Homely to control the system and if I remember correctly, the lowest COP we saw then was approx. 3.5 and most of the time, we have a COP of 4.5 or more. Regards, Toodles.

@pie_eater Is it one of these models? If so they all seem to do much better than 3kW at -7C OAT. Where did you get that figure from?

I had a quick look on heatpumpmonitor.org they have several octopus installed Daikins this one is an 8kw one and the owner states its output as 5.45 kW @ -3°C. you can see his stats here

https://emoncms.org/app/view?name=MyHeatpump&readkey=bfd97700dd78658dedd70ff6670b453c

@pie_eater Here:

I take it that you are looking at the red line and not the blue line.

@derek-m  I was looking at both specially the red line for costs . I've posted those images before but no one said anything. Is heating capacity not what I think it is? It's not an indicator of capability?  🫣

Posted by: @lucia

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@derek-m  I was looking at both specially the red line for costs . I've posted those images before but no one said anything. Is heating capacity not what I think it is? It's not an indicator of capability?  🫣

 

The blue line is how much thermal energy the heat pump should provide, the red line is how much electrical energy the heat pump would be using.

Ah helpful thanks . It’s the EDLA08E2V3 so yes I think that’s in the graph