Posted by: @mattc

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@majordennisbloodnok Thanks.  Similar installation cost again.  Looks like RHI was a really good deal.  The new BUS grant is £7500 (formerly £5000) which I thought was was pretty good but it seems small in comparison (not complaining - free money is free money).

It was a good deal, but the sting in the tail is that we had to stump up the whole cost up front and have the RHI grant repaid quarterly over six years. With the BUS scheme, the grant is paid at the start. We were lucky enough to be in a position to take advantage of the RHI scheme before it finished and I’m glad we did but not everyone would have been able to.

@mattc 

The heat loss was done for a temperature of -2.5°C and produced a figure of 7.31kWh for heat loss. That would work out at 175kWh for a day at -2.5°C.

The system design, ie radiator sizing, was done to produce this at a maximum 50° flow, and an estimated SCOP was produced (by Mitsubishi I think) based on the EPC (I think) of 15500kWh space heating and 3000kWh water predicting that we would achieve a SCOP of 3.8.

Our gas usage to to be around 12-13000kWh per year, and the heat pump uses near enough 10000kWh per year which is a good match when the boiler efficiency is accounted for.

In reality:

That dot at the top right (ignore the rightmost one) is the day that the AOT was -2.1°, and you can see that the house needed 80kWh to keep it an average 20°. So about half the calculated loss.

This is why I suggested using whatever data you can to work out your actual performance.

I should get a SCOP of 3.4 this year, though flow temps rarely got into the high forties even on the coldest days, my system seems to fail to run on temps below the mid thirties whatever the outside temp is. It could be that, like Mike-h, as the rads are sized for a higher temp flow they don't seem to lose enough heat to stop the heat pump raising the flow temp even at its lowest power.

@gunboatdiplomat 

Interesting comparisons.  It looks like we have very similar properties though ours is probably slightly better insulated as 80% of the outside walls are part of an extension we built in 2006, with the rest retro-fitted with cavity insulation in 2002.

I ended up enquiring with 6 companies, all of which gave an up-front estimate.  Octopus was the easiest (simple on-line form with immediate results) and the lowest by a long way, but from what I was reading at the time, people were seeing very long lead times for survey and installation, so I didn't pursue them.

I got 3 companies to do surveys, which are the ones I've reported.  Two of these did the survey remotely based on a combination of detailed floor plans and radiator measurements I sent them, plus in one case a Teams meeting including a walk around with a camera.  The third did an in-person survey - this is the one with the lowest estimated heat losses and the lowest quote.

@harriup 

Thanks again.  I've explored quite a lot of data (and plotted many graphs), but only have daily gas consumption figures from late January onwards (just caught the end of the last cold snap) and don't have anything with negative AOT values.  However, the relationship looks pretty linear (just like yours), which I believe concurs with the theory, so I'm pretty happy extrapolating down to lower temperatures.

Assuming 90% boiler efficiency and calculating an upper 95% prediction limit on the regression, I get a total consumption of 130kWh at -5C.  This is with the heating on for 17 hours (it's currently turned off between midnight and 7am) which equates to an average of 7.6kWh per hour through the day. 

This ought to be well within the capability of the 8.5kW Ecodan.  The 11.2kW unit would obviously be the safer bet, and would be fine at this temperature.  I'm just uncertain about whether there would be a performance penalty in milder weather when it only really needs to be putting out 4-5kW to maintain the temperature.

@mattc My 2p worth, Ecodan's don't modulate down well so avoid a larger unit if its not required as it will inevitably cycle badly at milder temps.  The lowest temp I can get my 8.5Kw unit to run at without cycling multiple times an hour is 32C, this is too high a flow temp above 2C ambient, so my unit has to either turn off when it hits the thermostat temp which means the house temp cycles up and down 0.5C not a massive pain but means the unit isn't as efficient as it could be as its running hotter than it needs to, but it can't run any cooler and maintain a stable flow temp without cycling.

@gary Thanks.  If you don't mind me asking, what is the size and/or heat loss for your property?  Or the usual daily heat output and COP from your Ecodan?  It would be interesting to know for comparison.

House is detached 180sqm 7kw heat loss from survey.  I get an SCOP of around 3.5 for heating.

@gary Thanks.  House is a similar size to ours.  It's interesting that you find even the 8.5kW Ecodan is overpowered. 

We got three surveys that came out with heat loss estimates between 8.1 and 13.5kW, hence my uncertainty about the size of the system needed.

Posted by: @mattc

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@gary Thanks.  House is a similar size to ours.  It's interesting that you find even the 8.5kW Ecodan is overpowered. 

We got three surveys that came out with heat loss estimates between 8.1 and 13.5kW, hence my uncertainty about the size of the system needed.

My property is 200sq m.  Two MCS surveys each taking 3hrs came out at 16kW.  They ignored fabric upgrades I told them about and double counted room to room loss.  Correcting for these gives 10.5kW. 

Working from my annual gas consumption of about 20MWh would equate to a 7-8kW actual demand which is what I measure on my smart meter (note this translates to about 4kW most of the time ie other than when it's not very very cold).  To prove this I have turned my boiler max output down and downloaded/checked my smart meter readings. 

I can get a spreadsheet calculation to equal 7 5kW by assuming 0.5ach instead of the default values MCS assumes.

So my strong advice it to be very sceptical about the higher figures you have been quoted and if you can, get a measurement.  Oversizing leads to excessive cycling and loss of efficiency and the spreadsheets suffer from gigo in a retrofit where the fabric has been non-uniformly upgraded.

@jamespa  Thanks. 

I'd already concluded that the higher estimates are excessive, which is why I'm going with the lower of the three figures that I've received.

To my mind, empirical data should always trump theoretical calculations that are based on uncertain assumptions and approximations, but I realise the MCS approach doesn't really allow for this.  From everything I've seen so far, I think the heat loss calculations are more useful for individual radiator sizing than for overall heating requirements.

Posted by: @mattc

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From everything I've seen so far, I think the heat loss calculations are more useful for individual radiator sizing than for overall heating requirements.

Thats precisely the same conclusion as I have reached. 

I have challenged MCS on the method they promote.  Their response was that they are aware of it's deficiencies but 'we don't currently have an alternative'.

All heat pumps will hit the point where their minimum output exceeds the heat being taken out by the heating circuit, not oversizing too much means that happens at a higher outside temperature. How the pumps handle the lower demand is really hard to pin down. I know my system when using the weather compensation curve worked just at its minimum power but cycled up to 6 times per hour. Switching to the room adaptive mode slowed the cycle to once per hour or so but with a more complex power consumption as it it often puts a burst of energy in before a pause. But overall this a more efficient mode - better COP. Looking at some similar systems on heatpumpmonitor, though, displays such dissimilar working patterns that it is hard to differentiate what elements are created by the way the house heating system is responding or is programmed.