Posted by: @editor

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This is just an observation, but after recording most podcasts with installers, we usually chat afterward, and while nearly all of them install Vaillant heat pumps in some capacity, they consistently say that the reported COP/SCOP figures from the control panel are overstated as are the performance data sheet specs for these units. 

Noted.  The problem I have with the report on openenergymonitor is that the samples given are all at fairly mild OATs (-2, -3) and the analyst is making the assumption that the heat pumps are operating at max capacity for a period of more than one defrost cycle.  I don't see how this assumption can be justified unless he also knows that they have been somehow 'stressed', which he doesn't (or if he does, he doesn't say so).  Its equally, possibly more, likely that they are responding to a demand which is less than their maximum capacity.  The average input powers in the samples reported are all over the place, which supports this interpretation, unless the spread between units is quite extreme. 

My own 7kW Vaillant has so far almost never run at maximum capacity for more than about 10mins at a time.  Since the house has remained warm I can reasonably presume thats because the demand is insufficient.  To stress it and prove the point, I would need the OAT to go down to perhaps -10 for a day, or -5 for 2-3 days, or I would need to turn up the WC curve (on a day when it was around -2) so that the room temperature was significantly higher, perhaps 25+ (and then of course compare with the Vaillant spec for the increased flow temperature).  I haven't done this and it wasn't done in the samples reported so far as I can tell.  And of course I would need to do some calculations on my emitter capacity to check that this wasn't the limiting factor.

Im not saying that the Vaillant figures are correct, but I am saying that we don't, in my view, have proof that they aren't.

My feeling about COP is that its inevitably a bit of an iffy measurement done 'real time'.  Measuring the input energy accurately should be easy, a £15 Shelley EM or indeed a £50 revenue grade meter will do this.  Measuring the energy out is more difficult as it depends on flow rate and deltaT, neither of which appear to be particularly easy to measure cheaply.  Heat pumps don't need to know either of these with any degree of accuracy, so they aren't going to use quality calibrated components.  Thus COP is unlikely to be accurate.  Of course the spec sheets can and presumably do depend on proper calibrated instrumentation, so should be accurate under the highly controlled conditions in which they are measured.  Doubtless these are 'optimal' in some way and so real-world values are likely to differ.   

I am currently comparing actual energy cost against what I would have paid with my gas boiler over exactly the same time period.  I can do that pretty accurately because I have good records of gas consumption vs OAT.  In the end thats probably more interesting than COP!

@jamespa  There are some questions that are not being asked. What room temperatures are trying to be achieved at 4C (21C?) and what are the temperatures dropping down to below 4C? What is the dT of flow to/from the radiators? Remember @heacol’s opinion from another topic that these problems are more than likely being caused by the immense difficulty of radiators being balanced correctly at 5C dT.

Thanks.  I think it’s clear from what OP says that the target is 21 or thereabouts.  The answers to the other questions aren't clear but there is only so much I personally feel comfortable asking at any one time!  The questions I have asked should, with a bit of luck, shed light on whether it’s a heat pump issue or a heating system issue, depending on what OP can provide.  I would say that, based on the limited info we have so far, it could still be either.  remote problem diagnosis is really challenging as you dont know what you dont know and there is so much you might need to know!

Posted by: @skidoc

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The house was built in 2010, so is relatively up to date with insulation etc and the extensions that have been added by me are very well insulated going above and beyond the regs, I get that it's a large house but and the distance from ASHP to the farthest away rad is about 20 meters in a straight line so by the time you factor in ups and down and lefts and rights it's clearly way more that that, it just doesn't get warm. 

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I'm sure you've looked into this yourself already but for the benefit of the rest of us....

You've drawn a distinct comparison between the insulation of the main building and that of the extensions (OK vs fantastic, respectively). How much hassle would it be to improve said "OK" insulation to better standards? Fairly obviously, reducing the heat loss is potentially just as useful an avenue to explore as trying to increase the heat pump's best efforts.

I've just checked the heat loss calculation for my system (7kW Arotherm+) and it estimated 8.3kW at -2.8C. I reckoned that was high compared with the historical gas consumption. One example of where the standard calculation is likely to be pessimistic is that most householders use curtains and anything other than the flimsiest curtain should help to reduce the heat loss for more than half the day during winter. The highest hourly output during last winter according to the Vaillant app was 7.6kW at -7C outside temperature and the total daily output was 104kWh. I suspect that Vaillant is over-cooking the numbers but the overall result is that the system has  spare capacity (104/24 = 4.33kW on a 24 hour basis).