What formula are you using now?
Itās the same standard assumptions and equation we are using elsewhere e.g:
T_condensing = T_flow + 2K
T_evaporating = T_ambient - 6K
Carnot_COP = (T_condensing + 273) / ((T_condensing+273) ā (T_evaporating + 273))
The difference is in how this is averaged over the data selection window. Iād like to try one other variation so will report back with more detail after Iāve done that 
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Feedback on lower than expected output from my view as a participant on the OVO part of the trial. Iām not a heat pump expert, though have come to understand them a little bit through necessity.
- Heat meter temperature sensor offset errors to do with installation placement.
- Quite possible in the early stages. There was basic instructions provided to the installers but installer knowledge and skill varied considerably, and several return visits to correct things, I assume because the installer was notified. My flow meter and temperature sensors were moved several times in the first six months (including swapping/correcting the flow and return temperature sensor placement) and a round issue correct on one of the data fields in the logger, heat output I think.
- Dirt in the Sontex Heat meters causing flow rate to be under read.
- Quite possible. Flushing the system wasnāt on the cards from my installer, might not be the same with all installs but was the case with some. Fortunately my system was relatively new/clean anyway. I know some people had follow up visits to flush their systems.
- Heat pump unit issues such as low refrigerant charge or air circulation.
- @JuliaCās system had low refrigerant that wasnāt detected until after the trial (makes you wonder what was checked on the first two servicesā¦), but realisitically more likely to only be on the split systems. Air entrainment certainly possible from comments weāve seen elsewhere on this forum.
- Perhaps impact of things like significant distortion in buffers?
- I donāt think buffers were very common. The Daikin suppied design that my installers were working to was for a volumizer on the return with no buffer tank, however some installs had pre-plumbing tanks that have a small built-in buffer tank.
- Some of the low % carnot systems have very rapid cycling, these may be struggling to get to an efficient operating point.
- My second and third attempts at an install had a poorly configured honeywell wireless thermostat that was causing the heat pump to cycle rapidly. These were fairly commonly specified on the Daikin systems across our SE part of the trial. I insisted they put the wired Daikin controller back during the final repair.
- There was, and still is, a general assumption that heat pumps are most efficient when run at the lowest possible leaving water temperature and lots of us (on the OVO trial through the self-help forum) tried lowering the running temperature (weather dependant curve) to improve efficiency and stop our homes over-heating. My initial install was a fixed 50C leaving water temperature, which I changed to using a weather dependant curve after a short while and a bit or research. We were given no advice from OVO, the trial or installers on this. I was running at 25C at 15C outside at one point, but through physically watching the heat pump display (remember we didnāt have access to the data) I noticed it was cycling every 10mins because it couldnāt maintain the low output. With the help of forums and me learning a little about heat pumps I was able to correct this, but many users didnāt understand or didnāt want to stand by their controller noting down readings every few minutes trying to figure out what was going on. Through this forum in particular, some of us now know this isnāt the best way to run for all heat pumps/emitter type, the larger Daikin in particular, as the heat pump does not modulate very low and they struggle to maintain a lower LWT.
It would be interesting to do a comparison between heat pump output and actual heat loss with this data and see how many homes had significantly over-sized heat pumps like mine. Iāve not this mentioned in the reports Iāve scanned, but I havenāt looked in detail.
Sorry, Iāve gone on a bit there.
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Iām currently (edit: slowly) pouring through my notes made during the trial and comparing with the EoH data, to try and discover what it was doing over that time.
The leak happened in the first summer after a few months of more or less successful running, I was assured it was not significant. Not until the first winter when I could see bands in the ice formed on the grill could I see actual evidence of my suspicions. It took another year before I was believed.
There was one service after the first winter but not done by an Fgas engineer so presumably the refrigerant might have been checked but definitely wasnāt.
By the time of the second service Iād got an independent opinion from Ken Bone and thankfully that was taken seriously and the refrigerant was weighed out and recharged. Alleluia.
Unfortunately the monitoring had failed before that and not fixed until a month before the end of the trial.
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Iāve been through the EoH data for my system EOH2283 and compared it to my photos and notes taken at the time - sorry itās a long saga but I hope it explains why my all new and expensive ASHP installation had such a poor COP of 2.47.
Itās all right now at a COP of 3.7 Daikin LT Split, Fetcham, Surrey.
TL;DR - probably due to a major refrigerant leak (which thankfully I saw happen) plus a load of other things not set up correctly.
Hereās my long explanation of the time it was being monitored for the EoH trial:
report.pdf (353.0 KB)
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Thanks @HydroSam @JuliaC a lot of insight there, thanks for sharing!
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Agreed re limitation on minimum flow temp for weather comp. As you both state this has to relate to the minimum modulation of the heat pump, especially for Daikinās and Mitsiās. Vaillant curves handle the cycling issue with their degree minutes control which removes the need to think so carefully about this.
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Both of you had initial fixed flow temps which seems very common, 50C and 42C. Interesting that your fixed temp was 42C @JuliaC, this being relatively low compared to the often default 50ā¦ but then if that was entered by the installer why was it left on fixed temp. What have you settled on for your design temp?
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Low refrigerant, interestingly @ColinS Daikin also had low refrigerant though probably not quite as low as yours, there does not seem to have been much of a change to the performance after it was topped up though. I wonder how common variation in refrigeration charge level isā¦ Colinās Daikin is a monoblock and I would have thought that this could be tightly controlled in factory production but it seems this may not always be the case.
Agreed
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Pretty happy with where I have got to now with the site: https://eoh.heatpumpmonitor.org
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There were a few loose ends around how the immersion heaters and backup heaters were taken into account. The EOH convention was for the whole system electricity consumption measurement to include the immersion heater and so SPF H4 calculation = (Heat output + Immersion) / Total electric consumption. While most systems matched this convention, there were a reasonable number that did not and so their SPFās were over reported. Iāve created a new column called AdjH4 (Adjusted SPF H4). This includes all the corrections that Iāve been able to identify, mostly automated now.
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I think Iāve been able to find most of the metering & configuration errors now. The ones that are easy to correct e.g whether the immersion heater is included or not in total electric consumption, or if the heat output from the heat pump included the boiler. Iāve fixed in that AdjH4 column. The total effect of these corrections is now available in the headline table top-right.
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The main result of all this is a tiny reduction in the ASHP Mean SPF for all systems of 0.02 and reduction of combined SPF of only 0.03. Which in terms of the overall result and conclusions from the study is arguably immaterial.
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By accounting for the immersion heaters correctly Iāve been able to reduce the number of systems below 35% carnot to 10%. There is the option to see what happens if you filter these systems out, the impact is relatively small. E.g ASHP mean SPF goes from 2.79 to 2.84. 35% is possibly on the low side, if you filter out systems below 40% carnot, the mean SPF for ASHPās goes up to 2.97, which while something is only 10% of the difference between the HeatpumpMonitor mean SPF and EOH.
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The MyHeatpump dashboards now show boiler output and also takes into account if the immersion heater is included or not in the standard electric consumption - the figures there at least in the power view mostly match the front page system list view.
The next step is to try and pick out some key findings!
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First part is as before, we calculate the ideal carnot COP for all time intervals e.g 1 year at 120s = 262,800 intervals over which we calculate cumulative totals:
// Calculate ideal carnot efficiency
$condensor = $flowT + 2 + 273.15;
$evaporator = $outsideT - 6 + 273.15;
$carnot_dt = $condensor - $evaporator;
$ideal_carnot = 0;
if ($carnot_dt>0) {
$ideal_carnot = $condensor / $carnot_dt;
}
The next part goes like this:
if ($dt>1 && $heat>0 && $ideal_carnot>0) {
// Calulate predicted elec consumption based on carnot efficiency
$kwh_carnot_elec += ($heat / $ideal_carnot) * $power_to_kwh;
// Calculate actual elec consumption and heat output when running
$kwh_elec_running += $elec * $power_to_kwh;
$kwh_heat_running += $heat * $power_to_kwh;
}
Finally after calculation for all time intervals:
$wa_prc_carnot = 0;
if ($kwh_elec_running>0 && $kwh_carnot_elec>0) {
$wa_prc_carnot = round(100 * ($kwh_heat_running / $kwh_elec_running) / ($kwh_heat_running / $kwh_carnot_elec),2);
}
wa_prc_carnot being weighted average percentage carnot.
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Just thought Iād share some interesting stats from the summary table
Adjusted SPF H4 by Refrigerant Type
Surprised R410a and R32 are so close, not quite the manufacturers claimed improvement, though install quality is possibly more of a factor here.
Adjusted SPF H4 by Brand
Adjusted SPF H4 by Region (Warmworks-Scotland, Eon-Newcastle, OVO-South East England)
A clearer difference here between how the three different regions were run.
And finally (though they probably wonāt like thisā¦)
Adjusted SPF H4 by Installer
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Perhaps they should all feel a little ashamed?
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Thanks @HydroSam great summaries!
Iāve added the ability to filter by multiple things at one, just add ; between each search term.
E.g ASHPās only: āwarmworks;ashpā
| E.ON | 2.62 |
| OVO | 2.68 |
| Warmworks | 2.92 |
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