Research question: Do we see a clear relationship between measured average flow temperature when the heat pump is running on the coldest day and annual SCOP in the HeatpumpMonitor.org data?
This is going to be a bit of a long thread that I will add to over time, looking first at aggregate stats before then looking closer at particular systems.
Theory: Heat pump systems on weather comp should show quite a clear relationship between design flow temp and annual SCOP for space heating. The SCOP calculator tool that I built here is a an example of how this can be calculated theoretically Heat pump SCOP calculator. It’s not the only factor in the calculator that affects the SCOP, the other factors in that calculator are the practical efficiency of the heat pump and the minimum modulation level which determines how low the flow temperature can go in mild weather. These would typically be 50% and 40% respectively for systems without sophisticated control of mild weather cycling.
Figure 1: All systems with valid data (minimum 290 days).
As is often the case with HeatpumpMonitor.org real world data, we see a wide spread of values. Clearly there is more to annual SCOP than the average flow temperature when the heat pump was running on the coldest day. See Heat pump oversizing data analysis thread for another example of wide spread of results.
All systems but one have average flow temperature when running that are below 45C, most are significantly below, the average flow temperature of all systems on the coldest day is 36C.
On average, systems had a measured average flow temperature when running of ~7C below the stated design flow temperature entered on the form. There are 2 systems with average flow temperature 20C or more below those entered on the form. 12 out of 59 systems were within 2C of the form design temperature. Actual experienced outside temperatures where on average 0.6C higher than form entered design outside temperatures.
System 65, next highest SCOP (H2 boundary) seemed to have a bit of data loss on the coldest day, though it was also off for that middle part of the day as can be seen from the flow and return temperatures.
Average flow temperature when running 33.1C including DHW cycle.
Average outside temperature -1.7C
We can see a little missing/incorrect DHW heat data probably due to an air issue earlier in the day, the carnot efficiency to reproduce the space heating part was ~45%.
Weighted average flow temperature from histogram tool for whole year: 31.20C
Coldest day:
Average flow temperature when running: 38.9C
Peak flow temperature on space heating: 42.0C
Average outside temperature: -2.5C
Carnot efficiency on coldest day 44.7% (fairly standard)
This is a good example of a heat pump running around 40C rather than mid 30s but still one of the highest performing systems - this is likely due to average flow temperatures over whole year being relatively low.
I’ve made a couple of improvements to the HeatpumpMonitor.org daily data tool. It’s now easier to access e.g mean flow temperature when running vs outside temperature for each system e.g Heat Geek Vaillant:
Perhaps the more informative temperature to look at would be average flow temperature when it’s around 5C outside, or average flow temperature for the outside temperature at which the most heat was delivered … which could be drawn as a histogram…
Added an option to show weighted average flow temperature to the histogram tool which plots flow temperature vs heat delivered. This might be the better measure as it should capture the average flow rate at which heat is delivered over the year, which should arguably correlate better than the coldest day… now to automate this calculation for all systems
Sheffield Viessmann: 30.08C, SCOP: 4.97 (H4)
Bristol Nibe: 32.77 C, SCOP: 4.66 (H2)
Heat Geek Vaillant: 31.20 C, SCOP: 4.62 (H4)
Glyn’s Samsung: 33.05 C, SCOP: 4.41 (H4)
System 170, 11 kW Ecodan, 41.67C, SCOP 3.6
System 77, Woodam surrey, Vaillant, 38.27C, SCOP: 3.7
For systems with DHW mode detection with the ability to filter out space heating only, the correlation between average flow temperature on the coldest day when space heating vs annual space heating SCOP is not much better.
Thought I’d add a few comments on the above as a starting point.
Looking at both the first chart (SCOP vs coldest day flow temperature)
and the 3rd from last: SCOP vs weighted average flow temperature for the whole year:
The weighted average flow temperature is showing a clearer relationship as we might expect but it’s still interesting how wide the variance is at a given average flow temperature. At face value a system averaging a weighted average flow temperature of 33C could have a SCOP of anything from 3.5 to 4.5, that’s quite a wide range!
Likewise you can select a SCOP such as 3.5 and see systems with weighted average flow temperatures all the way from 33 through to 43C.
Other factors are clearly having a significant impact on performance. It would be interesting to list different design factors and work out statistically how significant each of these factors are in determining SCOP::
Design flow temperature & average flow temperatures
Make and model of heat pump (and related effectiveness of control algorithms e.g for mild weather cycling)
Oversizing factor
Outside temperature differences
System volume
Hydraulic separation, type and configuration
Indoor temperatures
Type of emitter system
Monitoring error?
Someone with a data analysis background and with more skill than I could probably unpick the impact of these factors in a clearer way.
I think I will try and pick out specific systems that span a range at a given flow temp and try to and see if I can make any sense of why these are giving such different results.
