I managed to seperate out the standby electricity consumption of the heat pump and so the following monthly COP figures for space heating and domestic hot water (DHW) are specifically for the periods of time that the heat pump is running. This shows higher COP's in the summer than the winter as we would expect given higher outside temperatures. The combined COP chart includes this standby consumption and we can see that it drags down the summer COP's.
The average annual COP of 3.91 was very slightly lower than the 2019 figure of 3.93. This difference is not really significant but one reason may be that the heat pump provided 12 months of hot water heating rather than only the last two in 2019.
The following charts show the electricity consumption of the heat pump separated out again by space heating, hot water and standby. Standby consumption increases in summer as the heat pump spends a lot more hours on standby.
The higher space heating COP of 4.2 compared with 3.96 is primarily due to separating out the standby electricity consumption that was included in the 2019 space heating figure. With standby included, this would drop the apparent space heating COP to 3.94, but not change the overall combined heat pump COP result of course. This goes to show that how standby consumption is allocated can change the COP figures for any breakdown between space and water heating. The important figure for overall system performance is the combined COP of 3.91.
The water heating COP is perhaps surprisingly high, one of the key factors leading to this high performance is that we only heat the cylinder up to 40-42C twice per day. I meant to setup an automatic regular legionella pasteurisation cycle last year, but I only performed a small number manually as I'm still working on the heat pump controls. Theoretically extrapolating a weekly legionella pasteurisation cycle, using the heat pump to bring the cylinder to 53C (flow temperature 60C) and then the immersion to top the cylinder up to 60C suggests that the average annual DHW COP would drop by 5-8%, from 3.83 down to 3.56-3.64. A daily pasteurisation cycle would drop the DHW COP by about ~30% to 2.68. I estimate that the combined COP including space heating and standby would drop from 3.91 to 3.86 with a weekly cycle and 3.58 with a daily cycle. I intend to give more attention to Legionella protection this year, including testing the water to check bacteria levels, I will share the results in a separate blog post once I'm further along with it.
In the 2019 review I compared our heating consumption in that year to the modelled heat demand from a SAP based energy assessment. We used 45% less heating energy than the assessment suggested. I can account for 2/3rds of that as being due to lower average internal temperatures and higher outside temperatures for that year.
In 2020 we used 14% more heating energy, narrowing the gap between model and measurement from 45% to 38%. In 2019 our average indoor temperature was 17.7C (warmer during heating periods of course). In 2020 the average indoor temperature to 18.7C, partly reflecting increase in comfort and partly being at home more.
The stacks on the left show warmer than assessment external temperatures and lower internal temperature now make up about half of the difference rather than 2/3rds.
Calculating heat loss factors in W/K
To isolate the performance (or lack thereof) of the building fabric and air-tightness from all of the other factors that determine final heating energy requirements, I've worked out in the monthly table below an estimate for the heat loss factor in Watts per Kelvin (W/K).
The heat loss factor should be equal to the sum of all sources of heat, including gains from electricity consumption, body heat of occupants (metabolic gains), solar gains through windows and hot water gains, divided by the difference in temperature between the average internal temperature and the outside:
Heat loss factor = (Heating system input + Lighting, Appliances & Cooking electric consumption + DHW gains + Solar gains + Metabolic gains) / DeltaT
I've assumed that 100% of Lighting, Appliances & Cooking electric consumption ends up being used for heat and 50% of hot water heat. The metabolic gains are set at 60W per person (SAP assumptions). The solar gains are estimated based on solar PV data from a near-by home scaled by trial and error to provide a level set of W/K values through the year.
The chart on the left shows the resulting monthly heat loss factor values, varying between 151 and 205 W/K and an average of 173 W/K. The average is 15% lower than the heat loss factor calculated using the SAP assessment as shown in the graphic on the right. It's not perfectly precise but at least suggests that our heat loss is lower than the value calculated in the assessment, not higher.
The data for much of the analysis in this post was collected using our OpenEnergyMonitor monitoring system, I've written a more detailed post on the monitoring setup here: Monitoring. Here are the exported monthly temperature averages and consumption values for 2020:
Improved controls: In October I improved the heat pump controls to automate scheduling with a combination of room and flow temperature based set point profiles for each day. Rather than run the heat pump in a single continuous block, it now cycles at a low frequency much as any normal thermostat would work, the average flow temperatures can be lower this way when outside temperatures are milder and so the COP is higher. It would be better if the heat pump could modulate it's heat output down further, but Im limited by the minimum compressor speed of our heat pump.
Carbon Intensity: I would like to caveat the carbon intensity calculations, whilst I find it interesting, this kind of carbon intensity analysis is far from perfect. It doesn’t really capture what the aggregate effect would be if everyone consumed electricity at what are currently times of low demand and low intensity, nor does it capture a more complex view of long term electricity system decarbonisation and adoption curves of measures like switching to heat pumps or EV’s. Octopus agile is also a renewable tariff. Views differ of course on how these factors should be accounted for. I’ve added the above analysis as a point of interest more than anything.
Gas boiler comparison: Assuming a 80% efficient gas boiler, gas unit rate of 2.919 p/kWh, standing charge of 17.85 p/day (Octopus 12M tariff) and mains gas carbon intensity of 210 g/kWh (SAP), our heating cost assuming the same amount of delivered heat would have been £343.02 and CO2 emissions 1999 kg. Gas boilers also have electricity consumption associated with the central heating pump and standby requirements, SAP suggests between 156-165 kWh/year for the central heating pump. Standby may add another 50-60 kWh to this. If we assume 220 kWh of electric this would add £23.54 at 10.7p/kWh (Agile prices) and 42.24 kg of CO2 (at 192 gCO2/kWh). The total cost for 7615.5 kWh of delivered heat would therefore have been £366.56 and total CO2 2041.31 kg. This gives a heat unit cost of 4.81 p/kWh and CO2 intensity of 268 gCO2/kWh.