Heat Pump – First Operational Data

Click for a larger version. The heat meter estimates the heat delivered by the heat pump by measuring the flow of hot water [in kilograms per second] and the difference between the temperature of the water delivered by the pump (T1), and the temperature of the water returning to the pump (T2).

Friends, please let me tell you about the first data I have on the operation of the new heat pump – a 5 kW Vaillant Arotherm Plus.

[Edit 6/9/2021: Initially I stated this a 7 kW version because I had forgotten that in the end I opted for the lower power version]


As part of installation, I paid for a “Metering and Monitoring Service Package” (MMSP) which monitors: the heat delivered by the heat pump; the electrical energy it consumes; alongside the local internal and external temperatures.

All this data is measured every 2 minutes (!) and then whizzed into The Cloud where I can view and download it.

Just as importantly, the data is aggregated by Ofgem (Office of Gas and Electricity Markets) who can then assess real world performance of heat pumps ‘in the field’. And Ofgem will – I hope – eventually pay me for the data!

Aside from electrical power measurements, the key element of the monitoring system is a heat meter, whose operation is illustrated at the top of this article.

This clever device integrates several measurements:

  • The temperature of the water delivered by the heat pump
  • The temperature of the water returning to the heat pump
  • The flow rate of the water.

…to estimate the heat delivered by the heat pump.

Click for a larger version. The Sontex Superstatic 449 heat meter installed near the hot water cylinder. The meter indicates that since installation, the heat pump has delivered 78.533 kWh of useful heat.


At this time of year, we don’t need any space heating so the only way to assess the performance of heat pump is for heating domestic hot water (DHW).

The MMSP can detect whether electrical power is applied to the 3-way valve (see picture at the top) and so can tell if the hot water is being delivered to the radiators or the DHW tank.

The heat pump is set to heat the DHW tank between 3 a.m. and 4 a.m. each day. The data below is from the early hours of 13th August 2021.

The graph below shows electrical power drawn by the heat pump (watts), the thermal power delivered (watts), and the temperature of water (°C) versus time. The water temperature should be read against the right-hand axis.

Technical Note I have smoothed the power data by averaging it over 10 minutes to make it easier to see what’s happening.

Click for a larger version. Graph showing the operation of DHW heating cycle. The electrical power drawn by the heat pump (watts) and thermal power delivered (watts) are shown against the left-hand axis, and the temperature of the water (°C) is shown against the right-hand axis.

The first thing to notice is that the thermal power delivered by the heat pump is larger than the electrical power consumed. This is the ‘magic’ of heat pumps. The extra energy is drawn from the outside air which is cooled by about 3 °C in the process.

The second thing to notice is that initially the thermal power delivered is high (peaking at nearly 3.8 kW) and the electrical power is low (just under 1.0 kW). But as the water temperature increases from 30 °C to above 50 °C, the heat pump has to work harder (electrical power increases) to deliver slightly less heat.

This is the nature of heat pumps – they work best when heating lots of water through small temperature differences rather than heating small amounts of water through large temperature differences.

The ratio of the heat energy delivered to the electrical energy used is called the Coefficient of Performance or COP. This is shown on the graph below.

Click for a larger version. Graph showing the operation of DHW heating cycle. The Coefficient of Performance (COP) is shown against the left-hand axis, and the temperature of the water (°C) is shown against the right-hand axis.

For most of the heating cycle the COP is above 3 and almost reaches 4 when the water temperature is about 40 °C. The spike in COP at the end of the heating cycle is probably an anomaly caused by the smoothing of the data, and the fact that heat is delivered from pre-warmed pipes after the electrical energy was reduced.

Averaging the electrical power drawn by the pump over the whole day – the standby power is 12 W – the effective COP is around 2.8 i.e. the heat pump provided me with 2.8 times more thermal energy than the electrical energy I used to power it.

For those of you unaware of other household developments, this electrical energy came from a battery which stored solar power generated earlier in the day. So our hot water is 100% carbon dioxide free in the summer.

Future Improvement 

I am pretty happy with this performance. But I think it can still be improved.

Firstly, there is a four metre section of pipes between the heat pump and the DHW cylinder which are still not insulated. After insulation, more heat should be delivered and the overall COP should increase.

Secondly, I need to see how the temperature of the hot water in the taps and showers is affected by lowering the hot water storage temperature. I think somewhere between 45 °C and  50°C might be acceptable and that should improve the COP still further.

Finally, when used for space heating I am hoping to keep the temperature of the water circulating through the radiators as low as possible – perhaps 45 °C will be possible – which should again improve the overall COP. To achieve this I may need to change one or two of the older radiators. But that is a problem for the autumn.

For now I am enjoying the wonder of thermodynamics in action.

7 Responses to “Heat Pump – First Operational Data”

  1. Simon Duane Says:

    Not sure if this posting is intended as Heat Pumps#2, but thanks for the insights, Michael.
    I used to think that, if there is only a limited area available for (domestic) harvesting of solar energy, then solar thermal panels would always have an efficiency advantage over PV (~60% c.f. ~20%). Now I see that the energy that PV panels lose as heat (to the air, etc. instead of converting it into electricity) can be recovered, indirectly, using an ASHP powered by the PV panel.
    If heat is what’s needed, of course – other uses of the solar PV output are possible…

    • protonsforbreakfast Says:

      Thank you. I had not previously seen the figures for Solar PV vs Solar Thermal. I guess that explains their popularity. But Solar Thermal can only be used for one thing – Domestic Hot Water (DHW).

      In the summer our 20 m^2 of PV generates ~ 3 kWhe in the winter and ~ 15kWhe in the summer. For a solar thermal system (3 x more efficient but 20% of the area?) this would translate to 1.8 kWhth in the winter and 9 kWhth in the summer.

      Our use of DHW is around 5 kWhth/day on average through the year. So Solar thermal would be not quite enough in winter and too much in summer – and then all the energy would be wasted.

      In contrast, in winter I think using the heat pump with a COP of (say) 2.5 in the colder weather, 3 kWhe will be enough to meet DHW needs. In summer, our DHW needs are met easily and indeed the solar PV runs the whole household for 5 to 6 months and exports around 1000 kWhe to the grid.

      So I think in terms of functionality a Solar PV system and a Solar Thermal system are barely comparable. But their costs are also vastly different.

      Regarding the heating of PV panels in the Sun, this causes a significant loss of efficiency and cooling would be great – a combined PV/thermal system. But I think complexity is the killer here. And regarding using the panel as a source for an ASHP, ASHPs need LOTS of air to extract heat: for 4 kWh at a COP of 3, the ASHP needs to extract 3 °C of cooling from around 0.9 cubic metres of air per second!

      In any case I trust you are well and reducing CO2 emissions with gusto! M

      • Simon Duane Says:

        Yes thanks, all well if rather sedentary these days.

        At least one place https://www.dlsc.ca/index.htm uses a vast excess of solar thermal to charge their communal thermal store in summer, for winter extraction to meet their space heating needs. In the first few years, this led to impressive CoP values for their GSHPs. (I remember this cropped up in an earlier exchange we had.) More recently, I imagine that the seasonal imbalance means that no actual pumping of heat for winter space heating is required, it just needs to be tapped. By the end of summer, their store was said to reach 80 degC or so. Sadly there’s no longer a live display of system performance – I hope it’s still operational…

      • protonsforbreakfast Says:

        I nearly mentioned that idea in my reply but it was already too complex a reply.

        Yes, storing heat is a great idea but its tricky to get right and expensive whether you get it right or wrong!

        Anyway – good luck with your projects.

  2. carsort Says:

    Thank you for this post!

    Would you get a little more oomph (COP) if you ran the heat pump during the day when the outside air is warmer?

    Are you heating the stored water at 3am because you want hot water for showers early in the morning?



    • protonsforbreakfast Says:

      Thank you Suzanne:

      >Would you get a little more oomph (COP) if you ran the heat pump during the day when the outside air is warmer?
      Yes. That’s a good point.

      >Are you heating the stored water at 3am because you want hot water for showers early in the morning?
      We just picked a time when we could buy cheap rate electricity. At the moment (August) we still are nearly 100% solar/battery powered, but sometime in September that will gradually change. In the winter the idea will be to top up the battery and run the hot water cycle for the heat pump using cheap rate electricity.

      Then we will run down the battery over the day for all the normal stuff, but also using the heat pump for space heating. My calculations are that except on the coldest darkest days, we shouldn’t need to use full price grid electricity.


  3. calvinsclimate Says:

    Absolutely! Thanks for summarizing the data! Heat pumps do not create heat. They redistribute heat from the air or ground and use a refrigerant that circulates between the indoor fan coil (air handler) unit and the outdoor compressor to transfer the heat.

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