Real-World Heat Pump Efficiency Data – Part 1

Since I’m the kind of person who likes to measure things “because I can”, it seemed sensible to include Heat Meters on the outputs from the NIBE F1145 Ground Source Heat Pump and also to include an Electrical Sub-Meter on the input. Doing this makes it possible to compare the output power with the input power and calculate the real-world Coefficient of Performance (CoP) of the Heat Pump – a bit like recording all the fuel you put in your car so you can calculate its actual MPG.

The Heat Meters are Kamstrup Multical 302 units with wired M-Bus interfaces which are automatically read every 2 minutes as described in this Technical Article page. Data is published via MQTT and loaded into an InfluxDB database where it can easily be plotted using Grafana.

Two separate Heat Meters are required because there are two separate output pipes from the GSHP – one for the Central Heating and one for the Hot Water. (Within the GSHP unit there’s only a single heat source but there’s a diverter valve that sends water to the appropriate output pipe; the return pipe connection is shared.) While the downside is the cost of the extra heat meter, it does make it easy to see when the heat pump is in ‘heating’ mode versus ‘hot water’ mode – which is important.

The results are quite interesting and reinforce the basic physics of the heat pump operating principles. In summary, the data for my NIBE F1145 shows:

  • In ‘heating’ mode, the unit is delivering an instantaneous CoP of as much as 5
  • In ‘hot water’ mode, the unit is delivering an instantaneous CoP of as little as 3

Read on for further detail on how these numbers were derived.

Note that at this time of year the ground is still relatively warm and the ‘brine’ coming in from the ground loop is around 10 degrees, returning at around 5 degrees.

Heating

In ‘heating’ mode, the heat pump is configured to deliver water just hot enough to compensate for the heat loss from the house at a given outside temperature. For example, at 4 degrees outside it calculates it wants water at 31 degrees but since the F1145 does not have such a low setting it actually generates water at about 37 degrees. (However it measures how much it is over-delivering by keeping track of the ‘degree minutes’ of the water it produces and won’t turn on again until the average delivery matches its calculated target.) Producing water at 37 degrees, the heat meter records a power output of almost exactly 10 kW while consuming almost exactly 2 kW, giving a CoP of almost exactly 5.

Note that this is an ‘instantaneous’ figure and doesn’t take account of the ongoing low consumption of the GSHP when the compressor isn’t running (consistently showing as 60 W even with the circulation pump running at 30%). Note too that as the weather gets colder outside the water temperature required will increase and the CoP will tend to reduce.

Hot Water

In ‘hot water’ mode, the heat pump is configured to bring the stored hot water up to 50 degrees (except when running the special anti-legionella sterilization cycle where it goes to 60 degrees instead). To do this, it produces water at up to 55 degrees and when doing so the heat meter records a power output of 8.8 kW while consuming up to 2.85 kW – i.e. with a CoP of 3.09.

The ‘up to’ is because the heat pump ramps up its output temperature as the hot water tank heats up, so when the tank is only at 40 degrees the heat pump only bothers delivering water at 45 degrees where it has a much better CoP of around 4.5.

Summary

Overall, the conclusion is that the NIBE F1145 is performing in accordance with its (excellent) published performance figures and has been installed and commissioned well (kudos to Carbon Legacy for that). It’s significantly beating mains gas on both cost and CO2 emissions grounds.

 

Update 2018-02-16

As the winter has progressed and the ground temperature has reduced the efficiency figures are less impressive than they originally were. See here for a revised set of figures from February rather than November.

CC BY-SA 4.0 Real-World Heat Pump Efficiency Data – Part 1 by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

5 thoughts on “Real-World Heat Pump Efficiency Data – Part 1

  1. Wow, such a useful post. Thanks so much. It’s very difficult to find any real world data not from an installer. Love what you have done.

    • I really should do another similar post now we’re properly into winter. The ground has chilled down somewhat and it’s evident the earlier very high CoP was due to the relatively high ground temperature. The NIBE F1145 is still working well but just not delivering such a high CoP.

      • Hi,
        The technical data shows that at 0/35 your pump has 4.85 COP. It would be interesting to see what you are getting in the real world scenario now the borehole is colder. The COP figures in the technical data for the NIBE are much better than some of the other manufacturers. I am just about to pull the trigger on a 12-13kw GSHP but am torn between the NIBE and another manufacturers system. Mine is an older house so I would be looking at running the radiators at closer to 45-50 degrees. Your hot water temperature figures are therefore very interesting. It would be great to see the on/off cycle times of the compressor as well ie high kw load vs 60w. Are you achieving the 8kw output with the lower borehole temperatures? My poor wife has had to put up with my banning all use of the fire whilst I have sensors all over the house and oil boiler recording our house hold energy usage in preparation for the GSHP. If you can do an update it would be much appreciated. Thanks Andrew

        • Hi Andrew,
          I’d started drafting a post last week so I’ve just completed ‘Part 2’ and published it today.
          I’ve got ‘slinkies’ rather than a borehole (2 x 50m-long trenches) since I had the luxury of having that space available and no lawn or other garden features to preserve.
          The ground loops are still performing well – brine coming in at 5 degrees and going back at about 0.5 degrees – but the efficiency has been dented somewhat. I’m still getting the rated power output though.
          I hope you can see what you need from the graphs – they’re easier to read in a ‘live’ view than in a screenshot.
          Part 2 is at http://www.marshflattsfarm.org.uk/wordpress/?p=4619
          David

          • Ah, yes, I forgot you had slinkies. I am amazed you have got away with so few. The pleasure of a newly specced house.

            Thanks for the latest update, it makes great reading. It’ll be interesting to compare against borehole stats from my system down the line.

            I will be running a radiator system which I expect to be running continuously rather than cycling like yours. With my boiler it is turning on/off every 3 minutes or so to give an average of about 11kw!. The instantaneous hot water figures at 50 degrees are a little worrying. The f1255 which I am getting will likely have to run close to that temperature in the depths of winter. However, even that is cheaper than oil. I was hoping to rig up the power sensors in the same manner as yourself but I forgot that since it is an integral tank it will be difficult to achieve this. I might only be able to measure the COP on the domestic heating and not the hot-water tank. Like wise, diverting solar energy won’t work as it doesn’t appear to have an immersion in the tank itself. The advertised SCOP is almost 3.9 for heating at 55ºC and 4.04 for the hot water which I find hard to believe.

            It is interesting that solar radiation through the windows isn’t really taken into account for the MCS calculations. I wonder how much that helps through the year.

            Cheers
            Andrew

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