Real-World Heat Pump Efficiency Data – Part 2

This post is a follow-on from Part 1 which reported the experiences at the start of the heating season (at the end of November). After a few months of rather colder temperatures and the extraction of a considerable amount of heat from the ground, the Coefficient of Performance figures are still good but somewhat less impressive than before.

It’s evident that the temperature of the water coming in from the ground loop has a significant effect on the efficiency of the heat pump and the heat output it is able to deliver. At the end of November the ‘brine’ was typically coming in at 10 degrees whereas now it’s around 5 degrees. With brine at 10 degrees, the 8 kW heat pump was actually delivering 10 kW (as reported by the Kamstrup heat meter) whereas with brine at 5 degrees it’s ‘only’ delivering 8.5 kW. The heat pump experts tell me this is expected behaviour and it’s why an ‘open loop’ heat pump taking water from a lake or river performs better than a ‘closed loop’ unit which circulates the same brine around a closed circuit.

The headline performance statistics are now:

  • In ‘heating’ mode, the unit is delivering an instantaneous CoP of around 4.25
  • In ‘hot water’ mode, the unit is delivering an instantaneous CoP of as little as 2.75


In ‘heating’ mode the heat pump attempts to match the heat loss from the house by delivering water into the heating circuits at just the right average temperature. With colder temperatures outside this equates to warmer water, which tends to equate to lower efficiency (although actually the high thermal mass of the house reduces the impact of brief cold spells). During a typical ‘run’ it is still consuming almost exactly 2 kW (just like in November) but delivering 8.5 kW, for a CoP of around 4.25.

Hot Water

On cold-but-bright days, the hot water gets heated by the Immersun unit that diverts excess solar PV generation to the immersion heater, which has its thermostat set to 60 degrees. (It does this often enough that I’ve switched off the heat pump’s own anti-legionella sterilization cycle for the hot water tank.) On dull days the heat pump still kicks in to heat the stored hot water to 50 degrees. With the lower brine temperature (5 degrees in, 0 degrees out) it still consumes up to 2.88 kW but wth a reduced power output of 8.0 kW, for a CoP of 2.77.

Performance Graphs

The first set of graphs below are from a sample 24-hour period (2018-02-15T20:00 to 2018-02-16T20:00) which show:

  • Some of the heat pump temperatures, as reported by its internal control system
  • The heat pump power consumption, as reported by the sub-meter on its electricity input
  • The heat pump power output, as reported by the heat meter on the ‘heating’ flow pipe from the heat pump

These graphs also show the ‘cycling’ of the heat pump:

  • Overnight, it ran for 20 minutes every 1 – 2 hours
  • From 11:00 – 18:00 it shut down completely
    • This is because it was a cold but bright day: 0 degrees at 07:30 but then warming to 5 degrees by 11:30 and on to 7 degrees for much of the afternoon
    • As the outside temperature rose, the ‘Calculated’ (target) temperature for the heating circuit was reduced
    • Also, once the sun made it onto the polished concrete floor slab containing the UFH pipes the UFH stopped calling for heat
      • There are floor probes in the concrete slab which control valves on the UFH pipe loops to maintain the temperature of the slab at 23 degrees

GSHP Performance Graphs for a cold but bright day in February


The second set of graphs (below) show a ‘hot water’ cycle starting at around 2018-02-15T05:00. Note how the increased water delivery temperature (peaking at 55 degrees) corresponds to an increased electricity consumption (peaking at 2.88 kW) while the power output remains steady at 8.0 kW.

GSHP Performance Graphs for a hot water heating cycle in February

Thermographic Survey of Air Leakage

The air tightness of the building is still falling short of the Passivhaus requirement of 0.6 Air Changes per Hour at 50 Pascals of pressure – the last test came in at around 0.9. Clearly that means there’s more air leakage than there should be but the problem has been in locating where the leakage is happening in order to fix it.

The architect, the contractor and myself all made enquiries and it became evident that the most practical approach was to do a thermographic (i.e. thermal camera) survey following the procedure defined in “BS EN 13187:1999 Thermal performance of buildings – Qualitative detection of thermal irregularities in building envelopes – Infrared method”.

The main challenge with this procedure is that it relies on a significant temperature difference between the inside and the outside of the building (at least 10 degrees and ideally more) in order to show where air leakage is either heating or cooling the surfaces. Since the air tight barrier is on the inside, the basic approach is to:

  • Heat the inside of the building to a consistent temperature of more than 20 degrees
  • Use a blower door to de-pressurise the inside of the building so that external air is drawn in through the air leaks
  • Use a thermal camera to check for cold spots caused by the outside air causing cooling around the air tight barrier

For this to work best, the outside air wants to be as cold as possible – which also means the external surfaces must not be heated by the sun. The ideal time to do the test is therefore in winter (when the outside air is cold) and before dawn (when all of the external surfaces have cooled down overnight and not yet received any sunlight). The perfect time of year is around the end of December, when dawn is relatively late and the outside temperature is generally cold.

The equipment and skills required are quite specialised and only a few consultants offer this service. I contacted Apex Acoustics (largely on the basis of them having a Midlands office in Nottingham) who provide air tightness consultancy alongside acoustic consultancy. It turned out their thermographic expert is Mark Siddall (well known in the UK Passivhaus community) who is based near the Apex Acoustics main office in Gateshead.

The survey was completed this morning and showed a significant leak in a store room above the balcony and a few minor leaks elsewhere. I’ll update this post with some of the thermal camera photos when I get them from Mark.

The plan is now to fix the leak above the balcony and to check what that does to the overall air tightness figure, with the hope that it will be enough to enable Passivhaus certification.

Blower door doing its thing during Thermographic Survey