Air Source Heat Pumps, SCOP and humidity

My gas fired combi is getting a bit long in the tooth now, so I've been vaguely looking at the possibility of replacing it with a heat pump.

SWMBO isn't keen on the idea of ground source - we've probably got enough land for our relatively modest heating load, but having a mature garden on the top of it makes trenching a hard sell, so I've been looking at air source...

I realize that at times (very often in the cooler months I imagine) that the outdoor evaporator coils will be below 0°C - and so will ice up from condensation. Ice will not only block the airflow but insulate the coils, so needs to be removed. No problem I understand, the heat pumps detect this and automatically go into a defrost cycle where either the refrigerant flow is reversed (taking a bit of heat back from the heating water circuits and using it to melt the ice) or by direct electrical heating. From what I can tell all that's all included in the seasonal co-efficient of performance (SCOP) figures, so I can in theory still work out (roughly) how well the system should work overall.

But thinks I, having spent the last couple of days in fog, the SCOP benchmarks for EN 14825 are done according to the climates of the likes of Strasbourg, Athens and Helsinki; and it occurs to me that the British climate is often somewhat damper, so even if the temperatures match I would have thought we'd likely get a lot more condensation, and therefore a lot more ice - so it'll have to have much more frequent defrost cycles- which is going to consume more energy for no increase in output. So the SCOP figures provided by manufacturers may be well off what I could achieve in reality - which makes me a bit nervous.

My research so far seems to suggest that the test conditions only have to reflect the temperature profiles of Strasbourg, Athens and Helsinki, so far I've found no mention of humidity, so it might be possible that manufacturers could run the tests in a relatively dry atmosphere and get improved results and still comply with the standard.

Has anyone got any ideas as to how significant the defrost cycles might be on the overall SCOP?

   - Andy.

  • Let me start to open the can of worms.

    How well insulated is your home?

    How well is it draft proofed?

    How old is the building and is the whole build of the same age?

  • In reverse order...

    About 1910 stone built semi (with cellar) - apart from a (probably) slightly later loft conversion, no significant structural changes since.

    Pretty thoroughly draught proofed - double seals on all doors/windows - all holes around joists etc sealed. Ventilation is via a central heat recovery ventilation system.

    Internal thermal insulation retrofitted - approx 150mm celotex to outside walls and ground floor and similar sprayed to underside of roof, achieving a U value of about 0.15. Cold bridges mitigated as far as practical. Windows triple glazed, manufacturer says they achieve U of 0.8 (apart from a few Velux ones on the roof, which only come to about 1.0 I think).

    Wet underfloor heating already fitted throughout. I reckon worst case heating demand in the depths of winter, including ventilation losses, is about 3kW.  Gas supplier reckons I consume about 4,000kWh/year, but a significant chunk of that will be for domestic hot water rather than space heating (which of course is another issue with low temp output heat pumps).

      - Andy.

  • Hi Andy

    Have a look at this chap on YouTube

    He has 16 video about AirSource heat pump installs.  He talks about the design and he then does the install.

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