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.

  • I have had a wee look at BS EN 14825:2022 and it does not seem to mention the defrost cycle. It mentions active mode, thermostat off mode, standby mode, off mode, and the crankcase heater.

    It may not be a big problem - I would expect that the fan on the evaporator would keep ice at bay.

    Of course, a lot depends upon your location. We get very little fog on the sunny Hampshire coast and only a few freezing days per year.

    Any road, how old is, "a bit long in the tooth"?

  • Any road, how old is, "a bit long in the tooth"?

    The main heat exchanger is visibly corroding and spare parts in general are starting to get more difficult to obtain. Not sure exactly how old it is - it came with the house 13 years ago and I suspect was a few years old then - so probably knocking on 20 years old now.

        - 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.

  • I'd go for a replacement gas boiler. Why put all of your heat and light sources into one basket? We have a gas cooker for a reason, and that reason is because during the 70's power cuits we had a constant flow of neighbours coming round to boil a kettle. It seems irresponsible to depend exclusively upon the electricity supply for everything in the home, which is why we also retain an electric shower in case the gas boiler develops a fault. It is always a good idea to retain some energy diversity in the home. With a heat pump and electric hot water tank, you are screwed if the black or brown-outs begin as a consequence of all this NOT ZERO nonsense. Heat pumps won't work when the wind doesn't blow, nor the sun shines.

  • On a recent work trip I done in the more remote parts of Scotland I was told the homes must have a minimum of 2 sources for heat in a dwelling.  This is in case of an outage. 

  • Why put all of your heat and light sources into one basket?

    The gas boiler feeds into a thermal store, which is also fed by a solar thermal panel, wood burner stove and immersion heater - so I have a degree of backup already in place (the gas boiler won't run without electricity anyway). The log burner is on a gravity circuit and will boil a kettle directly anyway - so should be reasonably power cut tolerant. Torches and if needs be candles for light.

       - Andy.

  • during the 70's power cuits we had a constant flow of neighbours coming round to boil a kettle

    I recall a neighbour bringing round the Sunday roast.

    It is always a good idea to retain some energy diversity in the home.

    A very good point.

    If you have a hot water tank, at least you can cope for a while.

  • I see what you mean. Ours are more like 40 years old, but they are floor-standing cast iron ones so they should last forever. Whether the electrics will do so is another matter.