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.

  • Gas supplier reckons I consume about 4,000kWh/year


    Using rough figures from my own tariffs, at 30 p/day (which you will still be paying if you have a gas cooker, or even a gas fire) and 7 p/kWh, that comes to less than £400/year.

    I suppose that you could ignore your leccy standing charge, because you already pay that. 4000 kWh @ 30 p = £1200, but you would be using only a third of the amount, which comes to £400/year.

    On the face of it, you simply don't use enough energy to justify converting to ASHP.

  • 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.

    Interesting. I've read (I think in a MCS document) that the figures are normally based on "SEPEMO ‘boundaries" - which seem to include the electricity used by all the auxiliary parts usually inside the box (pumps, auxiliary heaters and so on) - so I was hoping that any electrical defrost heater would be included in that, likewise a reverse refrigerant system would reduce the cumulative heat output, so again should be counted. By big worry is how often the defrost cycle would be needed in reality, compared with lab conditions (not that I'd expect manufacturers to choose conditions that gave the most favourable result or anything).

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

    I'm not sure how much heat the fan would produce (low noise, low energy seems to be the order of the day), plus some of the ones I've been looking at seem to have the fan at the back/outlet side. I might suspect that things could well be arranged to reduce the problem (e.g. the front layer of the evaporator being at a slightly higher (above zero) temperature than the main part (but mostly still below air temperature), encouraging condensation to form there where it can drip away without freezing and so reduce the ice load on the main (cooler) part of the evaporator. 

    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.

    Unfortunately, I'm in the lea of the Pennines where cold and damp seem to be the norm in the bottom half of the year. Still there'll be  solution, I just have to find it!

       - Andy.

  • Don't let the greenwashing fool you, buy a gas boiler before they are banned!

    For most of us there is no viable alternative at the moment.  (Air source) Heat pumps are not working aswell as advertised, especially at lower temperatures.  Ground source can be effective but again has limitations as you need the deltaT, also huge installation costs and this is crucial for their effectiveness.

    The boiler in my house is over 30 years old and still going strong.  But I am worried when it fails I might not be able to buy a gas boiler replacement.

  • well, where we are now, is that it is gas heating in new build houses that will be phased out from 2025, and replacement boilers in existing housing from 2035 onwards. That in turn suggest to me that most older houses will still be running gas fired heating until perhaps the mid to late 2040s as the last decade or two of installations start to fail beyond repair, though a sharp rise in gas price may lead to the switch off of working systems.

    I think you have a good while to organize a replacement, and if you wish to stay with gas (and if you already have it I would at the moment) then there is at least a decade or so to do so.

    Mind you this is govt policy we are considering, so it may be cast aside at any point, but I'd be surprised if the dates go much nearer, I'd not be at all surprised by a slight delay when the deadline gets closer.


  • Yes thats true, I think the 2025 date will stick for new builds but the way things are going with targets and so called called climate crisis the farther away dates may yet come in!  But hopefully common sense will prevail and they will realise there is no viable alternative and scrap it.

    I'll buy a replacement before they are banned so will be running a gas boiler in my house at least until 2060, or until the gas supply is off or I am deed.

  • Weeell, there is a viable and very cheap alternative - but it certainly involves a drop in living standards and possibly lifespans- and that is not to heat houses very much at all.  This would revert to where we were before central heating - there is a reason for all those sleeveless jumpers in old photos, after all,- but we have had at least one generation, probably two,  that know nothing else but instant warmth, and would certainly struggle to adapt to that.

    At the moment folk talk about choosing to eat or heat, as if both are equally essential, while really to some degree food fuel and clothing can be interchanged, but then some folk think electricity and or the internet connection is essential too..... 

    It really depends how it pans out - even if climate change does not get us, the gas fields running flat certainly will at some point, so some adjustment will have to happen. If it is not planned, then simply being priced out  will do it, but just waiting for that to happen is not an outcome I would favour.


  • that is not to heat houses very much at all

    Modern houses should not need to be heated very much at all, which is why electric heat pumps may suffice.

    When I was a lad, many people did not heat their bedrooms and there would have been a coal fire downstairs. That's no good at least in England now.

    This year we have managed to get our gas consumption down to 60 MWh. I don't think that there is currently a viable alternative to gas.

  • Indeed - that coal fire would have been lucky if it was equivalent to 2 or 3 kW when well made up, and there would have been ice on the bedroom windows inside in the morning in winter - the only time a bedroom fire was lit if some one was really ill. - I recall that time too, but I was young..

    Of course if it is the same temp inside and out, there is not a lot of  point in double glazing either.

    I'm not advocating for this by the way, quite the reverse, but I wish to make it clear where I think the 'do nothing' scenario ends up.

    We can change the designs of future houses of course, the problem is more the rows and rows of hundred plus year old solid walled terraces that are the majority in many older cities. In the newer ones, the gas might have been original for lighting and then electricity was added at some point, then a phone line, and now in the next hundred years or so, for sure,  the gas is going to end.


  • Thats a really good point actually, better insulation and building standards mean less heat loss and thereby less demand for heaitng appliances and heat pumps can be more effective.

    But the flip side of that would also mean less gas consumption required!

    Plus if you factor in global warming (not that it really is much warmer in the winter) then there should be higher temperatures and again less heat demand, in theory at least.

  • In the newer ones, the gas might have been original for lighting and then electricity was added at some point

    Probably neither when our place was built in around 1902. The gas lighting seems to have been put in when the house was extended circa 1923, but there was no mains gas available at the time so it would have been only for lighting.

    Coal consumption would have been several tons per year then, about 5 cwt now.