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.

  • There is a lot of misinformation in this thread.

    Firstly, insulation of the house has nothing to do with HP's, it affects all heating sources, so I really dont know why HPs are singled out as 'they dont work on old properties', as, clearly they do! Of course insulation helps, and more insulation is the biggest thing to do to save energy.

    Ground vs Air. I used to be an advocate of Ground source, but no longer. I had a call out to one, it had frozen where the pipes came out of the ground, and the frozen ground went around 3 feet down. Research later showed that this was a known problem if the pipes are too close to each other in a vertical excavation, and also happens if they are too close to the surface in a horizontal format. The UKs typical air temperature is above 5 deg. C for 95% of the year, the ground will drop to to 5 deg before christmas on a horizontal excavation, and a bit later on a vertical piled array. They wont benefit from a daytime at 10 degrees, which is quite common, even in January. Vertical arrays are far better, but the install cost is beyond most normal people, and unlikely to ever pay back the install cost, even the excavations for a horizontal array at 2 metres deep may not be cost effective for 20+ years.

    Have a back up. Well, you've answered that, a gas boiler or immersion wont work if there is a power cut. A gas boiler will only consume around 250 watts power, so a generator can easily supply the power to make that run. If you have a heat loss of 3kW, then a larger generator, of 2kW or so would run the HP succesfully. Being as you have a log burner, with a back boiler,you are sorted anyway. Yes, a HP, and solar/solid fuel can be combined succesfully.

    Frost. Yes, they do frost up in humid conditions below 5 deg C. The defrost cycle will take some of the water from either the central heating, or hot water cylinder supply loop, run it in reverse (not actually in reverse, a valve opens inside the HP), run that for 2 or 3 minutes, and the defrost is finished. Of course, the COP does drop because of that, but, the HP should, if designed and fitted correctly, achieve a COP of 3.5 or higher. I was on a course last week, the Owner of the Training Centre is a bit of a researcher into HPs, and he regularly gets a COP of 6, all year round. He said on the day before, it was almost 9.

    Cost. Install cost can be almost free now with Octopus et al falling over themselves to fit them. However there is a downside to the cheap install cost. BG guarantee that their HPs will keep your house warm, but they dont say at what cost. From some inquiries, they set them to run at 50 deg.C which is not at all efficient. Octopus are similar in that they do a 'one size fits all' approach. If its cold, they just turn up the flow temperature, which gets the house warm, but adds energy costs.

    If you do want one, then it shoud be designed correctly, with a good heat loss survey done before anything else. From that, it can be decided if the current system needs upgrading. At a 3kW heat loss for the whole house (getting toward passive levels?), then you will probably be ok for the piping through the property if they are 15mm+. Radiators/emitters may have to be changed.UFH pipes are generally ok, as they usually run at a lower temperature anyway (27-30 Deg C?) The flow temperature will certainly be lower with a HP compared to a gas boiler. From the flow and return temperature is the Delta T, temperature difference. On a gas boiler it'll be 20-40 deg. on a HP it'll be 5 to 7 if designed correctly. This is what causes the need for larger emitters, not the flow temperature, basically, with a smaller DT, there is less heat given out on each revoltuion of the heated water, so, to make it up, a larger emitter is required.

    Running costs. Without good research, it can be hard to justify a HP against a gas boiler. Install cost is the big problem, as said, it can be fitted cheaply, with the 7.5k grant, can be almost free, but I wouldnt trust the Company who fitted such a HP. There are better Companies out there who do a far more detailed design and installation, and give proper after service (at the least, i would expect a revisit after a week, then a month, and then 6 months on to ensure all of the settings are optimised, being as you are technically minded, that could be something you do yourself). You would not get that kind of service from Octopus, or, indeed, any of the larger Companies who have jumped onto the £7.5k grant bandwagon.

    If you get a SCOP (years average) of 3.5, then the running cost will compete with gas. If designed properly, I think you could expect a SCOP of over 4. Get below 3.5, and gas will be cheaper. But, who is saying gas will not increase in price? It certainly wont decrease by much, whereas electricity could come down in price slightly, some suppliers are actually giving free power at certain times now as wind and solar need to be used up in their generating peaks.

    Whether to get one. The main advantage is their lack of emissions at source. Yes, there is still some coal, and a lot of gas generation, but that percentage is slowly falling, so you can say, without doubt, that you are running a greener form of heating, which will only get greener in future as renewables increase their production.

    Cost, install is certainly more than a gas or oil boiler. Running costs should be comparable to, or better than, a gas boiler. If it isnt, then the design and / or install has been done badly.Ongoing costs should be similar. They should be serviced every year (Grant do a 7 year warranty on their HPs if serviced to their spec), the service cost is typically £150. If you have a pressurised cylinder, that should also be checked every year, so combine the 2. Servicing costs are a little more than gas boilers, but, most gas services do not do a correct service, look into your service manual for any recent boiler, and there is a long list of items needing to be removed and cleaned etc, whereas most Gas Servicers clean out the combustion chamber, do a combustion and gas leak test and say it has been serviced. BG were notorious for this lack of actual servicing.

    It's a difficult choice, I'm getting a ASHP myself after looking into it a lot for my own house. Over the last few years I've done a lot of Courses about heating, and manufacturers courses on their own HPs. I wouldnt recommend one make over the other, but I'm going for a Daikin 4kW unit. Nibe was a close second, and has a better set of controls, Grant was in the frame too, but the training and back up from Daikin won the day, and the price was a bonus too, in being very slightly cheaper than the Grant, the Nibe was a lot more expensive, but I think it is a better overall unit, with better controls, but the extra cost did not justify that over the Daikin. Vaillant have been the market leaders for the last 18 months or so, but many of he other makers have caught up with them. Its a minefield, if you were buying a gas boiler, it is far easier to make a choice, as most of them are very similar, HPs are harder, as they all have their own proprietary controls, and the controls and setting them up are a big part of making them easy to use. If you are still looking next June, then get yourself along to the Installer Show at the NEC Brum, as all the Makers have their products on show in one big hall.

  • Today on Gridwatch templar Gas 47% Wind 30%.

    Wait for the blocking high pressure system and UK wind power stops. Usually happens during to coldest of winter spells.

  • That might be true for October (gas 27%, wind 33.7%), but not so for September (gas 33%, wind 24.9%) or August (gas 35.3%, wind 23.6%) or previous months.  It is rare that wind generation exceeds gas.  These are numbers averaged over the month; bear in mind that for significant numbers of consecutive days there was virtually no contribution from wind.

  • it is fair to say that if we did not have any gas fired electricity generation we would need a far better means of heat storage. But it is also fair to say that the fraction created by renewables, wind in particular, has increased massively over the last decade, and there is no reason to assume that trend will not continue - if nothing else rising gas prices encourage investment in alternatives.
    Gas  also skews the question about how best to heat the house - for 40 or so of the last 50 years or so, gas  has come from the north sea more or less at cost, while the last few years have seen the supply start to dry up, vast investment to try to keep it level ,and then the decline to resume anyway.
    I would not like to bet on the fraction of our electricity, or heating for that matter, coming from natural gas in the next 10 to 20 years doing anything other than falling further.
    I also do not think the politicians have a grip on any of this.

  • The solution is pretty obvious. Rather than the 400 billion so far chucked at wind turbines compared to the 200 million offered to Rolls Royce to get on  with small modular nuclear and then having to compete globally for the UK share. This country has no faith I  it's engineering any more.

  • yes, that sort of thing is very much worth pursuing. But really we need to be much braver and get things like the tidal stuff working as well - we have yet to realise that we are busy painting ourselves into a tight corner, and we really need to back as many potential escape paths as possible. Only backing one or two things, pretty much ensures that you miss the best,and may miss out altogether - betting on any single method is not going to cut it.


  • But really we need to be much braver and get things like the tidal stuff working as well

    Not much use on this part of the coast with a shallow slope to the sea bed and then the shipping lanes. Might be good, however, in Jersey. Then again, it is quite sunny, but not v. windy here. It's horses for courses and I agree that somebody should be investing in the future, but who?

  • We are being left behind by 15+ countries including Sweden, China, Russia and India who are current building 60 plus nuclear reactors. None of these players have any intention of going tidal, which is an impossible dream as is wind and solar. Anyway there is no rush or panic is there! We have plenty of time to work out and debate the correct logical path to follow that avoids bankrupting the country.

  • Our nuclear electricity generation programme is indeed in a sorry state and needs a kick start. But on its own it is not likely to be enough, even if we can get fusion going. There is so much investment needed in street cabling, transformers and so on to convert to electric transport and heating (resistance or heat pump) that we'd be stuck for many years even  even if the electricity was free at point of generation.

    Right now nuclear is somewhere between twice and four times the cost of wind, but the figures are so distorted by politics that direct comparison is dangerous.

    Again I really do not think the politicians have any sort of  grip on any of this.


  • Oh yes please, nuclear for me. It seems extraordinary that we have chosen to plug into France rather than build our own.

    Daughter survived school within sight of Sizewell, and Step-Daughter happily lives across the bay from Heysham. Windscale, 3 Mile Island, Chernobyl, and Fukushima have not exactly helped, but how many people died from coal-powered power stations?

  • Nuclear will keep you warm:

    Chinese long-distance nuclear heating project begins operation : Energy & Environment - World Nuclear News (

    Let's use the surplus heat energy.

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