The move to heat pumps

Cardew

Baxter100 said:

Is there any field based performance data which shows that a retrofitted ASHP/rads system in a normal home is capable of obtaining a COP of 300%?

I've searched the net a few times for actual performance data and there surprisingly isn't much out there. There's a couple of trials the Energy Savings Trust ran and a couple of University papers and these point to a seasonal COP somewhere between 1.7 - 2.5 for ASHP/Rads systems.

Both the trials run by the EST gave very poor results. The second trial was because of the very poor results in the first and ASHP manufacturers were allowed to change the installations and the overall results were still well below a COP of 3.0. There are old threads on MSE about these trials.

This is a very comprehensive 2017 report https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/606818/DECC_RHPP_161214_Final_Report_v1-13.pdf

You can jump to the overall conclusions on page 30 which states:

Overall conclusions

The analysis presented here demonstrates that median SPFH2 values for ASHP and GSHP in the cropped B2 sample are around 2.65 and 2.81 respectively. Approximately two thirds of ASHP and four fifths of GSHP in the cropped B2 sample met the EU criterion for being considered “renewable”.Filtering the data in alternative but equally plausible ways results in similar conclusions.

Many posters on here have paid well in excess of £10,000 to retrofit an ASHP.

The economics of retrofitting an an air to water ASHP will be totally dependant on any Government Subsidy.

 

Reed_Richards

Cardew said:

.....The economics of retrofitting an an air to water ASHP will be totally dependant on any Government Subsidy.

As it was with solar panels ten years ago.  If ASHPs were fundamentally expensive technology that can never come down in price if sold in greater numbers then I can see no point in the government bothering to subsidise them.  

Martyn1981

shinytop said:

Baxter100 said:

Martyn1981 said:

Kinda get the feeling that you've rigged that example to get the costs closer. I'd have thought upgrading radiators where necessary, or some basic insulation measures would make more sense, and then use a COP closer to 300%.

You'd also have to consider the need to generate and supply 3x as much leccy.

Also, what about the DHW demand for households that 'enjoy' 12 baths/showers per day, how would they cope?

Is there any field based performance data which shows that a retrofitted ASHP/rads system in a normal home is capable of obtaining a COP of 300%?

I've searched the net a few times for actual performance data and there surprisingly isn't much out there. There's a couple of trials the Energy Savings Trust ran and a couple of University papers and these point to a seasonal COP somewhere between 1.7 - 2.5 for ASHP/Rads systems.

Even at 10p/kWh, direct heating just isn't going to be affordable.  And there is a lot of scope for users to generate huge bills; thermostats a couple of degrees too high or Martyn's excessive use of HW described above.  If you double your £500 gas bill through being a bit wasteful that's one thing but a different matter when a £1500 leccy bill turns into 3 grand.  You always need to remember it's the public we're dealing with here.   

Yep. That enormous DHW demand, mentioned by Michaels some time back, seems to be the main reason that the numbers don't work for him. I'm also not sure if a normal sized household air-to-water HP would have the spare capacity to heat up around 1m3 of DHW per day too in the coldest months.

But then we've heard all these edge case examples as reasons why other technologies, such as PV 10yrs ago, BEV's 5yrs ago, etc, etc, won't work for most. I suspect the HP industry and technology will progress just fine in reality.

JKenH

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Baxter100

Cardew said:

Baxter100 said:

Is there any field based performance data which shows that a retrofitted ASHP/rads system in a normal home is capable of obtaining a COP of 300%?

I've searched the net a few times for actual performance data and there surprisingly isn't much out there. There's a couple of trials the Energy Savings Trust ran and a couple of University papers and these point to a seasonal COP somewhere between 1.7 - 2.5 for ASHP/Rads systems.

Both the trials run by the EST gave very poor results. The second trial was because of the very poor results in the first and ASHP manufacturers were allowed to change the installations and the overall results were still well below a COP of 3.0. There are old threads on MSE about these trials.

This is a very comprehensive 2017 report https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/606818/DECC_RHPP_161214_Final_Report_v1-13.pdf

You can jump to the overall conclusions on page 30 which states:

Overall conclusions

The analysis presented here demonstrates that median SPFH2 values for ASHP and GSHP in the cropped B2 sample are around 2.65 and 2.81 respectively. Approximately two thirds of ASHP and four fifths of GSHP in the cropped B2 sample met the EU criterion for being considered “renewable”.Filtering the data in alternative but equally plausible ways results in similar conclusions.

Many posters on here have paid well in excess of £10,000 to retrofit an ASHP.

The economics of retrofitting an an air to water ASHP will be totally dependant on any Government Subsidy.

 

Interesting study. In fact the SPFH4 value of Air Source to Rads systems is even worse at 2.41.

DECC considers the most useful measure of performance for the householder to be SPFH4, weighted over both space and water heating. This is a comprehensive metric that takes account of all electricity used by the heating system including the auxiliary heater, domestic hot water immersion and building circulation pumps

Even more interesting is that they removed all the poorly performing installations (SPF of less than 1.5 from the study). Including these drops the average SPFH4 value down to 2.33.

Martyn1981

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.
Plus with his concerns about the cost already, then what happens when the COP is lost for DHW.
I'm just not sure an electric system would work for him economically, until such time as the cost of gas more fairly reflects the 'cost' of gas.

matelodave

I reckon the biggest problem with any survey or study is people - those who are actually using the system.The study is supposed to compare systems in real installations but the standard of insulation in seemingly identical houses can vary, the temperatures that each family set are going to be different as will the times when heating is required, the consumption of hot water is different.

Just varying the number and quantity of hot water draw offs from the hot water tank has a marked effect on the efficiency of the hot water system and that is reasonably easy to measure but it's much more difficult to take all the other heating system variable into account.

It really doesn't matter how efficient the system is or how well it's installed if you can't rely on people using it properly. Likewise, unless you can monitor the settings of thermostats and other factors within a household, then the simple checking of input power verus heat output, even related to outside temperature, just wont give accurate or meaningful results. Unless you can take into account all the other variable interactions with the heating system (tweaking of thermostats, adjusting timings, number of people in a household, internal and external temperatures, insulation efficiency etc) then Pin v Pout is a pretty crude guestimate.

I've had my heatpump for nearly eleven years and I reckon my COP averages out between 2.5-3 but I can't be certain because there are many occaisions when my wife tweaks the thermostats up several degrees or overrides the time settings so it can end up running continuously at higher temps than it should until I find out an reset it

Same with hot water - she seems incapable of using the cold tap so everytime she washes something, even just rinsing a yoghurt pot, hot water is dragged out of the hot tank even if it never quite gets to the tap, but all that hot water sits in the piping gently heating the loft and getting cold until the next time a pot or her hands need a rinse.

Our tank must get at least 20 drawoffs of around 5 litres a go every day (it would be more if our taps didn't have flow restrictors) There's around eleven metres of pipe between our hot tank and the kitchen tap and about 15-16 to the shower, so thats a lot of hot water that gets wasted

So although these surveys and studies may help formulate policy, they really are nowhere as good as some people make out and should be read with the same degree of scepticism as any other "real life study" and not treated as gospel.

All that said, I'm really happy with the performance, efficiency and running costs of my heatpump but I'd still have mains gas if it was avaiable.

Interestingly, I borrowed a multichannel temperature data logger when I go my heat pump and generally the input power reasonably tracked the outside temperature and inside temperatures except for some very glaring anomolies when the input power was higher than normal (indicating that the HP was working harder or longer than the outside temperature would suggest).

It transpired that these were the days when it was wet and windy, although not everso cold, the hosue was cooling due to evaporation of the wet brickwork (we live in a very exposed part of the Cambridgeshire fens) and so the heating had to run longer and harder to maintain the internal temperature. It would be worthwhile understanding if these variables were accounted for in the studies.

QrizB

Martyn1981 said:

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.

The power required is pretty easy to work out.

A cubic metre of water weighs a tonne. The specific heat capacity of water is 4.2 joules per gram per degree C, 4.2 megajoules per tonne per degree C. If the water starts at 10 deg. C and is heated to 70 deg. C that's a temperature rise of 60 degrees and an energy requirement of 252 MJ.

A kilowatt-hour is 3.6 MJ, so heating the water will take 70 kWh. To heat it in 7 hours will require a 10kW immersion heater. Big by domestic standards (and drawing ~42 amps), but no larger than some modern instantaneous showers.

JKenH

Martyn1981 said:

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.
Plus with his concerns about the cost already, then what happens when the COP is lost for DHW.
I'm just not sure an electric system would work for him economically, until such time as the cost of gas more fairly reflects the 'cost' of gas.

I have a 140 litre and a 210 litre tank with 2 x 3kw immersion heaters in each with both bottom heaters connected to my IBoost and separate controllers for the top 2 elements. These are run on two 16A circuits so although I have 4 heaters maximum heating potential is only 6kw (not 12kw) so over 4 hours I could store 24kwh. I basically use the bottom elements only for 6 months of the year powered by the IBoost so little to no grid usage (see below*). My iBoost  tells me I have used 44kwh over the last 7 days and 194kwh over the last 28 days, so averaging 6-7kwh per day. That is for two of us at home all day, daily showers (sometimes 2 in hot weather) and washing up. The washing machine is cold feed only and we have used the dishwasher once in the last 28 days when we had visitors. 24 kwh could therefore cover the usage of 7-8 people over a 24 hour period, unless they are fond of baths, which we are not. To make this work you would need 2 separate 16A immersion heater feeds (or a single 32 A feed). 

Although our Herculag immersion tanks are extremely well insulated with spare pillows, blankets and duvets (so much so my wife complains the airing cupboards are useless for airing) my set up is not as efficient as it might be if we just used the one tank. The 140 litre tank feeds 2 bathrooms, one of which we use daily and the the 210 litre tank feeds the kitchen, utility room and a further two bathrooms, neither of which get used unless we have visitors. So I am heating up a 210 litre tank every day just to do the washing up. I think if we just used one tank we could probably save 1 kwh daily from cooling losses. 

*In late autumn/winter/early spring when the iBoost gives our bathroom immersion priority and little or none goes to the 210litre tank I give the latter a 45 minute boost overnight (top element only) on our 5.5p tariff (cost approximately 12p). In even gloomier weather the first bathroom immersion gets a similar boost. This time of year we can survive one gloomy day (like today) without a boost but after 2 days our bathroom needs a 12p boost but the 210 litre tank can go 5 days, with perhaps a kettle of boiling water (say 3p) to top up the washing up bowl. In summary, (pessimistic case) my water heating cost is around 25p a day for say 3 months when there is no spare solar, 12p a day for another 3 months and next to zero in summer, say £35 or maybe £40 max a year. 

Cardew

matelodave said:

I reckon the biggest problem with any survey or study is people - those who are actually using the system.

Agreed.

Both EST trials made the point that many people simply couldn’t/wouldn’t get to grips with operating an ASHP correctly.

However that should not have been a factor in the trials as ‘experts’ were on hand to ensure the systems were set up correctly; albeit they would not have been present 24/7. The manufacturers obviously had a vested interest in getting the best results – and in many cases those results were disappointing.

Despite these results manufacturers still talk blithely of ‘up to’ a COP of 4.0 to 4.5(they tend to say 400% efficient). Rather like my Ferrari(I wish!) can do 58mpg at 30mph down a hill!

I was just looking at website promoting green products that gives a comprehensive comparison between gas boiler and ASHP costs and it uses a COP of 4.3.

There have been many posts on MSE over the past 12+ years and there is a trend for those renting properties with an ASHP, or had an ASHP already installed in a property they bought, to be critical of their system.

On the other hand, those having paid good money to have a system fitted tend to be far less critical. This could be because some take more care to operate the system correctly, or are more realistic in their expectations.