I bought a Heat Pump

Reed_Richards

According to my documentation, the SPF for central heating (output water temperature 50 C) is 3.16.  For hot water (output water temperature 55 C to raise tank temperature to 50C) it is 2.89.  The design external temperature is 3.7 C.

@Cardew is missing the point.  If you have a radiator designed to give, say, 500 W of heat at its specified operating temperature then that is what it will do.  If the specified operating temperature is 50 C in and 45 C out then that radiator will be required to have a much larger surface area than one designed for 75 C in and 65 C out.  But there is no such thing as a slow 500 W and a fast 500 W. 

Reed_Richards

My heat pump is rated down to -25 C.  I presume this is the temperature at which you require 12 kW of electrical power in to get 12 kW of heat out.  

shinytop

coffeehound said:

Cardew said:

coffeehound said:

Can I ask please: what is the COP figure for a HP when it is raising the hot water tank to 55º?

Not sure of your question.

If it is COP = 1 or COP = 5 and the output is 1kW or 12kW it will still raise the water temperature to 55 C.

The outside ambient temperature will affect the COP of any heat pump.

Thanks I'm probably confusing the terminology but I'm wondering with what efficiency does the HP heat the DHW.  How close does it get to being no better than an immersion heater on E7 rates?

The COP of an ASHP depends on the difference between outside temperature the temperature to which you want to heat the water.   Some good information in this link.
https://heatpumps.co.uk/heat-pumps-information/what-is-the-cop/
The COP for HW is generally a bit lower than the COP for heating because HW requires a higher average temperature. However it would have to be quite cold before E7 was cheaper than a heat pump for 55 deg.  Such days will be far outnumbered by much more efficient ones in the UK.  

Reed_Richards

Mickey666 said:
Those are pretty scary figures! 

.....

Only superficially scary because when you think about it December, January and February are usually the coldest months of the year.   You will always spend disproportionately more on heating during those months and with an ASHP the disproportion is magnified.  At present I am averaging 38.3 kWh of electricity per day in total but my average for March was 22.1 kWh per day.  

shinytop

Mickey666 said:

Reed_Richards said:

Yesterday was very mild for February, my heat pump consumed 21.4 kWh of electricity in 24 hours.  My total consumption since about 10th December 2020 has been 3456 kWh which works out at 47.4 kWh per day.  My unit rate for electricity is 12.993 p per kWh so my heat pump has cost me £449 in fuel cost so far, an average of £6.16 per day. 

With an oil boiler you get no feel for day-by-day consumption but my average cost per year (over two years) was £920.  This equates to an average energy usage of 59 kWh per day assuming I got 10.35 kWh per litre of heating oil.  The worst case oil usage over December 2018 and January 2019 cost me £4.52 per day.

So so far I an paying more per day to run my heat pump, although as January this year was particularly cold it's not entirely a fair comparison.  I hope I can claw-back the difference over the milder parts of the year, which is what you would expect with a heat pump.   

Those are pretty scary figures! 
Scary for you because you've spent £17,000 installing a system that's more expensive to run than the oil alternative, implying that you'll never get any return on your investment.
Scary for everyone else because fossil fuel prices are only going to increase or be taxed into oblivion so we'll all eventually be paying similar costs to you for heating our homes.
It's not easy, or cheap, being green - though I can't help but feel the early adopters are rather paying through the nose for the privilege.

Nobody said going green was cheap.  Just look at the price of EVs.  Luckily, you and other taxpayers are funding most of my £16k ASHP central heating system installation bill.  And, for me at least, I'll be saving quite a lot vs my old NSH/immersion heater system.   If I could get 65% of the cost of an EV back from the taxpayer I'd have one like a shot.    

Like with ICE cars, we're all going to have to stop burning fossil fuels for heating eventually.  

Martyn1981

Reed_Richards said:

My heat pump is rated down to -25 C.  I presume this is the temperature at which you require 12 kW of electrical power in to get 12 kW of heat out.  

Cheers, but I was assuming that the 12kW rating was the output, not the input. Just like my 3.5kW A/C units draw about 700W.
But the -25C is nice to know, and anything above that will presumably have a COP of more than 1.
For my sister, moving to a small holding, the most important thing for her, was not only moving to electric so that the emissions can be reduced (hopefully to zero eventually), but also to not have to live in a house surrounded by diesel fumes, so oil boiler out, and ASHP in. Cool.

Mickey666

Reed_Richards said:

Mickey666 said:
Those are pretty scary figures! 

.....

Only superficially scary because when you think about it December, January and February are usually the coldest months of the year.   You will always spend disproportionately more on heating during those months and with an ASHP the disproportion is magnified.  At present I am averaging 38.3 kWh of electricity per day in total but my average for March was 22.1 kWh per day.  

I agree, but you may be missing my point.

A litre of heating oil gives about 10kWh of heat and costs about 40p, ie about 4p/kWh

Electricity costs around 14p/kWh or about 3.5 x heating oil.

Therefore, your ASHP has to be around 350% efficient (on average!) just to break even on its running costs.

Yes, your electicity consumption will vary throughout the year, but so would your equivalent oil consumption - for exactly the same reasons.  Yes, your COP will vary throughout the year depending on the weather conditions, but it's unlikely to significantly exceed 3.5 is it?

So, you have spent £17,000 on a heating system that will broadly cost the same to run as an oil-fired system.  It might even cost MORE to run than an equivalent oil-fired system.

If the running costs are about the same then you will not make any savings so will never recover your capital investment.  Your payback period is effectively infinite, ie never.

Mickey666

shinytop said:

Mickey666 said:

Reed_Richards said:

Yesterday was very mild for February, my heat pump consumed 21.4 kWh of electricity in 24 hours.  My total consumption since about 10th December 2020 has been 3456 kWh which works out at 47.4 kWh per day.  My unit rate for electricity is 12.993 p per kWh so my heat pump has cost me £449 in fuel cost so far, an average of £6.16 per day. 

With an oil boiler you get no feel for day-by-day consumption but my average cost per year (over two years) was £920.  This equates to an average energy usage of 59 kWh per day assuming I got 10.35 kWh per litre of heating oil.  The worst case oil usage over December 2018 and January 2019 cost me £4.52 per day.

So so far I an paying more per day to run my heat pump, although as January this year was particularly cold it's not entirely a fair comparison.  I hope I can claw-back the difference over the milder parts of the year, which is what you would expect with a heat pump.   

Those are pretty scary figures! 
Scary for you because you've spent £17,000 installing a system that's more expensive to run than the oil alternative, implying that you'll never get any return on your investment.
Scary for everyone else because fossil fuel prices are only going to increase or be taxed into oblivion so we'll all eventually be paying similar costs to you for heating our homes.
It's not easy, or cheap, being green - though I can't help but feel the early adopters are rather paying through the nose for the privilege.

Nobody said going green was cheap.  Just look at the price of EVs.  Luckily, you and other taxpayers are funding most of my £16k ASHP central heating system installation bill.  And, for me at least, I'll be saving quite a lot vs my old NSH/immersion heater system.   If I could get 65% of the cost of an EV back from the taxpayer I'd have one like a shot.    

Like with ICE cars, we're all going to have to stop burning fossil fuels for heating eventually.  

. . . which is proof of the fundamentally uneconomic technology of ASHP systems.  Anything can be made economically attractive to the consumer if they are given grants to pay for it, and good luck to you, but it doesn't make it a sensible solution in the long term . . . at least not until the general population is lulled into paying 3x, 4x, or more than their current domestic heating costs.

I agree that burning fossil fuels will have to stop eventually, but the costs of the alternatives are going to be a huge shock.  This is why governments the world over are having to give out massive grants and subsidies to smooth the way and, hopefully, prevent rioting in the streets when people cannot afford to heat their homes despite living on an island sitting on top of centuries worth of coal.

shinytop

Martyn1981 said:

Reed_Richards said:

My heat pump is rated down to -25 C.  I presume this is the temperature at which you require 12 kW of electrical power in to get 12 kW of heat out.  

Cheers, but I was assuming that the 12kW rating was the output, not the input. Just like my 3.5kW A/C units draw about 700W.
But the -25C is nice to know, and anything above that will presumably have a COP of more than 1.
For my sister, moving to a small holding, the most important thing for her, was not only moving to electric so that the emissions can be reduced (hopefully to zero eventually), but also to not have to live in a house surrounded by diesel fumes, so oil boiler out, and ASHP in. Cool.

I think it's the same as the A/C unit.  My 14kW ASHP has a Capacity of 14kW and an input of 4.8kW.  I think that's the same as your 3.5kW/700W A/C unit.  Its nominal/max running current is stated as 14.9/35A and it needs a 40A fuse so it's never going to have an input of 14kW.  I know for HW if it's struggling it'll just use its immersion heater; don't know if it does that for heating. 

coffeehound

Reed_Richards said:

According to my documentation, the SPF for central heating (output water temperature 50 C) is 3.16.  For hot water (output water temperature 55 C to raise tank temperature to 50C) it is 2.89.  The design external temperature is 3.7 C. 

Ah that's good that you are comfortably better off than with immersion heater; I wondered if the efficiency went off a cliff above a certain flow temperature.