I bought a Heat Pump

Reed_Richards

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.   

matelodave

Can you measure your heatpump's leccy consumption independently from the rest of the house. Mine used 42.95kwh on 12th February whereas yesterday when it was very mild it only used 10.92kwh  I've got an Efergy Engage energy monitor on the power supply to the heatpump which records and logs it's consumption. 

Mine is quite sensitive to external temperature variations (we have weather compensation activated) so although our peak daily consumption can get up to around 50 when it's very cold the average over the year is far less (in Jan it used 895kwh and so far this month its been around 560kwh) and in the summer when it's only heating the hot water tank it's down to around 3kwh orless a day. Our total annual leccy consumption is around 7200kwh, usually a bit less.

This how our total energy consumption varies over the year )ten year worth of monthly meter readings) It shows when it was really cold - the three blue ones in 2010 (Sept-November) were when the ASHP was fitted but we didn't have the controls installed or set up

Reed_Richards

Thanks @matelodave, that's very interesting.  These days you have no option, you must have a meter to measure your heat pump's consumption independently if you want to qualify for the RHI.  But I have to log the readings manually.    

matelodave

Those readings are my monthly leccy meter readings in my own spreadsheet.

The Efergy monitor isn't quite as accurate as actual meter readings (it's got a current clamp on the main feed to the HP) and is subject to the vagaries of odd dropouts on the interweb (the main one being power outages which reset my router - we have quite a few where we live, three last week in one day. Only short 3-4 minutes but it's irritating when the internet drops out and everything, including clocks etc has to reset or reboot. But it gives me a pretty good idea of what is going on and it's obvious if there's been a drop out so those days are discounted.

My RHI was set when it first started (about seven years ago) and is based on a nominal COP of 2.5 so I don't have to send in any info (except an annual declaration that I still own it and that is still works). In fact my last of 28 quarterly RHI payments is due in April and they'll total just over £5k.

The rules have changed multiple times since then so I don't know how they work it out anymore and my heatpump controller doesn't have the capability of remote access, recording or direct COP readings. I guess my nominal 2.5 is about right, probably a bit less in the winter but better in the summer but I'm quite happy with the performance and running costs. It took me the whole of the first winter to tweak the controls and understand the vagaries of how it works but apart from an odd adjustment it just gets on with it and runs when it want's to. Sometimes 24 hours a day and others, like yesterday hardly at all.

Reed_Richards

I've done a calculation on how my new heat pump would compare with my old oil boiler in heating the water in the central heating system up from cold to the specified operating temperature.  Actually the calculation was too complicated because the radiators will be losing more and more heat as they warm up so I simplified the calculation by assuming that did not happen.  As a result the real time it takes will be greater than the time I calculate but hopefully this does not matter for the sake of making a comparison between one and the other.

I have had to make some assumptions so I assumed the volume of water in the central heating system was 200 litres and "cold" was 10 C.  My heat pump has a 12 kW output and a specified output flow temperature of 50 C.  It replaced a condensing oil boiler with a rated 25 kW output and, presumably, an output flow temperature of 75 C.  I have different radiators now and that might have changed the volume of water but the big difference between old and new radiators is the effective surface area so I will assume the volume has not changed.

The heat-up time I calculate for the heat pump is 46.5 minutes.
The heat-up time I calculate for the oil boiler is 36.3 minutes.
The ratio of the two numbers is 1.28 and this will be the same whatever the volume of water in the heating system is.  The heat pump is much less powerful than the oil boiler was but does not have to get the water as hot so things even out somewhat. 

If you have managed to read this far, you're probably asking yourself: "What on earth is the point of all this?".  Well heat pumps have the reputation of being slow to give heat.  Once the radiators are at their specified operating temperature then if they are correctly sized it makes no difference what the heat source is and what their operating temperature is.  So if heat pumps really are "slow" it could only be in the time it takes to reach operating temperature.  The calculation for my house suggests that it might tale 1.28 times longer for the heat pump to get the central heating up to operating temperature from cold than it took my oil boiler.  That really does not seem too bad to me.  But I suppose I could have made some false assumptions; if so I will be pleased if somebody can point these out. 

Martyn1981

I thought that was really interesting, thanks.
At what COP is the output 12kW, and what would the time be at a COP of 1 (very cold outdoor temps)?

My sister is finally progressing towards ASHP after multiple Covid delays on renovation and a ridiculous catch 22 situation with the DNO, of 'we need to know what your heat pump and PV specifications are before okay'ing three phase', despite heat pump and PV installs depending on what the power supply will (or won't) be!!!!!
I think the latest plan now is 16kW 3-phase ASHP, and around 9 to 10kWp of PV depending on panel ratings.

Cardew

Reed_Richards said:

My heat pump has a 12 kW output and a specified output flow temperature of 50 C. 

.  Well heat pumps have the reputation of being slow to give heat.  Once the radiators are at their specified operating temperature then if they are correctly sized it makes no difference what the heat source is and what their operating temperature is.  So if heat pumps really are "slow" it could only be in the time it takes to reach operating temperature.  The calculation for my house suggests that it might tale 1.28 times longer for the heat pump to get the central heating up to operating temperature from cold than it took my oil boiler.  That really does not seem too bad to me.  But I suppose I could have made some false assumptions; if so I will be pleased if somebody can point these out. 

A couple of points:

Most heat pumps operate at a lower temperature than 50C for radiators; I suggest 35C is typical for maximum efficiency.

Even assuming your figure of 1.28 times longer to raise the water to the 50 C operating temperature is correct, surely that does not address the problem of being 'slow to give heat'.  The issue is not the time taken for the radiator water in a room to get up to heat, but the time taken to get the air temperature in a room to the required level(say 21C).

Edit

There is also the point made in the post above that the nominal output of 12 kW @ 55C will only be achieved at high ambient temperature. If the input to your Heat pump is, say, 4kW what will the output be to produce water @55C when the outside temp is say 2C 

coffeehound

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

Cardew

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.

coffeehound

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?