Is my COP rubbish?

[Bob77]

It's getting on for a year since I had my Daikin ASHP installed and I am wondering how my COP figures compare and whether they should be better.

 

Since installation these are the numbers I have:

Space heating: 

Energy in: 2753 kWh

Heat produced: 6437 kWh

COP: 2.34

 

Hot water:

Energy in: 1179 kWh

Heat produced: 2922 kWh

COP: 2.48

 

I thought that 2.5 is the low end of what I should be achieving, but both numbers are lower than this. Even in the very warm month of June, with only DHW being heated, the COP only came out at 2.4.

 

I had a problem (faulty flow sensor) when the system was first installed which meant that the unit was cycling on and off in the early part of winter but since then all seems to have been working well. I have weather compensation active when the heating is on, but the heating mode has been switched off since the beginning of June. During the cold part of winter we had building work going on and large holes in the wall, so the unit would have been working very hard back then, which I thought was to blame for the poor COP. I assumed that DHW operation during the warm months should improve my average COP but that doesn't seem to be the case.

 

Tank temperature is set to 51C (comfort) and 47C (eco), and the unit is set on "schedule + reheat", i.e. it will heat up to the comfort temperature overnight and then heat up to the eco temperature in the afternoon, but will also reheat if the tank gets depleted during the day. Should the COP be higher for hot water at this time of year? Are my tank temperatures too high? The water from the hot tap is never "too hot" for domestic use (so can't be coming out at 50C!).

[SteamyTea]

6 minutes ago, Bob77 said:

During the cold part of winter we had building work going on and large holes in the wall, so the unit would have been working very hard back then, which I thought was to blame for the poor COP

That is not going to help.

Do you know the flow temperatures for space and DHW heating?

[Bob77]

10 minutes ago, SteamyTea said:

That is not going to help.

Do you know the flow temperatures for space and DHW heating?

 

The tank target temperature (which I assume is the same as the flow temperature for DHW?) is set to 51C, and 47C for the "eco" secondary reheat during the day which is not always needed.

 

I will add the WC curve settings on here shortly - the unit is in the airing cupboard which is in our bedroom and I will lose points if I disturb my wife from her Sunday lie-in 🙂

 

But presumably WC is is irrelevant to the COP I am getting for the DHW? I am curious what COP people are getting in summer for DHW only. Is the warmer outside air offset by the higher flow temperature needed, which reduces the efficiency?

 

I wonder whether I should be heating the water during the daytime in summer rather than overnight. I have always done it overnight as we are still on Economy 7, but I am wondering whether to switch away from that. At the moment we use about 35-40% of our electricity on cheap rate which is just about on the right side of break even. But if I am losing efficiency by running the DHW when it is 15C outside instead of 25C then I should switch! I've just had a smart meter fitted so more tariffs will be available to me now.

[RobLe]

A cop of 2.5 is fine for dhw, although it’s a fair amount of total heat used.  Do you need to reheat the tank so often?  There’s a lot of pipe and tank loss from dhw heating.  We heat ours just once a night, in the cheap slot electric.

 

Space heating is a poor cop, likely indicating your rads are running hot.  What is the heating flow temperature (I’m guessing the installers set it high).  In the ideal world when you touch a rad you can barely tell it’s on, ours run at 32C.

[SteamyTea]

3 minutes ago, Bob77 said:

The tank target temperature (which I assume is the same as the flow temperature for DHW?)

Not really.  Energy has to be transferred from  the heating coil to the stored DHW, so there needs to be a temperature difference.

This gets a bit more confused as if the return temperature is too high, or too low, efficiency is also lost.

5 minutes ago, Bob77 said:

I wonder whether I should be heating the water during the daytime in summer rather than overnight

Generally there will not be much of a difference as there is not much of a difference in temperature (well not where I am, but other places may be larger).

I heat my water so that the heating period finished (I am on E7 to) as close to the time I want to use the water.  This reduces the thermal losses from the cylinder.

That is easy for me to do as I live alone, not so easy for a family.

 

[ProDave]

5 minutes ago, Bob77 said:

The tank target temperature (which I assume is the same as the flow temperature for DHW?) is set to 51C, and 47C for the "eco" secondary reheat during the day which is not always needed.

The flow temperature (known as water leaving temperature on my ASHP)  will need to be about 5 degrees hotter than the target tank temperature, so if your target is 51 degrees the flow temperature will need to be at least 56 degrees.

 

Most ASHP's won't give you a direct reading, of COP.  It is easy to measure energy in, but very much harder to measure energy out.  So I would not trust the readings yours gives you to be completely accurate.

[JohnMo]

First lower your cylinder temp. Do max temp of 47 - 48. That will give you a good increase in CoP.

1. Check to make sure your immersion isn't being asked to come on during water heating.

2. Also make sure the controller isn't asking for heat continually so it on off a lot.

3. When the heat pump is doing cylinder heating is it doing it in one run, or does it stop and start doing it?

 

What are WC flow temps and how are you operating?

Radiators or UFH ?

 

 

[SteamyTea]

Flow temperatures come up a lot with heat pumps.

Do you have a way of reading them reasonably well.

With DHW, you need to take the temperature reading as it enters the cylinder as well as exiting the heat pump.  There may be losses on the journey there.

[Bob77]

Thanks for the replies, everyone.

 

As far as I can see there is no setting for flow temperature for DHW, only the tank target temperature.

I have checked my WC settings - it is a two-point curve (if two points can form a curve!) with flow temp 50C at outdoor temp of -2C and 25C at outdoor temp of 23C. But as I say the space heating mode is off completely at the moment. We have UFH downstairs and rads (new and sized for the ASHP) upstairs.

 

The DHW is set with only two periods when it comes on - one overnight (to hit the higher temp) and a reheat in the afternoon  to hit the lower temp. It is also set to reheat if the tank temperature drops below 40C but that only tends to happen if we have multiple baths or showers.

 

Generally it only comes on during the scheduled hours. The back-up immersion heater shows zero hours of usage so it's not that.

I will take the tank temperature down a few degrees and see if that improves matters.

Edited August 27, 2023 by Bob77

[Beelbeebub]

what type of house do you have?

 

6,500kwh for space heating seems a bit low? For example the average UK household uses closer to 10,000kwh for space heating.

 

That figure seems to tally with a well insulated new house or maybe a flat or terrace.

 

If you had problems with the flow sensor over winter, not only might that cause inefficient running, but it would also affect your heat out readings as they are a product of the temperature difference between flow and return and the flow rate.

 

Do you have any annual heating estimates from before you fitted the HP (often on your bill somewhere)?

 

Does 6,500kwh tally with what you were using before? 

 

What happens to the CoP if you assume you used the assumed consumption from you bills? 

 

[ProDave]

9 minutes ago, Bob77 said:

Thanks for the replies, everyone.

 

As far as I can see there is no setting for flow temperature for DHW, only the tank target temperature.

I have checked my WC settings - it is a two-point curve (if two points can form a curve!) with flow temp 50C at outdoor temp of -2C and 25C at outdoor temp of 23C. But as I say the space heating mode is off completely at the moment. We have UFH downstairs and rads (new and sized for the ASHP) upstairs.

 

The DHW is set with only two periods when it comes on - one overnight (to hit the higher temp) and a reheat in the afternoon  to hit the lower temp. It is also set to reheat if the tank temperature drops below 40C but that only tends to happen if we have multiple baths or showers.

 

Generally it only comes on during the scheduled hours. The back-up immersion heater shows zero hours of usage so it's not that.

I will take the tank temperature down a few degrees and see if that improves matters.

50C seems hot for heating temperature?  does it really need to be that hot?

 

DHW flow temperature may not be such an issue.  If you monitor the flow and return temperatures as it heats the tank, you will probably find the return temperature is about the same as the tank temperature at that time, and the flow temperature is a few degrees higher.  As the tank warms up, both steadily increase.  It is physically impossible for the HP to run at much higher flow temperature as it's only a low power heater.

 

The setting on mime is maximum flow temperature, a maximum cap on how hot you let it get.

[JohnMo]

8 minutes ago, Bob77 said:

flow temp 50C at outdoor temp of -2C and 25C at outdoor temp of 23C.

That could well be the issue for the cop during heating, your flow temp is way to high. Are you trying to run the heating in short bursts or for long run times?

 

CoP at 50 (at -2) is about 2.5 at best, at 45 you will at 3. Ideally run at 35, 24/7, then your cop jumps to 3.2.

[SteamyTea]

What is needed is a chart with flow temperature, CoP and running hours in it.

 

No point saving 0.5 of the CoP if the running time is double.

[Bob77]

1 hour ago, JohnMo said:

That could well be the issue for the cop during heating, your flow temp is way to high. Are you trying to run the heating in short bursts or for long run times?

 

CoP at 50 (at -2) is about 2.5 at best, at 45 you will at 3. Ideally run at 35, 24/7, then your cop jumps to 3.2.

 

50C at -2C so it is not very often 50C. The WC has a slope of 1 so every degree above -2C outdoors, the flow temperature is 1 degree lower than 50C.

 

 

Generally during the winter the outdoor temp is between say 4C and 10C which equates to a flow temperature of between 44C and 38C. I did try having the 50C point at a lower outdoor temperature (-6C) but found the house a bit cold when the outdoor temp dipped to freezing.

 

I have the UFH controlled by timed thermostats so the heat pump is on during the cheap rate to heat up the floor in the early morning, then it is set back during the day. I was still experimenting with changing the settings during the first winter, and haven't yet worked out whether it is better to use the "storage heater" ethos of pumping a lot of heat into the slab overnight when electricity is cheaper, or to have it "always on". I have read varying opinions on this.

 

As I say last winter we still had a non-airtight house during the coldest spell so I need to test it more now that the building is finished.

 

1 hour ago, Beelbeebub said:

what type of house do you have?

 

6,500kwh for space heating seems a bit low? For example the average UK household uses closer to 10,000kwh for space heating.

 

That figure seems to tally with a well insulated new house or maybe a flat or terrace.

 

If you had problems with the flow sensor over winter, not only might that cause inefficient running, but it would also affect your heat out readings as they are a product of the temperature difference between flow and return and the flow rate.

 

Do you have any annual heating estimates from before you fitted the HP (often on your bill somewhere)?

 

Does 6,500kwh tally with what you were using before? 

 

What happens to the CoP if you assume you used the assumed consumption from you bills? 

 

 

It's a three-bedroom end of terrace, 1960s but we have added a side/rear extension which has almost doubled the ground floor space. Floor area is now about 1300 sq ft, up from 950 previously.

 

Bear in mind that this is not for a full year - the ASHP was installed in October. But it does cover one whole winter, so I suppose the space heating is for a whole year, more or less.

 

We had electric storage heating before (no gas at the property).

 

Our total annual electricity consumption was typically around 13,000 kWh in previous years, but I am not sure what proportion of this was space heating. The MCS estimate quoted annual energy requirements of 13,775 kWh for heating and 4,270 kWh for hot water - a total of 18,045 kWh which is clearly far more than we were actually using, but that was based on the larger footprint including the extension.

 

Edit: I just had a look at the EPC and it reckons the house should need 8,098 kWh per year for heating and 2,943 kWh per year for hot water. So basically 11,000 kWh.

 

We are certainly using less electricity than previously, even with the much larger house. Every month since we moved back in has seen lower consumption than the same month the previous year.

 

Monthly values (previous year in brackets)
Oct 2022: 599 kWh (639)

Nov 2022: 841 kWh (1178)

Dec 2022: 1293 kWh (1342)

Jan 2023: 1286 kWh (1753)

Feb 2023: 1094 kWh (1457)

Mar 2023: 1091 kWh (1147)

Apr 2023: 794 kWh (860)

May 2023: 553 kWh (786)

Jun 2023: 433 kWh (605)

Jul 2023: 405 kWh (440)

 

So I am not unhappy with the cost of running the system - it is clearly a lot cheaper than the old storage heaters, and much better in terms of maintaining a constant temperature rather than the old heaters which would be "boiling in the morning, run out of heat in the evening". I am just curious why the COP figures seem low.

 

I know that some heat pumps report the "heat produced" as the additional heat produced (i.e. on top of the energy input), which means you have to add 1 to the apparent COP. But I am pretty sure that Daikin doesn't report this way.

 

 

 

Edited August 27, 2023 by Bob77
EPC figures

[JohnMo]

Making a bit more sense now.

 

Just probably need to do a bit of fine tuning on the flow temps for CH to use the full cheap rate window and have the heat stored you need at the lowest input temp.

 

Your problem could be you are also trying to heat the cylinder and charge for the floor in only 7 hours.

 

You may need a simple spreadsheet, to work out best time to heat water and best flow temp to charge the floor, for the least money.

 

Have attached my ASHP tech spec, which should be good enough to get you going. It outside temp, flow temp, power output, power input and cop. If your heat pump is bigger than 6 have numbers up to 16kW.

 

[Beelbeebub]

1 hour ago, Bob77 said:

Our total annual electricity consumption was typically around 13,000 kWh in previous years, but I am not sure what proportion of this was space heating. The MCS estimate quoted annual energy requirements of 13,775 kWh for heating and 4,270 kWh for hot water - a total of 18,045 kWh which is clearly far more than we were actually using, but that was based on the larger footprint including the extension.

As you were using electric heating before, the 13,000 reading will be the absolute accurate measure. 

 

The question is how much of that is hot water. If you take 20% as typical your annual heating demand was about 10,000kwh, which would be typical for a house you described.

 

Though we are not a full year, we are mainly through the heating season. 

 

If you actually used 8,500kwh and the 6,500 is an error due to the flow meter problem, then your Cop would be closer to 3, which would be more in line with your flow temps. 

 

I'm not saying your system is perfectly set up, but it may not be as bad as the figures you posted suggest. 

 

I've been tricked in the past (not with heating systems) into long and complicated programs of improvements which were actually down to a measurement error in the first place.

 

[AliG]

1 hour ago, Bob77 said:

 

Monthly values (previous year in brackets)
Oct 2022: 599 kWh (639)

Nov 2022: 841 kWh (1178)

Dec 2022: 1293 kWh (1342)

Jan 2023: 1286 kWh (1753)

Feb 2023: 1094 kWh (1457)

Mar 2023: 1091 kWh (1147)

Apr 2023: 794 kWh (860)

May 2023: 553 kWh (786)

Jun 2023: 433 kWh (605)

Jul 2023: 405 kWh (440)

 

 

This table does indeed suggest a very poor CoP despite some of the hopeful comments re a sensor issue.

 

The electricity use will include normal household use presumably which clouds things, but allowing for say 300kWh a month for this suggests a very poor CoP for hot water.

 

The winter months are a difficult comparison as the house is also larger now.

 

CoP tends to fall dramatically at outside temperatures below 5C and with flow temp of 45C+. This is also when you will use most energy. It may be that these periods accounted for a disproportionate amount of heating demand so pushed the CoP down. You will use at least twice as much heating energy at 0C outside as at 10C outside, indeed maybe even three times as much depending on insulation and heat generated by the occupants and waste heat. You need your flow temp to be around 40C when outside temp is below 5C to get a heating CoP of around 3 and it sounds like you can't quite get it that low.

 

Your HW CoP of 2.48 is consistent with around a 55C flow temp. What makes it run in comfort vs eco mode? Reducing the HW flow by 5C would add around 0.5 to the CoP, you probably need your hot water set at 47ish to achieve a CoP of 3. To some extent is a choice of how hot you want your hot water versus the cost of hot water. As I normally shower at around 40C, 47C is plenty hot enough (you will lose some temp as it flows). But if you wash dishes by hand you might not feel the temperature is hot enough.

 

Basically you might be able to get the CoP into the high 2s but I suspect that you need better insulation levels to get into the 3s.

 

The best saving you can get with an ASHP is to move onto a cheap overnight electricity tariff and run your HW and as much heating as possible overnight. This will give you a way bigger saving than a slightly better CoP.

 

2 hours ago, SteamyTea said:

What is needed is a chart with flow temperature, CoP and running hours in it.

 

No point saving 0.5 of the CoP if the running time is double.

 

I am not sure what you are saying here @SteamyTea. A lower flow temperature and better CoP would use less energy even if the ASHP ran for longer as the amount the house is heated up is dependent on the toal energy input irrespective of the time it takes.

[Bob77]

2 hours ago, AliG said:

 

This table does indeed suggest a very poor CoP despite some of the hopeful comments re a sensor issue.

 

The electricity use will include normal household use presumably which clouds things, but allowing for say 300kWh a month for this suggests a very poor CoP for hot water.

 

 

 

We are not using anything like 300 kWh per month for hot water. Even in winter it is no more than half that, and in summer it is less than 100 kWh.

 

I have been logging the ASHP usage since the start of the year as well as the overall electricity use. We have two children including one who is not toilet trained and so we do rather a lot of laundry and tumble drying, which doesn't help our energy usage. (This is done overnight where possible, and we do dry outside when we can).

 

Monthly values - total usage, ASHP space, ASHP DHW, (non ASHP use in brackets)

Jan 2023: 1286 kWh, 635, 147, (504)

Feb 2023: 1094 kWh, 500, 136, (458)

Mar 2023: 1091 kWh, 435, 125, (531)

Apr 2023: 794 kWh, 220, 110, (464)

May 2023: 553 kWh, 53, 100 (400)

Jun 2023: 433 kWh, 5, 90 (338)

Jul 2023: 405 kWh, 0, 75 (330)

 

Actually that's quite eye-opening how little of our energy usage actually goes on heating and how much is on other stuff! Apparently the average three-bedroom household uses 2900 kWh/yr of electricity for non-heating purposes, or 242 kWh per month. We are at least 50% above that 😐

 

I will turn the hot water temperature down to 47C and see how it goes. (I won't tell anyone and will wait to see if there are any complaints about the hot water temperature!)
Regarding the comfort vs eco setting, I had it set to the higher temperature overnight, the logic being to get the heat into the tank at cheap rate for morning showers, and then if it needs topping up during the day we can probably make do with a lower temperature.

 

2 hours ago, Beelbeebub said:

As you were using electric heating before, the 13,000 reading will be the absolute accurate measure. 

 

The question is how much of that is hot water. If you take 20% as typical your annual heating demand was about 10,000kwh, which would be typical for a house you described.

 

Though we are not a full year, we are mainly through the heating season. 

 

If you actually used 8,500kwh and the 6,500 is an error due to the flow meter problem, then your Cop would be closer to 3, which would be more in line with your flow temps. 

 

I'm not saying your system is perfectly set up, but it may not be as bad as the figures you posted suggest. 

 

I've been tricked in the past (not with heating systems) into long and complicated programs of improvements which were actually down to a measurement error in the first place.

 

 

The 13,000 per year figure was our total usage, not just heating/DHW. Based on my figures above, we seem to be using around 400 kWh per month for other things, so you can probably take off 4000-5000/yr, leaving a heating/DHW demand of 8000-9000 kWh per year. Albeit that was with a lot less space to heat!

Edited August 27, 2023 by Bob77

[ReedRichards]

12 hours ago, JohnMo said:

That could well be the issue for the cop during heating, your flow temp is way to high.

 

Not necessarily!  Your system will have a design temperature, actually two - one for the UFH and one for the radiators, which will almost certainly be a higher temperature.  If your radiators were sized so that they need to run at 50 C when it's -2 C outside and the calculation for this design was correct then that is what you need to do or the radiator-heated floor will be too cold.  Of course, if your radiators had been sized so that they could run at 45 C when it's -2 C outside then should have given you a heating system with a better COP but maybe you didn't have room for radiators that large? 

[AliG]

20 hours ago, AliG said:

The electricity use will include normal household use presumably which clouds things, but allowing for say 300kWh a month for this suggests a very poor CoP for hot water.

 

That’s what I said. I was assuming 300kWh for general household use and 100kWh for HW.

 

I had to make some kind of assumption to guess at the change in energy usage for hot water, but I see that the actual HW figure is available now.

 

18 hours ago, Bob77 said:

We are not using anything like 300 kWh per month for hot water. Even in winter it is no more than half that, and in summer it is less than 100 kWh.

 

[JohnMo]

11 hours ago, ReedRichards said:

 

Not necessarily!  Your system will have a design temperature, actually two - one for the UFH and one for the radiators, which will almost certainly be a higher temperature.  If your radiators were sized so that they need to run at 50 C when it's -2 C outside and the calculation for this design was correct then that is what you need to do or the radiator-heated floor will be too cold.  Of course, if your radiators had been sized so that they could run at 45 C when it's -2 C outside then should have given you a heating system with a better COP but maybe you didn't have room for radiators that large? 

That being the case, he cannot expect that much better CoP. If an MCS install, all the expected efficiency, design temps and running costs should be laid out. If performing worse than that, something isn't right. If running as per those figures he getting the expected results. But cannot see why an MCS install would be in the mid 2s CoP.

[Lofty718]

2.5 CoP is terrible. 4 can be acheived nowadays

 

UFH should be running constantly at a low temperature, just set back slightly at night. Heating the floor like that with a heat pump is an absolute no-no

 

My gas boiler runs a weather compensation curve of 0.7 for radiators and 0.55 for UFH, yours seems too high at 1

[SteamyTea]

5 minutes ago, Lofty718 said:

UFH should be running constantly at a low temperature, just set back slightly at night

Except the electricity cost less at night, so raise the temperature and turn off during the day may be cheaper overall.

[Lofty718]

11 minutes ago, SteamyTea said:

Except the electricity cost less at night, so raise the temperature and turn off during the day may be cheaper overall.

It's not how wet UFH on a slab is designed to run. Blasting the floor with heat at night because that's when leccy is cheap is plain daft

 

The same with gas boilers, running on/off controls makes them inefficient.

Edited August 28, 2023 by Lofty718

[JohnMo]

4 minutes ago, SteamyTea said:

Except the electricity cost less at night, so raise the temperature and turn off during the day may be cheaper overall.

That's my current plan, fairly flat WC curve, 0,1 hysteresis thermostat set 0.5 degs below target temp.  Set WC to allow a 5 to 6 hrs slab heat time at start of heating season and about 8 to 10 hrs at the lowest likely temp.  So most electric usage coming from off peak E7 and if required draw on the battery in the day.

 

1 minute ago, Lofty718 said:

It's not how wet UFH on a slab is designed to run.

 

The same with gas boilers, running on/off controls makes them inefficient.

That said if you have a decent screed depth it works a treat. Last 2 years with gas boiler I tried every option going (will try same this year on ASHP), recording boiler efficiency and gas usage etc.  Batch heating overnight worked really well. Stable room temps, lowest gas usage of any heating pattern tried, basically flowing approx 35 degs (weather compensated) for 6 to 10 hrs overnight. So on for 6 to 10 hrs off at all other times, long run times, long off time. Found switching the heat off 0.5 degs below your target temp allows the final heat to percolate through the floor and not have any temp overswings