Some heat pump owners reduce the desired IAT during nighttime hours, which they may find beneficial and cost effective.
By implication - do most heat pump users maintain the same IAT during the night and daytime when not at home? I understand that the response time for a heat pump to heat a room is slower, but surely...
I can't speak for "most" heat pump owners but I can speak for my circumstances specifically.
One or other of us is working from home pretty much every day, so we don't really need to consider reducing heat output during the day (or turning the heat pump off altogether) most of the time. However, if we go out for the day on a weekend we do not normally change the heat pump's operation then either. Nonetheless, if we spend a night or more away from home we will certainly put the heat pump into holiday mode so that it is not heating an empty property but is still kicking in if it needs to do so for maintenance purposes (defrosting and so forth). We will normally then schedule it to start standard operating once again a few hours before we get back home; the place will still be slightly colder than it otherwise would be because the building's fabric won't have heated up fully but it's not a significant difference.
There are a few points worth making, however, now I've read back over the thread.
Firstly, a room doesn't have to be significantly slower to heat up using a heat pump; it's simply about how quickly you can transfer the heat and there are plenty of variables that will affect that. As several posters have pointed out on various threads, a fairly low flow temperature with a large surface area emitter will still transfer a lot of heat; for instance, our largest room has underfloor heating and it heats up quicker using the heat pump than the bedrooms did even when fed by the previous oil-fired boiler. That's simply because even though the radiators are fairly large they still aren't as good at heat transferrance even at high flow temperatures as the UFH is at low flow temperatures. Net result is that if you want your rooms to respond quicker the flow temperature isn't the only thing that could be played about with. Not, of course, saying that "playing about with" emitters is a cheap game but it does demonstrate how no two properties or even rooms are necessarily equal.
Secondly, I forget which poster mentioned it but the point was made on another thread that outside temperature has less of an effect on COP than outside humidity given how much energy a heat pump uses with its defrost cycles. That will inevitably make seasonal guides that much more awkward to produce. A cold, dry winter may well be cheaper to heat through than a slightly warmer wet winter and trying to forecast whether the next winter will be cold or not AND wet or not is quite an ask of a rough and ready tally table.
Thirdly, that lovely battery of yours could have a huge effect on the third area that could make a big difference and that's your electricity tariff; it has done for us. You mention that your day rate is 33p/kWh and your night is around 14p. Whilst filling up the battery at night and using it during the day is pretty obvious there is a lot more tinkering that can be done, especially in the winter. We moved to a time of use tariff for import as well as export and it's not surprising that the 4pm - 7pm stretch is always significantly more expensive than the rest of the time. As a result, if the solar PV isn't sufficient on a particular day to get our battery full enough to tide us over that expensive bit then we supplement the charging with leccy from the grid ahead of the expensive time. That is obviously a more common strategy in the winter than the summer but it means our monthly average import price pre kWh is around 16-17p - not much higher than your cheapest rate. Perhaps slightly oversimplifying (and certainly stating the bleedin' obvious) but whilst the tariff won't help with a heat pump's COP if you can halve the cost per kWh you can run the heat pump for twice the time for the same overall cost.
105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs
Thirdly, that lovely battery of yours could have a huge effect on the third area that could make a big difference and that's your electricity tariff; it has done for us.
Unfortunately, I have already baked in the savings. I fill my battery everynight. In Jan/Dec my current electricity usage matches my battery size. So without an extra battery, i cant have any additional low tarriff electricity. My tipping point is April, where this month (and the next 7 months) my DHW is run off my immersion at night and my total energy bill is £0 for the month. Although I could probably make a bit more of a saving by continuing to burn gas for the DHW, there is a secondary advantage, is that the (uninsulated) pipework between the boiler and cylinder runs below the main bedroom and can overheat in milder weather.
So the issue for me is as to whether I can run the heat pump at a higher cost than gas, during the winter. But make sufficient savings through the other three seasons. Having made the easy changes, the next steps are more dificult...
@majordennisbloodnok In hope that this does not muddy the waters but may throw a little light… During the last few months, I have used a setback temperature for the early hours; we have good insulation and as may be seen in the screen shot from my Homely Dashboard, there is very little temperature drop during those hours. This suggests to me that with good insulation, a setback does not necessarily impact on the IAT overnight. Regards, Toodles.
Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!
@majordennisbloodnok In hope that this does not muddy the waters but may throw a little light… During the last few months, I have used a setback temperature for the early hours; we have good insulation and as may be seen in the screen shot from my Homely Dashboard, there is very little temperature drop during those hours. This suggests to me that with good insulation, a setback does not necessarily impact on the IAT overnight. Regards, Toodles.
Looking at the data it would appear that your heat pump is still working and maintaining the IAT during the setback period, so the setback does not appear to be being performed.
Thirdly, that lovely battery of yours could have a huge effect on the third area that could make a big difference and that's your electricity tariff; it has done for us.
Unfortunately, I have already baked in the savings. I fill my battery everynight. In Jan/Dec my current electricity usage matches my battery size. So without an extra battery, i cant have any additional low tarriff electricity. My tipping point is April, where this month (and the next 7 months) my DHW is run off my immersion at night and my total energy bill is £0 for the month. Although I could probably make a bit more of a saving by continuing to burn gas for the DHW, there is a secondary advantage, is that the (uninsulated) pipework between the boiler and cylinder runs below the main bedroom and can overheat in milder weather.
So the issue for me is as to whether I can run the heat pump at a higher cost than gas, during the winter. But make sufficient savings through the other three seasons. Having made the easy changes, the next steps are more dificult...
You will know better than me about the detail but I'm still to be convinced, @graeme. The point I'm making relies on having a time of use tariff rather than a peak and night time rate. For instance, if we look at the 24 hours from 11pm tonight until 11pm tomorrow night (and speaking in generalisations) the price I'm going to be charged for any electricity I use will vary each half-hour but will be bumbling around the low to mid teens pence until about 6:30am, the 6:30 - 8:30 slot will be up to about 20 pence-ish, it'll drop back to low teens or around 10 pence-ish until 4pm and then it'll stray up into the mid to high twenties until 7pm and then drop steadily down from about 20 pence to mid teens again. If this was your tariff, all your battery needs to do is keep you from importing in the morning blip and the 4-7pm peak. If it does that then your daily tariff is effectively not an average of the day's rates but an average of the cheap bits. If you're trying to run a heat pump in the winter, you could easily drain your battery and refill it twice in a day but still at cheap points, and any time the heat pump is doing its thing and your family are doing theirs outside those two expensive points you're still consuming at the "low tariff rate". It's just a mind-shift trying to get away from the idea that you have one chance to fill the battery and then that's got to last you; with an agile tariff that's just not the case.
All that said, where you are absolutely right is your last paragraph. Running a heat pump alongside solar PV and a battery is a great setup for potential savings on your heating bill. However, it REALLY polarises your bills; when they're expensive they're real eye openers and when they're cheap they're often negative but you need that bit of confidence that the averaging over the year will work in your favour. All I can say is that when we did it we managed to shave about £1,000 per year off our energy bills.
105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs
@majordennisbloodnok Apologies - now I understand what you are doing and why it requires a leap of faith. I will need to delve into the data in more detail. I am on Octopus Flux - so a switch to Agile would be easy. I have hesitated because the controls on my inverter (Solax) are a bit limited. But I need to look again in more detail at the inverters "peak shaving mode" and the flexibility I have about charge/discharge times.
All I can say is that when we did it we managed to shave about £1,000 per year off our energy bills.
Thats really impressive. But wouldnt apply to myself as I am looking at an energy bill of £700 total this year. But nonetheless, every penny counts as they say.
@majordennisbloodnok Apologies - now I understand what you are doing and why it requires a leap of faith. I will need to delve into the data in more detail. I am on Octopus Flux - so a switch to Agile would be easy. I have hesitated because the controls on my inverter (Solax) are a bit limited. But I need to look again in more detail at the inverters "peak shaving mode" and the flexibility I have about charge/discharge times.
You might want to look into the Solax modbus integration with Home Assistant and control it from there.
105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs
That link is very interesting. So when we see claimed SCOPs for ASHPs the manufacturers are actually quoting the figures measured by this European certification company? Or are there other bodies with their own standards?
I don't recall spec sheets for ASHPs giving SCOPs for three different climates, so presumably if manufacturers are using Eurovent and they only provide one figure, it is the one for Eurovent's 'Average climate', which is based on temperatures in Strasbourg? This is going to be closer to UK climate than the other two Eurovent bases, which are Helsinki and Athens, but it still isn't going to be a very good match for most of the UK, which will not be a cold as Strasbourg in winter.
The Strasbourg SCOP is calculated on the basis of equal weightings of four outdoor temperatures: -7, 2, 7 and 12. That is, it is X/Y, where X is the sum of the kWh heat output of the ASHP to maintain 20C indoors temp at those four outdoor temp scenarios and Y is the sum of the kWh electrical input in those four scenarios. Mathematically, they seem to be modelling the ASHP as spending equal amounts of time operating at each of those outdoor temperatures, over the course of a year. So, assuming 8 months when it is delivering room heat, they are modelling 2 months at a steady -7C outdoor temp, 2 months at 2C, 2 months at 7C and 2 months at 12C. I would think that this is going to give a much worse SCOP than is realistic for most of the UK, though Eurovent don't say what humidity levels they use, so perhaps the high humidity in the UK would partially offset this.
Might these figures be a way for @graeme to estimate an answer to his question? I seem to remember some manufacturers' spec sheets (sorry, can't remember which) provide COPs for the four different outdoor temperatures, so this would give a sense of the likely ratio between the winter COP and spring/autumn COP.
The Strasbourg SCOP is calculated on the basis of equal weightings of four outdoor temperatures: -7, 2, 7 and 12. That is, it is X/Y, where X is the sum of the kWh heat output of the ASHP to maintain 20C indoors temp at those four outdoor temp scenarios and Y is the sum of the kWh electrical input in those four scenarios. Mathematically, they seem to be modelling the ASHP as spending equal amounts of time operating at each of those outdoor temperatures, over the course of a year. So, assuming 8 months when it is delivering room heat, they are modelling 2 months at a steady -7C outdoor temp, 2 months at 2C, 2 months at 7C and 2 months at 12C. I would think that this is going to give a much worse SCOP than is realistic for most of the UK, though Eurovent don't say what humidity levels they use, so perhaps the high humidity in the UK would partially offset this.
Might these figures be a way for @graeme to estimate an answer to his question? I seem to remember some manufacturers' spec sheets (sorry, can't remember which) provide COPs for the four different outdoor temperatures, so this would give a sense of the likely ratio between the winter COP and spring/autumn COP.
Yes they are a help of sorts. My outdoor temperatures are 1C(low)-6C(high), 2C-9C, 8C-16C, 11C-19C. Average these to 3.5C, 5.5C, 12C, 15C. A good bit warmer than Strasbourg. Except as you suggest, the impact of humidity. The house doesnt require *any* heating (for six months), so the later two figures are redundant to me. However:
That is for heating. How does one measure the efficiency of using a heat pump for hot water which would be for 12 months? That is raising from an external temp to a stored temp. Is it simply the same as SCOP @ a given stored temp?
"When the temperature outside is around 7 ℃, the average heat pump should have a CoP of around 4.5, however this figure can drop to 2.3 when the temperature outside decreases to -7 ℃." So, maybe you should assume a COP of about 3 in UK winter.
