You are not comparing apples with apples. Your house is fairly unusual in that it appears to cool very rapidly and (possibly) have a low thermal mass.
I think you are making assumptions here. Review the data for Jan 2024 above, and posts passim that include setback periods. The house has a relatively high thermal mass for its size, unsurprising because of a solid ground floor, and thick solid stone walls. they are both slow to heat up, and slow to cool down.
I am. The assumptions were based on what you had said, were reasonable based on the limited information, and were necessary to explain the measured results from your oil system given your insistence that the house heat loss is 12kW. Since you (crucially)now doubt the latter and have provided more information, I would no longer make the assumptions.
You haven't attempted to do this for the ASHP because your lifestyle has changed.
NO! While it is true my lifestyle has changed (I've retired from normal work), the driver for how I run my heating is my heating system has changed. Having a heat pump is what causes me to have mostly always on heating. I even made determined attempts to add setbacks, in other words move back towards timed heating, only to be told repeatedly that I was wasting my time, although in due course they were some occasional reluctant admissions that in certain scenarios, it may not be a total waste of time.
Nevertheless you havent tried optimising your heat pump set up for your previous lifestyle, which required the house to be heated only a very small proportion of the time. I dont doubt that there are savings to be made were you to do this, I dont think we know how much!
I know it can be seen by many as heresy to say anything negative about heat pumps, but I am of the opinion that honesty is the best policy. Heat pumps do have many good things going for them - and heck, I even have one! - but for many people, saving money isn't one of them. Better to manage expectations than have irate post heat pump installation homeowners throwing bricks through windows!
I dont currently think personally that heat pumps should be sold as cost saving for those with gas boilers. Roughly the same cost is more accurate unless you can take advantage of a special tariff. I agree that honesty is the best policy, but still dont think your n=1 example can be used to draw general conclusions applicable to anything like a majority. Unfortunately I dont think we yet understand the best way to operate a heat pump when there is an extreme timed requirement/opportunity of the type you had previously.
This post was modified 2 days ago 3 times by JamesPa
I have been looking at data for my heat pump, it's just a mine field. Sun being behind thick cloud or thin cloud seems to be enough to throw results off no end.
Two days data, house at exactly the same temperature inside
Day 1 average daily temp 9.6 degs, used 4kWh of electricity
Day 2 warmer at 10.5 degs but used 6.5kWh of electricity.
Only difference one day (day 2) overcast the other bright. Try analysing that in 6 months.
I have been looking at data for my heat pump, it's just a mine field. Sun being behind thick cloud or thin cloud seems to be enough to throw results off no end.
Two days data, house at exactly the same temperature inside
Day 1 average daily temp 9.6 degs, used 4kWh of electricity
Day 2 warmer at 10.5 degs but used 6.5kWh of electricity.
Only difference one day (day 2) overcast the other bright. Try analysing that in 6 months.
Is that throwing the results off, though?
If your house benefits from thermal gain in sunny weather, and that thermal gain more than offsets some reductions in outside temperature, is the data not just accurately demonstrating that rather than displaying anomalies? I hasten to add, I'm not suggesting that actually is the reason for the data; just positing one theory in order to demonstrate the results may not be thrown off at all but simply valid data.
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
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs
If your house benefits from thermal gain in sunny weather, and that thermal gain more than offsets some reductions in outside temperature, is the data not just accurately demonstrating that rather than displaying anomalies?
I know the solar gain is the reason. My point not made well enough, is all this data is being gone though based on OAT, IAT and kWh input. A week of sunny weather not captured in the data, makes it all a bit meaningless. -3 and sunny may half the kWh input compared to -3 and overcast.
It's just as easy to dismiss elevated kWh as an anomaly, but they are the real data, the low stuff is the anomaly as it's miss represented by solar gains.
I have been looking at data for my heat pump, it's just a mine field. Sun being behind thick cloud or thin cloud seems to be enough to throw results off no end.
Two days data, house at exactly the same temperature inside
Day 1 average daily temp 9.6 degs, used 4kWh of electricity
Day 2 warmer at 10.5 degs but used 6.5kWh of electricity.
Only difference one day (day 2) overcast the other bright. Try analysing that in 6 months.
With your 3kW house (lucky you!) its not surprising that solar gain has a major effect, but then nor does it matter too much if you oversize the heat pump by a factor of 2. With a more typical 8kW retrofit solar gain has much less of an effect and it certainly does matter if you oversize the heat pump.
Nevertheless even with you house I hope that a scatter diagram over perhaps 6 months gives useful info. And if you really wanted then doubtless solar irradiance can be found somewhere on the internet.
Here is my average daily consumption power (gas use/24h) plotted against degree days. Its not perfect but its not bad, and certainly good enough to reach some reasonable conclusions.
I once did an exercise with hourly data, where I had IAT, OAT and power consumption. By making allowance in the calculations for 'history' I got an R coefficient better than 0.9 (This was on someone else's house from either this forum or buildhub, not mine for which I dont have the IAT data).
The fundamental point Im making is that there is data out there which can be used, yet we ignore it.
This post was modified 2 days ago 3 times by JamesPa
@jamespa FWIW, this relates to a 12 week period for my Gas boiler heating beginning the end of October last year.
I think this all helps in understanding my building's performance. I suspect that two areas that tend to most derail my detailed recordkeeping are those sunny days which give significant solar gain in the east-facing rooms with significant areas of glazing and windy days which increase the air-change losses. I can, perhaps, add TRVs to limit solar gain effects but I think I've already reached my 20/80 threshold (effort versus effect) for draughtproofing.
If your house benefits from thermal gain in sunny weather, and that thermal gain more than offsets some reductions in outside temperature, is the data not just accurately demonstrating that rather than displaying anomalies?
I know the solar gain is the reason. My point not made well enough, is all this data is being gone though based on OAT, IAT and kWh input. A week of sunny weather not captured in the data, makes it all a bit meaningless. -3 and sunny may half the kWh input compared to -3 and overcast.
It's just as easy to dismiss elevated kWh as an anomaly, but they are the real data, the low stuff is the anomaly as it's miss represented by solar gains.
Understood, and thanks.
If that's the case, then, perhaps it might be worth capturing some weather data too. I've found several web sites available that can give daily historic data for a given location, allowing to at least show if a day was overcast, cloudy, broken sunshine, full sun etc. and that would at least allow you to interpret the other data you're gathering in a better context.
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
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs
@jamespa FWIW, this relates to a 12 week period for my Gas boiler heating beginning the end of October last year.
I think this all helps in understanding my building's performance. I suspect that two areas that tend to most derail my detailed recordkeeping are those sunny days which give significant solar gain in the east-facing rooms with significant areas of glazing and windy days which increase the air-change losses. I can, perhaps, add TRVs to limit solar gain effects but I think I've already reached my 20/80 threshold (effort versus effect) for draughtproofing.
Looks interesting, what are the axes as it looks like consumption goes up as temperature goes up, IE the wrong way round.
Looks interesting, what are the axes as it looks like consumption goes up as temperature goes up, IE the wrong way round.
Degreedays on the bottom and daily energy on the left. It suggests a 12kW heat source may be appropriate. This compares with a heat loss of 11kW from Heat Engineer and 13.8kW with an MCS spreadsheet. Interestingly, the sum of the ΔT50 figures for the existing radiators is 12.3kW so I anticipate having to replace them all and maybe add another in the kitchen/diner which presently has two large column radiators.
I also received this in an email from Loop Energy in November last year, which is quite interesting:
Degreedays on the bottom and daily energy on the left. It suggests a 12kW heat source may be appropriate. This compares with a heat loss of 11kW from Heat Engineer and 13.8kW with an MCS spreadsheet.
Degreedays on the bottom and daily energy on the left. It suggests a 12kW heat source may be appropriate. This compares with a heat loss of 11kW from Heat Engineer and 13.8kW with an MCS spreadsheet.
That sounds like a result to me!
Further thoughts on this....
In recent years my gas use (including DHW, gas hob and very occasional wood burner effect gas stove) has been around 30,000kWh. It was 35,000kWh in 2021-2022 (was it a cold winter?). Using the '2900' rule of thumb, this latter figure suggests I need 12.1kW. Is this a coincidence?
The 12kW I derived from my graph makes no allowance for less than perfect boiler efficiency as it is based on gas volume and calorific value. Would this be oversize for my needs? Given what I read on here about 'label ratings' and how maximum output varies with other conditions, I believe that 12kW represents a pragmatic compromise. If it is too high then I'll have to hope that the ASHP will modulate to a sufficiently low level. If too low, then I'll be reminded that it's my fault for a handful of days a year.
Degreedays on the bottom and daily energy on the left. It suggests a 12kW heat source may be appropriate. This compares with a heat loss of 11kW from Heat Engineer and 13.8kW with an MCS spreadsheet.
That sounds like a result to me!
Further thoughts on this....
In recent years my gas use (including DHW, gas hob and very occasional wood burner effect gas stove) has been around 30,000kWh. It was 35,000kWh in 2021-2022 (was it a cold winter?). Using the '2900' rule of thumb, this latter figure suggests I need 12.1kW. Is this a coincidence?
As you can see there is furious disagreement (or healthy debate) going on, so I would suggest you read what everyone is saying and make an assessment what to consider.
My view is this: I've never heard of 2900, unless you live somewhere thats pretty cold (do you?) Around here (South of England) its 2200 or even 2000 (the latter giving a bit of margin).
If you dont live somewhere cold, I would explore why your gas consumption is so high as it seems prima facie inconsistent. Do you have the temperature turned up a lot or is there some other reason (eg very poor boiler efficiency), and how does that correlate/sit with your degree-day data, or do a vast amount of cooking or use vast amounts of DHW. Your summer consumption might give you a clue where to look.
The 12kW I derived from my graph makes no allowance for less than perfect boiler efficiency as it is based on gas volume and calorific value. Would this be oversize for my needs?
Maybe if the boiler efficiency is very low which, if its rather old or spends all its time cycling it might be. On the other hand if its very new and operating so its always condensing, the actual demand could be 10% higher than the gas consumption indicates. You have some information to take an educated view and might take some comfort from the fact that the spreadsheet comes out with a similar result.
Given what I read on here about 'label ratings' and how maximum output varies with other conditions, I believe that 12kW represents a pragmatic compromise. If it is too high then I'll have to hope that the ASHP will modulate to a sufficiently low level. If too low, then I'll be reminded that it's my fault for a handful of days a year.
Ignore the labels, you really do need to check the actual capacity at your design temperature which might be quite a bit more or quite a bit less!
This post was modified 2 days ago 4 times by JamesPa
