How did you come to the conclusion you need 12kW (what calculation did you do?).
Pre-installation, standard heat loss calcs done by me and also separately by my installer (latter had one typo which did matter but was easily corrected). We both got the same result, as did other who did competent calculations.
Here (in this thread) I am using simple logic: if the IAT stays constant, then the heat loss is equal the the heat delivered (and vice versa), and if we know one, then we know the other. Thus, if you know your historic fossil fuel heat delivered (kWh use corrected for efficiency) and plot the heat delivered against OAT and (if the correlation is OK, as it is in my plot), then you can get a very good idea of the heat loss at design temps. I thought you had made very similar proposals, ie use historic energy use to predict actual heat loss. I am just trying to demonstrate my version of the idea in practice.
Midea 14kW (for now...) ASHP heating both building and DHW
(1) my demonstration of my method for using energy in/out to determine heat loss, using my heat pump as an example, no fossil fuels involved in my example, but the principle, I suggest, holds for fossil fuels, with necessary adaptions (using my heat pump was just an example)
and
(2) @jamespa's 'divide by 2000' rough rule of thumb to convert from historic kWh use (in the example here, fossil fuels) to heat loss and so heat pump rating
They overlap, but they are not the same thing, rather two ways of trying to estimate heat loss.
Midea 14kW (for now...) ASHP heating both building and DHW
Thus, if you know your historic fossil fuel heat delivered (kWh use corrected for efficiency) and plot the heat delivered against OAT and (if the correlation is OK, as it is in my plot), then you can get a very good idea of the heat loss at design temps.
Agreed, so long as you average over an appropriate period, say 6-24 hrs. If you consider instant heat delivered or heat delivered over a short period, particularly if you practice part time heating, then it is distorted by the power needed to catch up, which generally you will want to avoid doing with a heat pump if you want it to be efficient.
Likewise if you heat only for a few hours per day and plot against oat for those hours you get a distortion.
All of the above is true unless you live in an uninsulated barn which cools very quickly when the heating is turned off.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@jamespa - @jamespa - you are still avoiding my question! Is it not the case that the divide by 2000 rough rule of thumb can give very misleading results in some cases?
Midea 14kW (for now...) ASHP heating both building and DHW
You said the above, your chart stops at 4.5, if it was 12 that would make sense. So confused by you saying 12kW and the 300% efficiency statement????
That chart shows heating energy in for my heat pump. Given an efficiency of 300%, then an energy in figure of 4 will give an energy out figure of 12.
The points are real data, as measured from my heat pump, over long periods. The raw data is minute data, converted to hourly for the plot shown.
If this is the case and you were previously using 10MWh per annum in oil, then your heat pump is running very inefficiently not at a COP of 3, or something else has changed.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Agreed, so long as you average over an appropriate period, say 6-24 hrs
I'm actually doing better than that. The chart has thousands of data points, and the ('trend') line in effect averages/summarises all those data points.
Midea 14kW (for now...) ASHP heating both building and DHW
Yeah, this isn't roof or wall insulation, bit hard to explain. Imagine the joists that separate the ground floor ceiling from the upper storey floor, the ends of those joists are just open to the outside air. In fact they have a covering of tiles, but essentially that was the total insulation, just a tile. This made the downstairs ceilings and upstairs floors very cold, but the upstairs floors are carpeted, so that made it less obvious. And let's be honest, how often do you touch the ceilings to see if they are cold, you don't, right?
So we have basically been losing warm air from downstairs almost directly to the sky. I have lifted all the tiles, put rockwool insulation between the joists at the exposed ends. It will be quite easy to open that up again to show anyone, though the guy that did our EPC nearly a decade ago when we got solar PV never bothered to look.
That sort of heat loss isn’t in any calculation I know and therefore there will be no improvement to show. BUT as a result the std survey won’t find it as an extra loss either! You have the real benefit of a warmer lounge which is what really matters.
Running your heating for most of the day and not overnight is not too far off an overnight set back. It depends on how quickly your house looses the set temperature. Ours has a large thermal mass so at an outside temperature of -2C it dropped 5C inside overnight and then took 8 hours to warm back up to 20C. We were doing the practical ashp simulation that you propose with human WC turning the boiler up and down every few days. This 8hours to warm up showed we certainly needed larger radiators for a successful ashp installation. Our lounge was the worst room in this, mostly because we had a large sofa due to overall geometry, in front of the only radiator.
2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC
@jamespa - @jamespa - you are still avoiding my question! Is it not the case that the divide by 2000 rough rule of thumb can give very misleading results in some cases?
Doubtless it is the case, for example if you live in an uninsulated barn heated part time, or want to run a heat pump in the same fast and furious way as an oil burner might be run.
But the suggestion was for the benefit of someone in a 1960s house to which the rule of thumb is likely to apply (as a sanity check only), on the assumption that they wish to run their heat pump at a reasonable level of efficiency.
Agreed, so long as you average over an appropriate period, say 6-24 hrs
I'm actually doing better than that. The chart has thousands of data points, and the ('trend') line in effect averages/summarises all those data points.
Not doubting the data, just trying to reconcile your oil consumption with your heat pump consumption whilst assuming your house loss has remained the same.
By average I mean average the heat input not average the data. If you have thousands of data points all from periods where you only heat for say 3hrs, the data is good but the heat input measured as instantaneous power isn't representative of the total energy delivered over a timescale comparable with the warm up time of the house.
What was your heating pattern with oil and with ashp and how quickly does your house cool if you switch heating off.
This post was modified 5 months ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
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