Is this hot water use normal?

Hello all,

Yesterday I decided to look at my hot water and space heating demand for last year (2024).

Here it is on OEM zoomed to last year:
https://emoncms.org/app/view?name=MyHeatpump&readkey=9b87718992b6ed5756d94d4288059a6f&mode=power&start=1704002400&end=1735689600

Space heating:
HP electric input - 1,803 kWh
HP heat output - 6,119 kWh
SCOP - 3.39

Hot Water:
HP electric consumption - 3,013 kWh
HP heat output - 9,968 kWh
SCOP - 3.31

EPC guestimation:
Space Heating - 21,422 kWh
Hot Water - 2,023 kWh

Two things jump out at me:

  1. We are using much less on Space Heating than our EPC thought we would but the inadequacies of RdSAP calculations used for the EPC are well known.
  2. We have used a huge amount of heat for hot water both compared to our space heating and the EPC guestimate.

Background on our system
We have an Ecodan R32 8.5kW heat pump heating up a a 250L (actually about 230L after volume of coil is removed) slim cylinder.

We are a family of 5 although 3 of those are under 10 and normally share a (admittedly long) shower each night. I have quick and quite low temperature shower but my wife has longer and hotter ones.
Main hot water use is a thermostatic shower. Occasional baths (probably average once a fortnight). Kitchen mixer (but not thermostatic) tap for washing up what can’t go in the dishwasher each evening. Handwashing is done with cold water (IKEA do great single lever mixer taps where the middle position is cold).

I have been setting the hot water temperature to somewhere between 44°C and 48°C during the year as I experimented.
I’ve also been changing when we heat the water a bit. Normally this is done using a temperature drop on a probe maybe 1/3 of the way from the top of the tank combined sometimes with a scheduled run during cheap electricity.

We have solar PV so a mixture of direct electric diversion (when the excess is low) and triggering the heat pump (when the excess is high) have been used to increase the tank temperature 5-10°C more than the normal target temperature on sunny summer days (after the battery has been filled). This makes the value even worse and some of the energy isn’t recorded in the figures above.

Initial thoughts and questions

  • Have I done something wrong in my set up of OEM to cause my hot water to be inflated (although I’ve never noticed anything strange looking at the daily graphs)?
  • Could that be a correct amount of hot water use or is something going wrong somewhere?
  • I plan on putting a thermostatic valve on the output from our hot water cylinder to limit the water temperature. This should reduce standing losses in pipes, allow more dilution down when the tank is at higher temperatures in summer from diverted PV and mean the kitchen tap uses less hot when washing dishes. Will this really make much of an impact on nearly 10MWh!?
  • More long term I would like to install a heat recovery on our shower waste but this will have to wait till next any renovations are done on that room.
  • Could I install a flow or even a heat meter on the output from our cylinder to see the energy usage? Any suggestions? Could this even be used to more accurately trigger DHW runs?
  • Anything else anyone can think of.

Don’t have so many showers, that’ll be what’s using the energy. Pro rata it’s similar to our hot water energy use.

I do not trust those numbers over the entire year, as they are affected by data sampling.
Do you have cumulative kWh in separate feeds for DHW vs. space heating?

Looking at the DHW output for the past week (100 kWh) and multiplying by 52 comes out at a more reasonable 5,200 kWh. Space heating for the week is considerably higher (500+ kWh), though is not constant through the year.

Picking a random week in July shows only 55 kWh of heat put into the water tank, so the total for the year is likely to be lower than the figure above, so maybe 4,000 kWh?

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According to HeatpumpMonitor.org your numbers are actually:

Heating Consumed: 3840 kWh
Heating Produced: 13282 kWh

Hot Water Consumed: 1072 kWh
Hot Water Produced: 3202 kWh

Which seems much more typical.

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I’d say your figures have gone a bit squiffy.

Looking at the period 1/7/24-1/8/24, which is almost exclusively DHW, your consumption is around 3kWh/day, so approx 1000kWh for the year, leaving 3800kWh directed towards heating.

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Ah ok, thanks.
Is that a problem with the emoncms My Heat Pump app or just something that you have to accept with sampling?

Maybe it would be good to be able to be able to change the daily view to have per month and per year bars as I assume these are based on accumulated numbers.

Yes those look better.
Hot water is a little over the SAP estimate and the space heating is still well under.
This doesn’t surprise me as I bet thinks like the airtightness assumptions in RdSAP are well out compared to reality.

Yes. The app is only able to fetch 8000 data points for any given view, so as you zoom out it starts to miss a lot of data and the aggregates get squiffy.

The data shown on HPMorg is collected daily, so doesn’t have the same issue.

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