Running an ASHP on a ToU tariff

[SteamyTea]

As it is winter, the same questions are coming up, again, again.

I decided to have a look at my temperatures to see what is happening at different times of the day, even though I don't have an ASHP, I thought it would be an interesting exercise anyway.

I sample, and log, temperature data via my energy monitor (approximately every ten seconds) and via a RPI and a DS18B20 outside (every 600 seconds).

For analysis purposes I take the mean temperature over each hour and use that.  The standard error of the mean is smaller than the resolution of the DS18B20, so I am not worried too much about accuracy being way off.  For this analysis I am using daily means taken from the hourly means, while not perfect, they are easily good enough to get a decent understanding of what is really happening.

Over the last 11 months, the daily mean internal temperature has been 20.9°C (the secondary glazing fitted last year has raised my temperature about 1°C compared to previous years), the minimum has been 17°C (Jan 8 when I was away) and the maximum 25.4°C (was that heat wave, on June 24, I grew up in the tropics so don't find it excessive).

This is shown, by the red line, in the chart below.

 

 

The chart below shows the outside air temperature.

 

 

Another way to look at this is temperature differences between the internal temperature and the external temperature, again it is the red line.

 

 

 

You may have noticed that there are two other series on these charts.  These show the temperatures between midnight (0 hour) up to 8 AM and 8 AM to midnight.

Over the year the mean temperature difference has been 9.8°C, with a minimum difference of 1.1°C and a maximum difference of 19.6°C.

The 0 hour up to 8 AM (near enough my E7 window) mean difference is 10.4°C, minimum of 4.5°C and maximum of 19.5°C.

The non E7 times the mean temperature difference is 9.4°C, minimum of 2.4°C and maximum of 19.6°C.

Now I do not know how much of a difference the coefficient of performance is affected by, at worse, a 2.1°C difference when the mean temperature is 9.4°C (E7 window), compared to 10.4°C (non E7 window).

 

Now this year, I turned my heating off on the 1^st April.  Thought that would be a laugh.

 

So below are the same charts adjusted to just my heating times.

 

 

 

 

Taking just the numbers from the temperature differences, which are the important ones as they show how hard a heating system needs to work.

The mean temperature difference is 13.4°C, minimum 10.2°C and maximum difference 19.6°C.

During the E7 window, the mean difference is 13.5°C, minimum 9.2°C and maximum difference 19.5°C.

Outside the E7 window, the mean difference is 13.4°C, minimum 9.5°C and maximum difference 19.6°C.

 

With those differences I don't believe there is anything to worry about regarding the CoP of an ASHP.

This analysis is, obviously, only for my location and my house heat loads, and I am not sure how well it would describe other locations and houses/households.

 

One way to try and make a universal model is to look at the energy usage over the heating period (1,463 kWh) and adjust that to floor area and see how it compares.

For my house (~50m²), for 90 days heating (and DHW/Cooking/Laundry/Lighting/etc) it works out at 0.325 kWh/m², which is 13.5 W/m².  I am not sure of that is good or bad to be honest.  It seems quit low to me for resistance heating.

 

If the above is plotted, then the correlation between temperature difference and energy usage shows that for every 1°C increase (in difference), and extra 70W of power is needed.  That is an extra 1.4 W/m² or over the 90 days, 3 kWh/°C.

 

 

Looking at the rest of the year, to 01/12/2023 (non heating), shows that for every extra 30 W, the temperature difference increases by 1°C.

This is nonsense as it is the non heating season and the house is ventilated a lot more (windows open).  What is actually happening is that the house is at a relatively stable temperature and the OAT is varying.

 

What needs to be done here it the 36 W/°C (from the trend line function calculation) needs to be subtracted from the 70 W/°C trend line in the above chart.

This leaves 34 W/°C needed to heat the house.  As a power per square meter number that is 0.7 W/°C, which is very little.

The important point there is that my house is thermally stable, even though it is a timber frame.  It can 'store' enough thermal energy for short term OAT variations (just as well as Cornwall has extremely variable weather).

 

 

I am not sure if any of the above is going to be of use to anyone else, but the main point is that for a few quid, basic temperature and energy monitoring can show a lot of useful information when deciding to change a heating system.

There is enough information on here to build your own monitors and it amazes me the amount of people that don't monitor and spend a couple of hours analysing the data.

 

 

 

 

 

 

 

 

Edited December 4, 2023 by SteamyTea

[JohnMo]

This has all the tech data for a typical ASHP, shows detailed info on CoP for heating and EER for cooling at different outside and flow temperature etc.

 

 

Technical-Bulletin-i-32V5-1.pdf

[SteamyTea]

26 minutes ago, JohnMo said:

his has all the tech data for a typical ASHP, shows detailed info on CoP for heating and EER for cooling at different outside and flow temperature etc.

Shall have a look when I get time/bored.

Currently looking at the temperature and energy distributions.

[S2D2]

I think the question is less the OAT changing and more how much you have to push the flow temperature to get a reasonable extra amount of energy into the house in the cheap period, which has a larger impact on deltaT and so CoP.

 

You can backtrack from the price to what an acceptable CoP drop is but I gave it up as too much hassle with A2A. I know others have had success with A2W and a big slab to store the extra energy.

[SteamyTea]

7 minutes ago, S2D2 said:

I think the question is less the OAT changing and more how much you have to push the flow temperature to get a reasonable extra amount of energy into the house in the cheap period,

Yes.  The best correlation was with AT difference, rather than just OAT.  Probably more true in places that have prolonged periods below 0°C.

The problem with cramming in as much energy in as shorter time is that it requires a larger output heat source and room for the storage.  This penalises smaller houses.

 

For my climate, as I said in my analysis, ToU probably does not make much difference.

Out of the 2152 hours I have had my heating on, only 676 hours were 4°C or less (31.4%).

325 hours and I did not need heating as it was over 9°C (my mean switch on temperature) that is 15% of the time.

Of course, having storage heaters I don't have the luxury of stopping output fully, so the room temperature just rises a bit for a few hours.

Most of those few hours, the excess heating is during the day, when it is most useful and does not cause excessive heating,

My mean room temperature, when it is 4°C or below, it is 18.6°C, when it is 5°C and above, it is 20°C (which is my target temperature).

 

[TerryE]

Nick, really interesting stuff but I am a little confused about how this relates to "Running an ASHP on a ToU tariff".  Perhaps you could link the dots for this old dummy?

[TerryE]

Nick,  surely any form of efficient ToU heating should rely on a number of factors:

• Your house as a system should have a reasonable thermal inertia, passive or active, so that you can decouple the timing of heat input from the largely externally-driven heat losses.  This BTW is an intrinsic property of my house design with its passive warm-slab, and 30 cm cellulosic wall insulation: I can pretty much top up the heat in the house at any ToD and the worse that I see in room temperature ripple over the day is ~1 °C.
   
• To make use of a ToU tariff you must have some predictive model, say covering the next 24 hrs, of what your input needs are going to be based on an external weather forecast for temperature, and possibly other factors such as sunshine, windspeed, etc. plus a corrective feedback trim depending on the delta between your target average room temp, and the historic actual temp over the last 24 hours, say.  For us the external temp forecast and a trim correction works fine.
   
• If you are using resistive heating then relating heat requirements to electrical input is trivial, but with ASHP you will need a reasonable CoP predictor based on external weather etc.  so that you can map your predicted heat requirements to electricity input.

I do this type of modelling automatically at 11PM each night and then use the Octopus Agile tariff for the next 24 hours to allocate an optimum heating plan for the coming day.  So here was my Agile cost for Christmas Day.  OK, we were visiting my daughter and family  so this was a "sustain day" where the system was only maintaining the house and HW temperatures, but the 23 kWh the system used only cost me 37p.

 

Edited January 26 by TerryE