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Do setbacks save energy without compromising comfort?

cathodeRay · 2023-11-05T10:16:26Z

An answer appears to be yes. I say 'an answer', rather than 'the answer', because this is one night in one building, with the weather conditions that prevailed at the time. Other buildings and/or other prevailing weather conditions may - very probably will - give different results. But here is a documented record showing that I saved around 5-6 kWhs last night by applying a 26 degree to 12 degree setback between the hours of 2100 and 0300. Result (chart shows the last 48 hours, the first 24 hour no setback period for comparison with the second 24 hour setback period; I have also done the long overdue conversion of the energy in/out plots to bars rather than lines, with each bar showing the preceding hour's energy in/out value; note the bars are overlaid not stacked, ie each bar starts at zero): There is zero energy use for heating during the setback. As the current normal energy use is roughly 1 kWh per hour, that means I saved 6 kWh of energy. Now comes the 64 million dollar question: how much extra energy did I use during the recovery (which happened fairly promptly, the room temp was back to the desired room temp of 19 degrees by 0700, having fallen to a nadir of 18 degrees during the setback, not a lot I agree, a consequence of a large thermal mass and mild but by no means unusual temperatures)? I emphasise extra energy because in continuous running (no setback), the heat pump would, in the prevailing conditions, use about 1 kWh per hour, what I want to know is how much extra energy I used during the recovery period. The answer is, in the prevailing conditions, very little. The recovery boost is managed by my python auto-adaption script (see below for details), which increases the weather compensation curve endpoints (and so the LWT) when the room temp is below where it should be, as it was at the end of the setback. If you look at the first hour after the setback, the energy out (red bar) is visibly higher (as is the LWT in the upper chart) but the increase in energy in is minimal, 1.24 kWh for that hour, compared to 0.84 kWh in the hour before the setback, and 1.1 kWh in the second hour after the setback. That gives around an extra 0.24 kWh used, over and above the around 1 kWh I would have used anyway. I therefore saved 6 - 0.24 = 5.76 kWh last night by using a setback, and comfort was not compromised, the room temp was where it should be when I got up at 0700 this morning (and yes it was a bit cooler in the middle of the night, I know because I have to do what most middle aged and over men have to do during the night). A result, I hope you will agree. Some things to bear in mind. 1. As noted above, and emphasised again here, this is one building over one night in the weather prevailing at the time. I will add more data once I have it, and it has something to show. 2. I call it a setback, because it is a setback of the room stat from 26 to 12 degrees, but its effect is to act as an on/off timer, because in normal circs the house will never fall to 12 degrees, meaning the heat pump will never come on during the setback. A heat pump on/off timer could used to do the same thing. 3. The parameter monitoring is pretty tight, data collected over modbus at one minute intervals, meaning we can really see what is going on. All the data bar the room temperature comes from the Midea controller, and so far as I have been able to verify its accuracy, it is not far out. The room temp (IAT) comes from a third party sensor in centre of the living area of the house. 4. I am currently running my DIY auto-adaption. This uses a python script that checks the difference between the actual room temp and desired room temp every hour and adjusts the end points of the weather compensation curve according to the difference. The current logic moves the ends of the curve one degree up or down from the baseline endpoints for every degree the actual room temp is below of above the desired room temp respectively, eg if the room temp is one degree below the desired room temp, the ends of the weather compensation curve go up by one degree. The maximum increase in the endpoints is limited to a three degree rise, to stop the heat pump melting itself down. One effect of this auto-adaption is that it can boost the output when the room temp is below where it should be, eg at the end of a setback period, and provide a so called setback recovery boost, meaning the heat pump doesn't take forever to get the room temp back to where it should be. The auto-adaption can also reduce output eg if there is solar gain, increase it if there is significant wind chill, for example from draughts from leaky window frames.

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Air Source Heat Pumps (ASHPs)

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cathodeRay

(@cathoderay)

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Topic starter 16/01/2024 12:35 pm

Posted by: @scrchngwsl

↑

I've had success with this £6 sensor attached to a Raspberry Pi

Yes, I came across Raspberry Pi solutions, but it means adding a whole new layer of complexity which I think I could manage technically (can solder without burning holes in things), but I do like simplicity. The modbus setup I have is one cat5e cable connected via a RS485 to USB adaptor to a USB port which also supplies the voltage needed to run the XY-MD02 sensor, with the Midea wired controller (and any other sensors I may add) daisy chained to the modbus cable. Python's minimalmodbus module then collects the data. It is simple robust and reliable.

The XY-MD02 sensor (paid £11 on ebay) is about 3" x 2" x 1" and looks like this:

As you can see, it is clearly not weatherproof, and even direct sunlight could I suspect cause false readings. I did think about some sort of cowl for it, but again it may lead to aberrant readings if there isn't enough free flow of air. Finally, after yet another search online, I have come across a couple of 5V modbus enabled temperature/humidity sensors that claim they are waterproof to IP67. They are either obviously from China, or likely from China, given the delivery times, but one way or another I'll get one and hook it up. I will then - hopefully - have the true OAT.

Midea 14kW (for now...) ASHP heating both building and DHW

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cathodeRay

(@cathoderay)

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Topic starter 16/01/2024 12:58 pm

Posted by: @mikef

↑

How can this be a coincidence?

It is certainly interesting that the numbers should be so similar. Looking at the last week's use, to 0700 yesterday (Monday), it is still in the same ball park, 1.13. I am currently taking daily readings from the external meter, and will post results once I have a weeks data. It will be interesting to see how the current cold weather affects things.

Midea 14kW (for now...) ASHP heating both building and DHW

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cathodeRay

(@cathoderay)

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Topic starter 16/01/2024 1:01 pm

Posted by: @derek-m

↑

I hope that the above makes sense.

Thanks for such a detailed description in words. It really does help my understanding of what you are doing.

Midea 14kW (for now...) ASHP heating both building and DHW

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