@scrchngwsl so would it be fair to say while the HP is modulating to a reducing temperature, eg. to the set back temperature of say 16c down from 21c it is still technically on? And therefore contributing to its COP stats?
Having a setback definitely makes my COP worse on auto adapt because it bumps up the flow temp to heat up the house in the morning. When I'm on overnight setback the ASHP is effectively off because the house temp never drops that far. During this time, the energy produced is zero and consumed is a tiny amount.
We've discussed this many times, and will no doubt do so again, but a setback followed by a boost lowers overall consumption over a sensible time frame (say a day or a week), despite seeing worse COPs at times. All other things being equal, a lower set temp = lower overall cost, but that has to be set against the lower efficiency during any post set back boost period. From what I remember from last time, the gains (savings) made during the set back always (unless something crazy happens) outweigh the efficiency losses in the boost period, net result over a sensible time frame = savings.
One of the reasons I am spending so much time on getting at the the Midea controls is to find a way to incorporate the same basic idea as Ecodan auto adaption, a short period of boost (slightly higher LWT) after a setback, into my heat pump controls. At the moment, the Midea native controls cannot do this, and the house takes days to recover design temps, so no setbacks for me at the the moment.
Midea 14kW (for now...) ASHP heating both building and DHW
We've discussed this many times, and will no doubt do so again, but a setback followed by a boost lowers overall consumption over a sensible time frame (say a day or a week), despite seeing worse COPs at times. All other things being equal, a lower set temp = lower overall cost, but that has to be set against the lower efficiency during any post set back boost period. From what I remember from last time, the gains (savings) made during the set back always (unless something crazy happens) outweigh the efficiency losses in the boost period, net result over a sensible time frame = savings.
One of the reasons I am spending so much time on getting at the the Midea controls is to find a way to incorporate the same basic idea as Ecodan auto adaption, a short period of boost (slightly higher LWT) after a setback, into my heat pump controls. At the moment, the Midea native controls cannot do this, and the house takes days to recover design temps, so no setbacks for me at the the moment.
When one thinks about the actual thermodynamics, without considering the comfort level, then reducing the indoor temperature overnight should reduce the overall energy consumption, since the heat loss will be less, which should equate to lower energy consumption.
I suppose that much depends upon the rate of heat loss, the thermal mass of the home, and how quickly it is required to return the indoor temperatures back to the desired level. In a well insulated home, with a relatively high thermal mass, it could take quite a number of hours for the indoor temperature to fall from say 20C, to a setback temperature of 18C, and unless it is very cold outside the setback temperature may not even be reached before the heat pump restarts. The quantity of heat energy lost during the setback period, is approximately the amount that needs to be replaced during the warm up period, so the longer the warm up period, the less hard the heat pump needs to work.
Let's consider what may be happening.
I suppose that the first thing to consider is how the setback is achieved. If the setpoint of an on - off type thermostat is lowered to 18C, this would probably stop the heat pump immediately, whereas if the setback is performed by an offset to the WC, or lowering the setpoint in AA mode, it may take a little while before the heat pump actually stops.
Once the heat pump has stopped supplying heat energy to the home, the indoor temperatures will gradually start to fall. The rate at which the indoor temperatures reduce, will be dependent upon the DeltaT between indoors and outside, the level of insulation, and the quantity of heat energy stored in the thermal mass of the building. At the time of switch off the heat emitters will also contain a quantity of heat energy which will still be transferred into the rooms. It is therefore difficult to quantify the exact rate of cooling to be expected, since the rate of heat loss from the building will start to reduce as the indoor temperature starts to fall, and hence reduces the DeltaT if the outside air temperature remains constant, but also the rate of heat energy being supplied by the heat emitters could initially increase slightly as the indoor temperature starts to fall. It is the usual thermodynamic balancing act, where the heat energy is determined to flow from a higher temperature to a lower one, with any form of insulation doing its best to try to stops this.
So the heat energy being lost from the building will initially cool the internal fabric, which in turn will cool the air coming into contact, as this cooler air flows around the rooms it will absorb heat energy from the heat emitters and furniture etc. and eventually cool the water in the central heating system.
When the heat pump restarts it therefore could have a mammoth task, first it needs to warm up the internals of the heat pump itself, then it needs to start warming the water in the central heating system. Any initial heat energy is quickly swallowed by the cold pipework and heat emitters, who then have to start warming the air in the rooms. This warmer air then starts to contact the cooler furniture and internal fabric, so soon has to return to the heat emitters for more heat energy. As the internal fabric of the building warms, the external wants its share of this extra heat energy, and not to be left out the outside air is also demanding. To feed all these hungry mouths the heat pumps needs to work harder, and generate a higher LWT, if it is to have any hope of getting the indoor temperature back up within a reasonable period of time.
Just considering the heat energy balance, lowering the indoor temperature overnight will undoubtedly reduce the overall heat energy loss, and from the building point of view, the actual heat loss will remain lower until the original 20C is achieved.
In the real World, switching off the heat pump will drastically reduce the energy demand for that period of time, but then this will need to be higher during the heat up period, which could mean working the heat pump much harder during the cooler, less efficient part of the nighttime. Whether it would be more efficient to warm up slower, at a lower LWT, over a longer period of time, would be more efficient than warming up quicker, at a higher LWT, over a shorter period of time, is better, only detailed testing may prove. It could be that during certain weather conditions one way is more efficient, whilst different weather conditions may favour the other method.
Personally I don't think that there will be vast differences in energy consumption between either way, or even running the heat pump continuously throughout the night. But I would be happy to be proved wrong. 😡
@derek-m - in general I agree with your assessment. If I can add some real world observations:
(1) I have only ever had one night with a setback, because the following day the house was cold all day. Without auto adaption, the Midea unit simply can't warm the house up quickly enough, it takes hours if not days to get back to the desired temps (this is the main driver for my attempt to control the heat pump through Home Assistant, so I can set up a form of auto adaption).
(2) I did the setback by setting the main room stat to 16 degrees, from I think it was 2200 to 0600. Sure enough, the heat pump stopped, and stayed stopped - the high thermal mass meant the house cooled very slowly, and never actually got to 16 degrees. Last night, not a particularly cold night (~7-8 degrees), during those over night hours, my heat pump used on average 2kWh per hour. Applying that to a setback period of 8 hours, that gives a saving of 16kWh during setback on a not very cold night. I would save more on a colder night.
Now the question is, were I to add in a boost period after a setback, would it use an extra 16kWh, over and above what it would normally use? 16kWh over a few hours (say four hours) is really quite a large increase. On these sums, the first four hours of the day, during the boost period, the heat pump could use 6kWh for four hours, and still break even. If in total the boost uses less than 16kWh, then overall I am quids in.
This is the key question: does the boost use more extra energy than normal during the boost period than the setback saves during the setback period?
I have yet to do this (setback followed by boost) for real, too much of a faff around to adjust the LWT manually, partly because I don't know what the LWT increase needs to be, and it could take days of experimentation to find out. I don't want to aggravate my HPDHD (heat pump deficit hyperactivity disorder). If/when I get HA to adjust the set LWT by time of day (it needs to take the normal weather comp set LWT, and then add say 5 degrees), I will try it, and see what happens. I will also have the setback period happen a bit earlier, maybe 2100 to 0300, as the low rate of cooling means I probably won't notice it as I got to bed, and there is a reasonable time for boost recovery in the morning before I get up.
Midea 14kW (for now...) ASHP heating both building and DHW
Also I’ve noticed reference in this thread to a ‘Boost’ in the morning whereas we have been doing a rise in 1 degree increments from 5am in an attempt to emulate what that Homely app does- a gentler take-off like a high aspect glider.
And in your example I t’s becoming clear that the setback probably needs to tie in with the speed at which heat migrates away from the building to avoid a long recovery.
To be honest our bedroom is too hot with a downstairs thermostat setback of 17c. So with a well insulated house nestling at 16 degrees in the early morning perhaps using something similar to auto adapt won’t surge on the power if it’s only got to raise the temperature by 1c to get to 17 degrees, then 18. Etc.
the difficulty might be setting a schedule with that many incremental changes?
Have a look at this. The two graphs are my house after a setback, one on Mitsubishi AA, one on WC. Both have similar (but not identical) ambient temps. AA has a target room temperature and it's supposedly 'learned' the best way to achieve this. WC just gets the flow up to target as fast as possible and keeps it there.
With AA, the house is up to temperature by 9am then it's steady state (apart from HW at 3pm). With WC, it's more like 1pm (note I had HW at 9am also)). So to compare the effect of a boost, we can compare consumption 5-9am in both cases. AA was 9.1kWh, WC was 6.2kWh. So the cost of a boost to get the house up to temperature is circa 3kWh. Steady state consumption is about 1.4kW so the 5 hour (12-5am) setback saves c.7kWh.
In my case, even with a boost, I'm saving c.4kWh. I think AA uses quite a bit more energy overall but that's another story.
If we consider what is happening just in heat loss terms. If the calculated heat loss of the building is say 12kW, with an indoor temperature of 21C at an outside temperature of -3C. I think that it has previously been established that the variation in heat loss is probably linear, which would indicate that at an outside temperature of 9C, the overall heat loss would be approximately 6kW.
Considering the 'perfect World' situation, where the outside air temperature remains constant at 9C throughout the test period. The indoor air temperature is initially 21C with the LWT at 35C, and the heat pump is using 2kW, so giving a COP of 3.
The heat pump is now switched off.
Hour. Indoor temp. Heat Loss. (kWh) Energy Saved. (kWh)
1 20.9 5.95 2
2 20.7 5.85 2
3 20.5 5.75 2
4 20.3 5.65 2
5 20.1 5.55 2
6 19.9 5.45 2
So over a period of 6 hours the indoor temperature has dropped by 1.1C, the building has lost a total of 34.2kWh of heat energy, but has saved 12kWh of electrical energy.
To bring the indoor temperature back up to 21C, it is necessary to replace the 34.2kWh of heat energy lost. If the heat pump is just switched back on with the previous settings, it would be developing 6kWh of heat energy, which with the present rate of heat loss (5.45kWh) would provide a surplus of 0.55kWh. Even if this energy surplus could be maintained, it would take over 62 hours to replace the lost energy, but as the indoor temperature starts to increase, the surplus will invariably reduce.
Over what time period would it be acceptable to replace the lost energy, 1 hour, 2 hours, 3 hours or more?
If the heat pump is rated at 14kW, then when operating at full output it would provide a surplus of just over 8kWh, which would therefore take over 4 hours to bring the indoor temperature back to 21C. If during this 4 hour period the heat pump is drawing 5kW of electrical energy at a COP of 2.8, which would be 3kW above the normal requirement, then the extra consumption would soon eat away at the 12kWh saved.
Obviously much of the above is hypothetical, but still based upon real World experience, so could be not too far from the truth.
The best method of reducing energy consumption will no doubt vary from home to home. In the above example reducing the indoor temperature from 21C to a constant 20.5C could potentially save 6kWh of heat loss throughout a full day.
Boosting the indoor temperature and/or producing hot water, during the periods when there is a higher outside air temperature during the day, should provide better efficiency.
Using a limited setback during the periods when the outside air temperature is at its lowest, then recovering when it is warmer outside, could be another method.
Obviously improved insulation to reduce overall heat loss is always going to be beneficial, as will balancing the heat emitters and optimising system operation.
it is necessary to replace the 34.2kWh of heat energy lost
@derek-m - I am not sure about this. That's surely the delivered kWhs needed to keep it at a constant temp during the set back, not the energy needed to reheat after a set back. I think you may also have missed the fact the heat loss is on the output side of things (post-COP as it were) while the energy saved is on the input side of things (pre-COP as it were), getting back say 36kWh post-COP only needs 12kWh pre-COP, given a COP of 3, if that makes sense. Or perhaps I am being very thick.
There is also @kev-m's interesting real world measurements, suggesting setback followed by boost does produce real world measurable savings. For higher heat loss buildings, the pro rata savings could, perhaps should, be even greater.
Midea 14kW (for now...) ASHP heating both building and DHW
I think AA uses quite a bit more energy overall but that's another story.
So I'm wasting my time considering trying AA over WC? Is that what you're saying? Because that's how I read that. I was considering giving AA a try to see if it would realise a financial saving.
Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
14 x 500w Monocrystalline solar panels.
Have a look at this. The two graphs are my house after a setback, one on Mitsubishi AA, one on WC. Both have similar (but not identical) ambient temps. AA has a target room temperature and it's supposedly 'learned' the best way to achieve this. WC just gets the flow up to target as fast as possible and keeps it there.
With AA, the house is up to temperature by 9am then it's steady state (apart from HW at 3pm). With WC, it's more like 1pm (note I had HW at 9am also)). So to compare the effect of a boost, we can compare consumption 5-9am in both cases. AA was 9.1kWh, WC was 6.2kWh. So the cost of a boost to get the house up to temperature is circa 3kWh. Steady state consumption is about 1.4kW so the 5 hour (12-5am) setback saves c.7kWh.
In my case, even with a boost, I'm saving c.4kWh. I think AA uses quite a bit more energy overall but that's another story.
Hi Kev,
What is the setting of the 'Time Interval'? It would be interesting to see the same test with a longer time interval.
I think AA uses quite a bit more energy overall but that's another story.
So I'm wasting my time considering trying AA over WC? Is that what you're saying? Because that's how I read that. I was considering giving AA a try to see if it would realise a financial saving.
It's hard to say. Some people think it's better. It's a little complicated for me as I have 2 zones and they heat and cool at different times. It's definitely worth a try.
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