I would state that last nights cycling was due to the weather being milder, hence the heat demand was below the minimum output of the heat pump. My previous comments were in relation to your previous charts where the OAT and Set LWT did vary quite considerably.
the LWT increases about [above?] the the Set LWT value until it reaches the upper deviation setting, which you have set to either +4C or +5C. When this setting is reach[ed] the heat pump stops operating, and does not restart until the LWT falls to the lower deviation setting, which I think is -9C.
I can't actually find any of these settings in the Midea Engineering Data manual. There is something called dT1SH, which 'sets the temperature difference between the heat pump leaving water temperature (T1) [what we call LWT] and the heat pump leaving water set temperature (T1S) [what we call Set LWT] above which the heat pump provides heated water to the space heating terminals'. The default appears to be 5, and mine is set to 5. Translating this into English, it seems to say the trigger for starting the compressor is when the actual LWT differs from the Set LWT by the amount in the setting, in this case 5 degrees. For example, when the Set LWT is 50 degrees, and the compressor is off, it will come on again when the actual LWT reached 35 degrees. This isn't borne out by the data, which shows the compressor restarting when the difference is around 7 degrees.
Nor can I find any setting for the difference between the Set LWT and actual LWT that causes the compressor to stop running when it is running. Something clearly does turn it off, at the point when the actual is about 4 degrees above the set in the above chart, but I have no idea what it is, and it does not appear possible to alter it.
I would state that last nights cycling was due to the weather being milder, hence the heat demand was below the minimum output of the heat pump.
A bit warmer than of late but hardly mild at 6 degrees most of the time. This is in the range in which ideally it should run in steady state, too warm for defrost cycles, and too cool for the over-supply of heat problem (can't modulate low enough) at truly mild OATs.
What I am beginning to wonder is whether this is the normal state for Midea heat pumps, and possibly others, once above the defrost range, they can't modulate their output very well at any OAT, and almost always over-shoot, and so have to use cycling to modulate output. In effect, the cycling we see and thought we understood that happens at truly mild OATs also routinely happens at lower OATs, possibly all the way down to the defrost range, where defrosts dominate the picture.
No one has ever raised this possibility before because, as far as I know, no one, at least no end user, has collected the Midea data I have, at the frequency I have.
We need more data!
Midea 14kW (for now...) ASHP heating both building and DHW
@cathoderay My heat pump is running for 9hours straight and i can see by compressor starts that at below9-10deg, i have zero cycling. could be just midea issue.
But how does it measure, and respond to, the demand?
In short, the RWT. The emitter loss determines how much the water temperature falls during its passage through the emitters. The heat pump will (or at least should) respond to this by putting in just enough heat to restore the flow temperature to its design value. That's pretty much the fundamental feedback loops in any wet heating system.
As a secondary feedback loop some pumps also respond by adjusting the flow rate to keep the deltaT across the emitters constant. It will remain constant until the demand changes, at which case the changed demand will change the deltaT/RWT, and the heat pump will adapt its output again.
And why is it currently (if my above assessment of what the data represents) choosing to run at 'min' level, even perhaps a bit below that?
That I cant currently answer that definitively, but unless there is a fault its because its all the heat that is needed (and, unless your IAT is going down then it is in fact all the heat that is needed). Can you point me to the graph which illustrates what you are concerned about, I must confess not to have followed all of the most posts as @derek-m seemed to be responding
PS If you are referring to this graph it looks to me like mild cycling because the actual demand is less than the minimum capacity, unless I'm missing something. In this case you would expect it to be running at minimum level.
This post was modified 1 year ago 3 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.
Certainly, or it could just be my Midea unit. The problem is that unless and until we have more data, we just don't know.
I'm hoping others who collect minute by minute relevant data either over modbus or by some other means will post some data. @kev-m has already kindly posted some, and it also shows cycling, albeit less frequently, but still nonetheless present. I hope @kev-m won't mind if I reproduce his two earlier charts again here for ease of reference:
Midea 14kW (for now...) ASHP heating both building and DHW
@jamespa I take it you mean my house will be yoyoing in a sense with regard IAT so by having setback. My counter point is it doesn't affect our current lifestyle as we are warm when we are there. I get what you mean theoretically but i suppose in my case its theoretical as its not having any real world impact on our comfort. The lost energy is generally going to be replaced when OAT is higher so heatpump cop will be better. Im just trying to articulate that running 24/7 may not be a universal fit.
I completely agree with the last sentence, and have said so on this thread, but... (Almost) without doubt there are circumstances where setback saves energy, but there are also circumstances where, at least in principle, it doesn't. That's essentially what I said in my initial response - be a bit careful interpreting the results and look out for contraindications (eg turning up the WC curve because you feel cold).
In your particular case, where you don't attempt to recover for breakfast, It's much more likely to save money than a case where you do attempt to recover for breakfast. Equally its much more likely to save money when the daytime temperature is significantly higher than the night time temperature.
Until we explore this a bit more we cant currently put definite boundaries on it. @derek-m has created a model which enables some theoretical exploration, @cathoderay is trying to do some experiments, and we are collectively realising that its more difficult, particularly to do the experiments, than we might first have thought!
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.
Can you point me to the graph which illustrates what you are concerned about,
It's not on a graph, it's in the raw data, which appear to suggest my heat pump spends most of it's time running at the 'min' capacity level, even when it is cool/cold outside, eg this morning's output was reported (by the wired controller) to be around 7kW, which is just below the 'min' capacity at the prevailing OAT/LWT combination. My concern is that it doesn't ramp up to 'norm' or even 'max' when it could and should, to warm the house better in the cold spells (and hence, perhaps, the cold house).
I am only to aware that this wish to have a higher capacity (output) somewhat flies in the face of what I have been suggesting recently, that it is over-capacity (over-supply of heat in the short term) that causes cycling...
Midea 14kW (for now...) ASHP heating both building and DHW
Can you point me to the graph which illustrates what you are concerned about,
It's not on a graph, it's in the raw data, which appear to suggest my heat pump spends most of it's time running at the 'min' capacity level, even when it is cool/cold outside, eg this morning's output was reported (by the wired controller) to be around 7kW, which is just below the 'min' capacity at the prevailing OAT/LWT combination. My concern is that it doesn't ramp up to 'norm' or even 'max' when it could and should, to warm the house better in the cold spells (and hence, perhaps, the cold house).
I am only to aware that this wish to have a higher capacity (output) somewhat flies in the face of what I have been suggesting recently, that it is over-capacity (over-supply of heat in the short term) that causes cycling...
If the house is warm then its spending its time at min capacity because that's all the energy you need to keep LWT within the set bounds.
Sure, cycling is (usually) caused by over-supply, but the over-supply is (usually) caused because the demand (determined by the emitters/house) is less than the minimum output that the pump can modulate to. Thus it operates at its minimum output and because the demand is less than the supply the RWT and LWT progressively increases. After a while the LWT reaches the upper bound that is programmed into the control system, so the pump shuts off until it has dropped below the lower bound programmed into the control system. Again a basic feedback loop. My boiler does exactly the same.
The most obvious inference if its cycling due to demand at current OATs is that your pump is over-sized and/or has a poor modulation depth.
Last night it was on Weather Compensation, no setback.
If you would care to provide the minute by minute raw data for last nights chart, I will run it through the Modeling Tool as see if it predicts that cycling should occur on your system.
If the house is warm then its spending its time at min capacity because that's all the energy you need to keep LWT within the set bounds.
Sure, cycling is (usually) caused by over-supply, but the over-supply is (usually) caused because the demand (determined by the emitters/house) is less than the minimum output that the pump can modulate to. Thus it operates at its minimum output and because the demand is less than the supply the RWT and LWT progressively increases. After a while the LWT reaches the upper bound that is programmed into the control system, so the pump shuts off until it has dropped below the lower bound programmed into the control system. Again a basic feedback loop. My boiler does exactly the same.
The most obvious inference if its cycling due to demand at current OATs is that your pump is over-sized and/or has a poor modulation depth.
My point is we are not in these conditions, warm house in mild conditions, where we know about and expect cycling as the only way further to modulate output downwards, instead we are in single figure OATs here, in the zone where the heat pump should be able to match the demand, ie the inability to modulate down low enough to match a very low demand is not a problem.
My heat pump definitely isn't over-sized, we can rule that explanation out. Perhaps the answer is after all in the engineering data tables. If in the prevailing conditions at the time (ie as in the chart posted earlier, OAT 6, LWT ~45), then the max/norm/min outputs are given as 14290/11103/7859 respectively. Assuming a linear relationship between OAT and heat loss, and given my heat loss is 12.4 kW at -2, then the heat loss at an OAT of 6 will be around 7.5kW ie just below the 'min' - in other words the heat pump does have what I think you mean by poor modulation depth, and so is forced to cycle even when in ideal world it shouldn't. Using the max/min numbers, it can only reduce to 55% of its max output.
All of this is really just another way of looking at what I proposed earlier, that at least for Midea heat pumps this is the normal state of affairs, they always cycle: at mild OATs, which we already knew about, because they can't modulate down enough, but also at lower OATs, because the same still applies, they can't modulate down far enough, and so over-shoot (it's only by doing detailed minute by minute monitoring that you get to see this), and then at low single figures, defrost cycles kick in.
Midea 14kW (for now...) ASHP heating both building and DHW
If you would care to provide the minute by minute raw data for last nights chart, I will run it through the Modeling Tool as see if it predicts that cycling should occur on your system.
Will post the data after I have had some lunch.
Midea 14kW (for now...) ASHP heating both building and DHW
My point is we are not in these conditions, warm house in mild conditions, where we know about and expect cycling as the only way further to modulate output downwards, instead we are in single figure OATs here, in the zone where the heat pump should be able to match the demand, ie the inability to modulate down low enough to match a very low demand is not a problem.
Do you know it isnt matching demand, so far as I can see your IAT is staying constant which suggests it is.
Edit - Oh - I see what you mean, cant match demand in the sense that it cant go low enough (as opposed to high enough).
Well yes you know this is the case if its cycling (unless the cycling is due to some other cause), and you can check by reference to the engineering tables, assuming you know your actual house heat loss (see next question).
My heat pump definitely isn't over-sized, we can rule that explanation out. Perhaps the answer is after all in the engineering data tables. If in the prevailing conditions at the time (ie as in the chart posted earlier, OAT 6, LWT ~45), then the max/norm/min outputs are given as 14290/11103/7859 respectively. Assuming a linear relationship between OAT and heat loss, and given my heat loss is 12.4 kW at -2
How do you know what your heat loss is at -2. Did you measure it? If not how can we rule out the possibility its oversized?
All of this is really just another way of looking at what I proposed earlier, that at least for Midea heat pumps this is the normal state of affairs, they always cycle: at mild OATs, which we already knew about, because they can't modulate down enough, but also at lower OATs, because the same still applies, they can't modulate down far enough, and so over-shoot (it's only by doing detailed minute by minute monitoring that you get to see this), and then at low single figures, defrost cycles kick in.
If you look back I said oversized or poor modulation depth. So I grant 'they cant modulate down far enough' (ie poor modulation depth) may be the problem. If all you need at the current OAT is 7kW, and the minimum output is 7.9kW, then thats what the engineering tables are telling you - that it cant modulate down far enough.
To determine which is the principal cause we need to answer the previous two questions. Having said that we already know that it cant modulate down far enough, what we don't know is whether you could have got away with a smaller pump. As it happens I have a feeling (which could be confirmed or not by looking at the engineering tables) that the minimum output is the same for all (or many) the Mideas in any given range, because they are in fact all the same hardware. If this is the case then a smaller Midea would have lead to the same cycling, and you might as well have the larger one.
This post was modified 1 year ago 4 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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