Posted by: @cathoderay

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(a) the heat pump is currently in this cold weather operating at the limits of its capabilities (the old 12.4 kW loss vs a 11.4 kW output from my so called 14kW heat pump)

I think you may be right there. It wasn't something we really considered and took the designer's assurance that we could just light the wood burner if it got too cold (without considering the soaring price of wood!), so I feel that I got lucky that my "8.5kW" pump can actually produce that down to -10, although the COP does fall rather alarmingly below zero.

Posted by: @cathoderay

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But that might be nonsense, because all the heat pump ever knows about is ambient temp. It has no way of knowing what the room temps are...

That's something that irritates me about the ecodan controllers: they do know about both ambient and room temperatures, so why does the auto-adaptive mode act so needlessly aggressive when it's only a degree or two below target?

If you had an open home automation system that can control your Midea (I don't know if they can), you could connect up as many temperature sensors as you wanted and really make a complicated and confusing mess of your flow temperature calculation! I've tried a couple and seen plenty more on other forums. I'm sure there's some ideal mix of weather compensation, target temperature-at-time, energy prices and so on, but I'm also pretty sure that I will never find it for myself! So I've settled for automating the sorts of buttons I was pushing last winter when I noticed things not going as I wanted...

Posted by: @cathoderay

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@kev-m, @mjr, @mookyfoo, @derek-m - thank you all for your comments. I did do the setback over Sunday to Monday night (to 16 degrees on the kitchen room stat which is just a calling/not calling for heat control between 2200 and 0400). It worked insofar as the heat pump was off for that period, and I probably saved around 12kWh of electricity, and it also tells us, because the heat pump didn't come on, that the kitchen stayed at > 16 degrees, so not really a major drop in warmth, but on the Monday (yesterday) morning it took forever to get back up to temperature, and if fact never got to design temp all day. By noon it had reached 18.0 degrees, and pretty much stayed there all day. Design temp is 19.0 degrees. 

Because of this failure to recover, and the forecast colder weather, I turned off the setback last night and had the heat pump running all night. This morning, the kitchen is still only at 18.0 degrees.

I think this failure is because of a combination of things: (a) the heat pump is currently in this cold weather operating at the limits of its capabilities (the old 12.4 kW loss vs a 11.4 kW output from my so called 14kW heat pump) and (b) the relatively high thermal mass of my building. It cools down slowly (didn't even get down to 16 degrees overnight Sunday/Monday), but it also takes forever to heat up again. Basically, when its cold outside, say 5 degrees and below, the heat pump can just about supply enough heat when the house is in steady state to keep it in steady state, but when it has to raise the temp from a setback, it really struggles. I think during this cold spell I am going to have to run it 24/7.

If its a bit milder, I might try a setback again. Previously i asked how to boost the heat output during the rewarming phase (the have a second weather comp curve for this phase question) but I now wonder whether that might possibly be answered/worked around the other way round, ie have the curve set to what is needed for the boost phase, and then achieve marginally lower output in the steady state phase by the use of TRVs in a number of less frequented rooms. These would start to close down a bit more than they do now as the rooms got to temp, and the reduced flow would in due course cause the heat pump to marginally reduce its output, as the warmer RWT from the plate heat exchanger 'fed back' the lower heat demand to the heat pump.

But that might be nonsense, because all the heat pump ever knows about is ambient temp. It has no way of knowing what the room temps are...

         

Your heat pump actually does know, indirectly, what is happening to the room temperatures, by as you state, 'the feedback' from the RWT.

If your rooms are not achieving the desired temperature now that the outside air temperature is colder, it could be that your WC curve requires modification. An easy way to check would be to set the WC offset to +1C, and see if the room temperatures increase.

@mjr - I have given some thought to adding some sensors/automation, but as you say it is far from clear if they can be readily hooked into a Midea system. Furthermore, even the most basic systems look rather complicated to me, and I am not one normally afraid of technology (even if I am afraid of giving my wifi password to a Chinese company). @derek-m's as ever useful comment confirms there is already some feedback, the question is how to make best use of it.

@derek-m - I have been tweaking my WCC (weather comp curve), next step if it doesn't keep up with this colder weather is to up the low ambient end LWT a degree at a time and see what happens. It's a slow process because of the long lag intervals between making a change and the effect, if it happens, being visible. The next few days, with temps forecast to be at or below zero for much of the time, will be an excellent opportunity to tune the system for these more demanding conditions.

@batalto - I've just seen the Midea unit go through a defrost cycle on the Midea controller, I just happened to be looking at the LWT when it happened. The LWT/RWT numbers changed over a minute or two to end up as LWT 20 / RWT 30 - and that is not a typo! It was the pump in vampire mode, sucking heat out of the building! The ambient went from 1 to 5 degrees, nice for the garden, I suppose, and the confirmatory clue was a 'frost' symbol appearing next to the compressor symbol on the bottom left of the display. The whole thing lasted maybe 3 or 4 minutes, and ten minutes or so later, the ambient was back at 1 degree, and the LWT climbing back towards the upper 40s. Rather curiously, the rads seemed not to cool down very much. I'll have a look at the Midea app later to see if there is any clue that it happened in the app, or whether the controller is the only place inside the house where you can see it happening, and then only in real time.

@cathoderay that does make sense. It's a small area to cool and it's literally a heat exchanger. It would make sense it can be very quick. I can't imagine it actually takes that much overall heat out of the water given the volumes and the heat needed. It's probably only 1kw to defrost it. Given you know the flow temps and the time, you could probably have a stab at calculating the actual number.

Posted by: @batalto

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Given you know the flow temps and the time, you could probably have a stab at calculating the actual number.

The other thing I noticed out of the corner of my eye but didn't record was the flow rate went up. Mine is normally glued to 1.4 something, but I think it went up to 1.8 something. That might affect the calculations. What I was really hoping was to find a way of seeing them in the app, and if possible determining how detrimental they were over a given say 24 hour period.  

Posted by: @cathoderay

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@mjr - I have given some thought to adding some sensors/automation, but as you say it is far from clear if they can be readily hooked into a Midea system.

I happen to know that Mideas can be monitored and controlled by modbus - it's how Homely works. I think I even saw a repo for it somewhere in the depths of Github where someone had reverse engineered some of it, but I might be remembering something else. Do you have a home automation system like Home Assistant or Open HAB? 

Posted by: @oswiu

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I happen to know that Mideas can be monitored and controlled by modbus - it's how Homely works. I think I even saw a repo for it somewhere in the depths of Github where someone had reverse engineered some of it, but I might be remembering something else. Do you have a home automation system like Home Assistant or Open HAB? 

There is certainly a lot of modbus stuff in some of the Midea manuals, but I have yet to do my homework on what modbus is, though I'm guessing it is some sort of communication protocol like RS232 and USB (Universal Serial Bus). If someone has reverse engineered it for Midea, that's promising.

I haven't got any home automation stuff yet, as I said earlier, I am not afraid of tech, but I am very nervous of being over complex. It took a long time for me to stop driving my Morris Minor Traveller, and very reluctantly move on to a Mk 3 Golf Diesel Estate. My current car, a Passat, constantly annoys me by trying to be too clever, and its systems are way to complex, and not that clever either. I'm also very wary of the IoT (internet of things), I see all sorts of very nasty Big Brother potential there. Strings, wires and levers for controls, yes, black boxes and wifi - not so sure, need persuading. I probably will be persuaded, but I am not going to rush in blindly! Part of this is because in my experience, modern tech usually fails to live up to its promise. The Midea app, for example, is pretty limited to say the least, and much of it is unreadable on the screen without a magnifying glass. Why Midea don't make a desktop version is quite beyond me. As for not providing any way of recording historical data short of using pencil and paper - what's that all about?

I will look into the available home assistant options. Any pointers towards some simpler, more robust systems? or where a good place to start is? To a beginner, the area looks like a field full of rabbit holes of unlimited depth.

Interesting readings this morning. Ambient -2 to -3, Set LWT 53 (this is where it should be with my current WCC, 53@-2) and actual LWT touches 53 degrees at times but not all the time and defrost cycles, there was one about half an hour ago, give the LWT a nasty jolt. At the same time, recent kWh usage has been eye wateringly high: 63kWh yesterday, and today has already (by 0800) used 34kWh. Upstairs is just about at design temps, but downstairs isn't. Kitchen is 17 to 17.5 degrees against a design temp of 19 degrees. Despite Graham Headroom's assertions to the contrary, I don't think there is any slack in the system at all. I am going to briefly put the set LWT up to 55 to see if it can get there while it is -2 / -3 outside.  

@cathoderay could you alter the dips and change your unit into a 16kw?

Oddly we used less power than you yesterday - but maybe the fabric of the house is more insulated

I have checked the Midea controller every 5 mins or so over the last hour having set the LWT to 55 degrees. What I observed was: ambient -2 most of the time. At least 2 defrost cycles, don't last very long (a few minutes) but when they happen LWT drops down to the 20s, rest of the time LWT low 50s most of the time, with on one occasion 54 degrees. It never showed 55, the set temp. kWh use 0800 to 0900 was horrendous at 5kWh, equivalent to 120kWh/day, though previous hour was 'only' 4kWh, a mere 96kWh/day. At today's cap prices, this is around £38/day - unsustainable.  

@batalto - we all but posted at the same moment. I have considered flipping the dips, but am holding back, because kWh usage is already unsustainably high, see above. What I have established is the unit is almost certainly working flat out at its 14kW setting, the headroom guff is stuff and nonsense. 

@cathoderay we are consuming 2.6kw at the moment, minus 400w for the house and its probably using 2.2kw on the pump. I'd have to check my temps. Have you considered removing your heat exchanger and just straight piping the system?