Posted by: @transparent

↑

What do you think @jamespa ?

It does sound like the house has a large thermal lag.  Bear in mind that the thermal lag of the house can be long even if the radiators can cool quickly, this is as much (actually more) about the house as it is about the emitters.

I can see how any whole-house control loop can realistically deal both with solar gain (which @clockworks mentions) and a house with long thermal lag, unless it has a model of the house and individual control over each emitter (and even then it might be impossible).  I also dont know what thermal lag/range of thermal lags autoadapt control loops assume to be honest.

I disabled room influence/autoadapt on my Vaillant a few days after it was installed, and have it running on pure, open loop, WC.  The result is much more stable than with room influence enabled and much, much more stable than my boiler ever was.  I have one room where solar gain is a 'problem' (is free energy ever a problem?), the radiator in that room happens to be a fancoil so it turns off the fan of its own accord, if it weren't a fancoil I would probably fit a TRC acting as a limiter.

I haven't followed this thread in full but I understand that the tweaking is principally designed to reduce cycling.  The problem with that is simple, if the house loss at any given temperature is less than the minimum output of the heat pump, the heat pump has to cycle whatever you do with it.  You can 'tweak' the frequency by playing with controls, but not the duty ratio.  Looking at the plot posted upthread this appears to show a time when the loss only just exceeds the minimum output; the duty cycle isn't that much less than 100%.

If the house has a long thermal lag then part time heating might turn out to be more efficient when it would otherwise cycle.  Otherwise Id personally leave it to do its thing without trying to bend the control algorithms built into the machine.  There is probably an adjustable 'min cycle time' parameter which could be turned up if short cycling is common and personally I would change that rather than trying to get some external controller/thermostat to do the job.  Best generally to keep these things as simple as possible.

There is a setting for "room temp control interval", but I'm not sure if this just applies in AA mode for reading the wireless sensor, or whether it applies in all modes for any thermostat switching.

It's been running along steadily in AA mode since about 7am, basically tracking the outdoor temp as it steadily rose. It's still cycling just as much as it did in WC mode though - once the OAT is steady.

It really does look like my cycling issue is just down to the relatively high minimum output of the heatpump being over the heat requirement of my house.

I think I'll go back to WC mode. Hopefully I can leave the wireless remote connected so that it can be used to nudge the operating temperature up or down with the buttons, but it'll be out of the feedback loop, and not try switching off the heatpump?

In WC mode, I could just set the curve a couple of degrees higher than necessary, and use the Evohome controller to limit the rooms and drop the call for heat if the rooms are all too warm. The Evohome seems to operate at +/- 0.5 hysteresis, so finer control than the Mitsubishi wireless remote. It's also limited to a maximum of 3 cycles per hour, and will respond by switching on when just one room drops 0.5 degrees below target.

I've currently got 2 big rads open all the time, and could increase that to 3  without compromising the way the Evohome works. Still the danger of cycling as the heatpump hits target temp - hence turning it up higher than necessary.

I know this is not how it's supposed to operate, but it might be the most economical way to get stable temperatures and the least amount of cycling. Run harder,  50% duty cycle, and a startup every hour or two, rather than every 20 minutes.

Posted by: @clockworks

↑

I know this is not how it's supposed to operate, but it might be the most economical way to get stable temperatures and the least amount of cycling. Run harder,  50% duty cycle, and a startup every hour or two, rather than every 20 minutes

The plot you posted was more like 80% duty cycle, unless you also increase flow temperature (compromising efficiency) then you cant, at the outdoor temperature of the plot, achieve 50%.  Whatever you do duty cycle has to be able to vary from 0% to 100% to match the load. 

I hope I wont offend if I ask - are you sure you arent over-thinking this and that you wouldn't be better just letting the heat pump controller do what it should be good at?  Do you actually have any evidence its a problem?  All heat pumps cycle at least some of the time and, given that they have a modulation ratio of 3:1 at best, even one that is perfectly matched to the load will only not cycle between about 4C (below that it cycles due to defrost) and about 12C.

@clockworks @transparent wrt thermal inertia - the control parameter I mentioned which I have been tweeking for my own installation (see previous post in this thread) is specifically relevant to this. 

Ref: fine tuning heat pumps  See 'Kelvin minutes'.  In case that's useful. 

I appreciate this is Stiebel Eltron, not Mitsubishi, but do you have access to Mitsubishi technical support?  To see how they do this?  

I would very much agree that less is more wtr controls.  I only use WC.  No room controls.  All zones / TRVs etc are fully open.  My kitchen has high solar gain and is one of best insulated rooms in the house - so I compensate for that through balancing the system, ie, reduced flow rate for the kitchen UFH which effectively reduces the heat output.

I'm pretty sure that I am overthinking it. It's just that so many people say " low and slow, cycling is bad", etc.

If by that, they actually mean "leave it alone, don't keep switching it on and off with the thermostat " then I've got the wrong end of the stick. I do tend towards fixating a little.

If the heatpump is designed to tolerate cycling every 20 minutes unless it's cold outside, then I'll be quite happy to leave it to get on with it.

I'm happy with how evenly warm the house is, and with how much it's costing to run. Better than my old oil boiler, and not costing much more.

Basically, I'm looking for reassurance that it's all OK, and there's nothing to worry about.

Posted by: @clockworks

↑

I'm pretty sure that I am overthinking it. It's just that so many people say " low and slow, cycling is bad", etc.

If by that, they actually mean "leave it alone, don't keep switching it on and off with the thermostat " then I've got the wrong end of the stick. I do tend towards fixating a little.

 

If the heatpump is designed to tolerate cycling every 20 minutes unless it's cold outside, then I'll be quite happy to leave it to get on with it.

I'm happy with how evenly warm the house is, and with how much it's costing to run. Better than my old oil boiler, and not costing much more.

 

Basically, I'm looking for reassurance that it's all OK, and there's nothing to worry about.

All heat pumps cycle, in fact the 'non-cycling' temperature window is quite small even if the heat pump is perfectly matched to the house.  Because the typical modulation ration is at best 1 in 3 and below 4C cycling will occur due to defrost, running without cycling between say 4C and say 12C is about as much as you can realistically expect and I would be willing to bet the window is smaller than that for a majority of installations.

Listen to your fridge, its on and off all day.  Same technology.  How often do fridges fail?

Obviously Im not in a position to make promises but I genuinely don't think you have much to worry about!

Count me reassured .

I'll switch back to WC mode, and remove the wireless remote if necessary.

I'm reasonably confident that once I've replaced one slightly undersized radiator, got the hallway radiator replumbed the right way round with new valves, and topped up the insulation over the bathroom and lounge, the rooms will be reasonably close in rate of heatloss.

As it stands, I'm having to run a little hotter to keep the worst rooms up to temperature.

I left it in AA mode overnight, with the wireless remote set to a target temperature of 19.5, and with the Evohome TRVs limiting the temperature in most rooms.

Wireless remote in the hallway, and the hallway radiator "open".

Heating schedule set to "off" between 20:00 and 23:30.

OAT dipped to 9 degrees, back up at 10 degrees before dawn.

When the heating came on, the flow temp started off 5 degrees above the WC curve. That's fine, it "knew" the house needed more heat to make up for the "off" period. This is what I want it to do - run a bit harder during the 7p hours, after being off for 3.5 hours when it's 27p kWH.

By the time I got up just before 7, all the rooms were within 0.5 degrees of the setpoint, and everything was stable.

The graphs posted show that AA mode is actually working - as long as the target temp is very close to the actual temp, and the OAT is stable. The flow temp is up and down a little, and it looks like it's being controlled by the compressor duty cycle - same 20 minute cycles, but with much longer "off" periods than when in pure WC mode (a fixed 4 minutes)

Interesting little "blips" on the power consumption graph. Looks

like it fires up the pumps after being idle for a few minutes, checks the temps, and then starts the compressor.

I'll leave it alone for a few days, and see how it copes with swings in OAT. I'm not sure if it went crazy yesterday morning because the OAT dropped rapidly, or because I had the target temp set at 20 degrees - a temperature the hallway was realistically never going to reach.

I'm seriously impressed with the cost so far today - 80p and 6.26kWH since midnight, and that includes DHW up to 49 degrees. 

After a week of using AA mode, I've switched back to pure Weather Compensation mode.

I had a couple of instances when the house started to get too cold - OAT rising slightly, wireless controller bang on the setpoint, ASHP flow temp lowered by 5 or 6 degrees. The thermal lag in the hallway meant that the temperature seen by the wireless controller didn't drop, so it kept lowering the flow temp - ended up going way too low for most of the rooms.

It also made no real difference to the "cycling".

When it was "in sync" with the house, AA mode did seem to be a bit cheaper. Too unpredictable though.

I think AA mode would work if it had multiple sensor inputs, and could take an "average" across the whole house. Or, maybe it'd work if I kept all the internal doors open to equalise the temperatures? Keeping the whole house at 21 degrees might get expensive though

Posted by: @clockworks

↑

After a week of using AA mode, I've switched back to pure Weather Compensation mode.

I had a couple of instances when the house started to get too cold - OAT rising slightly, wireless controller bang on the setpoint, ASHP flow temp lowered by 5 or 6 degrees. The thermal lag in the hallway meant that the temperature seen by the wireless controller didn't drop, so it kept lowering the flow temp - ended up going way too low for most of the rooms.

It also made no real difference to the "cycling".

When it was "in sync" with the house, AA mode did seem to be a bit cheaper. Too unpredictable though.

 

I think AA mode would work if it had multiple sensor inputs, and could take an "average" across the whole house. Or, maybe it'd work if I kept all the internal doors open to equalise the temperatures? Keeping the whole house at 21 degrees might get expensive though

For what its worth I had exactly the same experience with my Vaillant heat pump and have settled on operating in pure WC mode.

One of the challenges for the designers of AA mode (or whatever your heat pump calls it) is that it attempts to respond to changes in IAT.  However any change it makes in response have a delayed effect (because of the thermal mass of the house), leading potentially to instability in the control loop.  This is particularly true with houses that have UFH in a slab, and much less true if you have fancoils throughout.  Thats one reason why weather compensation works so well, the heat pump is responding directly to a leading indicator of the heat loss (the OAT) rather than to a lagging indicator (the IAT) - so there is no loop delay.  

You may or may not be aware that there is a whole branch of engineering called control theory which deals with this sort of (horrible) problem.  Where the characteristics of the system remain constant (other than the one variable for which you are trying to correct for) its possible (in many cases) to design a control system that stabilises well, using three terms (proportional, integral and differential) with appropriate weightings.  When the characteristics of the system (other than the one variable that you are trying to correct for) change in a timescale comparable to the response time of the system, this becomes increasingly difficult.  WC circumvents this problem by responding directly to the main variable that we are trying to correct for.

Out of interest @jamespa did you need to adjust your WC heat curve setting at all? 

Posted by: @ajn9000

↑

Out of interest @jamespa did you need to adjust your WC heat curve setting at all? 

Difficult to say as I was still in the process of adjusting my WC settings at the time I made the change.  What I know is that the house temperature is significantly more stable having turned off room influence.  This is likely however to be house specific!