Update:

Yesterday I deleted out all the time periods and set it to manual 20°C (again), and last night it resolutely drifted down to 17°C and this morning is now trying to recover to 20°C. It's like there are some stored settings in the main control unit that aren't getting overwritten, whatever I do.

Posted by: @josephiah

↑

Update:

Yesterday I deleted out all the time periods and set it to manual 20°C (again), and last night it resolutely drifted down to 17°C and this morning is now trying to recover to 20°C. It's like there are some stored settings in the main control unit that aren't getting overwritten, whatever I do.

I don't know if this applies to Vaillant controllers, but I believe with some heat pumps it is necessary to switch the controller off, and then back on again, for setting changes to be updated and activated. You could try that.

@derek-m Thanks. Will give that a go later. It seems like that shouldn't be necessary (as it would rather defeat the purpose of having a wall/app programmer), but I guess it could flush out some spurious data.

Resetting the control unit made no difference at all. I can make immediate changes no problem, but they are overridden by the phantom schedule settings that now seem to be hardwired in...

Installer hopefully coming out this afternoon to have a poke around.

A few more quick experiments last night/this morning:

1. Resetting the control unit yesterday evening had no discernable effect. Same setback to 17°C and slow recovery towards 20 this morning. Set upstairs setback differently, but it too reverted to the 17°C then recovery.

2. Changing target temps manually causes more or less instant changes to HP behaviour, as it should do - seems to be working correctly.

3. Tried adding some new scheduled settings this morning (high/low/high at 10min intervals on both circuits). All seemed to work as expected, with both wall panels and app synchronising fine, and zone valves and flow temperatures doing sensible things at each step.

So it all looks fine apart from the apparent unclearable phantom settings overnight - weird. (It suggests that it might be possible to workaround to assume extent by adding scheduled times to counteract those. But without being able to view what those are, we'd be fumbling in the dark, and we're left with a rather unpredictable system!)

Assume you have checked this but in case you haven't...in the weekly planner you set the desired temp during different time periods of the day, if you have gaps in those time periods then the HP does one of two things if it is in Normal heating mode it defaults to the set-back temperature that you set in those periods, however, if its in Eco heating mode it ignores setback completely and will let the house cool down until the next heating period

Is your HP turning on during the set-back periods?  If it is, you just need to increase the set-back temp higher or as you stated you could just set a 24 hour schedule with lower temps at night or if it isn't it could be in Eco mode and won't be heating regardless of how cold it gets.

From the manual Eco/normal mode is under Menu/Settings/Installer Level/Installer Configuration/Set-back mode then you should see Eco and Normal options

@josephiah

When the clocks changed, did you manually change the time on the controller or did it occur automatically? Try changing the time back to what it would have been then delete the schedules.

The other thing to check is that the firmware is up to date.

Thanks for the suggestions, both.

@gary Yes, it behaves as you would expect for the 17°C setback - switches off for a couple of hours, then when it reaches 17 it comes on at a low level periodically to maintain temperature, then warms back up from 530 or whenever it is.

So I'm certain it's not a system capability or heat curve issue.

@derek-m I don't recall, I'm afraid, but I don't think the start of this issue coincided with that. In any case...

My installer had a thorough look around the whole system this afternoon. Nothing else obviously wrong, which is a good start. Once he understood the actual issue, agreed with me it was weird, and nothing to do with the design or current heat curve settings. Performed a full factory reset which should hopefully have cleared any odd settings out. We'll try it over the next day or two on a constant temperature and see what happens.

Now that I know a little more (from hovering over the engineer and bugging him with questions) about which bit of the system does what, I'm not surprised that resetting the main control unit didn't do anything.

The user settings (time schedules, etc) are dealt with on one of the other units (so might have been interesting to reset just that one...), and the heat pump control unit just deals with the lower level heat pump functions, i.e. it just does as it's told by the programmer and controls the heat pump.

Update:

After installer performed full factory reset, we tried first night with constant temp - worked fine.

Then I applied overnight setbacks 1°C lower than daytime. All changes applied with wall unit to rule out app conflict. Didn't work - is now setting back to something like 15°C instead, and not on my timings. As before, other changes, including setting new scheduled times, work fine. Grr.

Any further suggestions welcome!

Posted by: @josephiah

↑

Update:

After installer performed full factory reset, we tried first night with constant temp - worked fine.

Then I applied overnight setbacks 1°C lower than daytime. All changes applied with wall unit to rule out app conflict. Didn't work - is now setting back to something like 15°C instead, and not on my timings. As before, other changes, including setting new scheduled times, work fine. Grr.

Any further suggestions welcome!

If you run your heat pump on a fixed LWT, the IAT will likely vary with changes in OAT. Applying a 1C setback may have just made matters worse.

If you have a fixed LWT of say 40C and the OAT is say 10C, then it is probable that the heat emitters can supply enough thermal energy to meet the demand, so the room thermostat may need to start and stop the heat pump to stop the IAT getting too high. If the OAT now falls to say 2C overnight, the heat demand will be much higher and may exceed the quantity of thermal energy that the heat emitters can supply at a fixed LWT of 40C. The room thermostat, even at a lower setting, could be running the heat pump continuously, but because the LWT is now too low to supply the increased heat loss, the net effect will be that the IAT starts to reduce.

This is why it is necessary to run a heat pump in WC mode, so that it raises and lowers the LWT to meet the thermal energy demand.

Thanks, understood.

It's running on WC and modulates the temperature sensibly. There plenty of capacity in the system, and the building warms fine during the day. (And then loses to much temperature overnight due to the excessive setback.)

Pretty certain the issue is with the demand signal being (incorrectly, or not at all?) passed from the programmer to the heat pump controller, rather than the system's response to that demand. I.e. it's not that the system is trying to heat the house and falling short; it's not trying (clear on our consumption charts, which I'll maybe post if I get a chance).

(Since we haven't yet experienced a correctly enacted 1°C setback, I don't have enough info to compare that yet!)

Have tried resetting the gateway, and if that doesn't help I'll be badgering the installer and/or Vaillant again tomorrow, because its now firmly in the "this control unit just isn't working" category as far as I'm concerned.