Posted by: @adrian

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I think you can set the temperature to very low 5C in the time weekly timer.

In general terms programming a target temp shifts the WC curve which also features a 'minimum flow temperature' parameter.  So will programming 5C target drop the ft to the value of 'minimum flow temperature' or go further than that.  I have a feeling it may be the first of these, in which case 'minimum flow temperature ' needs to be set sufficiently low.

@davidb 

I have given some thought to this and I can think of 4 methods to do something like what you want, each with slightly different characteristics. 

Method 1 - simplest but not perfect

Program a timed reduction in set temperature to say 5C.  This will reduce the target flow temperature (because that's what changing the set temperature does) probably to the minimum.  Make sure your minimum is sufficiently low.  

Method 2 - perfect but needs extra hardware

Use a separate timer to switch the call for heat input of the heat pump

Method 3 - simple but may compromise performance and/or comfort

Use extended mode. 

Method 4 - perfect but a bit complex

Set the target temperature a degree or two above your actual target.  If necessary (it probably will be) turn down the WC a notch so that your actual target is reached but not exceeded on wc alone, without using the fire. 

Once you have done this engage extended mide.  It wont have any effect until you turn the fire on, when it will and will turn the hp off.

The reason you will likely need to adjust the wc is that raising the target temp shifts the WC up, so you need to bring it back to the right value (and one which won't normally trigger a switch off) by turning it down a notch.

If this were my house I would go with #4 notwithstanding the slight complexity.  The temperature on tge display is only a number after all, it's what it feels like that matters.

@jamespa Thanks for your considered response. 4 options is well past what I thought possible.  
Anyway I have gone simple just now, partly to get experience using the time control.  Until now everything has run on manual - low and slow.  2 time periods, midnight to mid day on at 18.5C  Mid day to 22.00 at 8C  This ran yesterday, warm when we woke, heat went off at 12.00 and stove took over.  All good.  At 22.00 heat came back on so I felt comfortable I understood what was happening.  As the house was at 20 I was going to have heat on when not required so reset the time so the 8C target temp was reset from 22.20 to 23.50.  I was then surprised when the heat did not turn off as shown on the app.  I thought it might be the lag in updating the app.  This morning the energy graph shows it never want back off.  I am confused and wondered if you or anyone else had any helpful insights?

Posted by: @davidb

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@jamespa Thanks for your considered response. 4 options is well past what I thought possible.  
Anyway I have gone simple just now, partly to get experience using the time control.  Until now everything has run on manual - low and slow.  2 time periods, midnight to mid day on at 18.5C  Mid day to 22.00 at 8C  This ran yesterday, warm when we woke, heat went off at 12.00 and stove took over.  All good.  At 22.00 heat came back on so I felt comfortable I understood what was happening.  As the house was at 20 I was going to have heat on when not required so reset the time so the 8C target temp was reset from 22.20 to 23.50.  I was then surprised when the heat did not turn off as shown on the app.  I thought it might be the lag in updating the app.  This morning the energy graph shows it never want back off.  I am confused and wondered if you or anyone else had any helpful insights?

Not sure exactly what your time pattern was when it didnt go how you expected but, if you still had midnight to midday programmed at 18.5 I would have expected it to turn back on at midnight. 

Dont forget that the timer is really a daily timer, there is a sort of wizerd to program the same for several days, but the underlying data I am pretty sure is 'stored' per day.  This introduces ambiguity over the definition of midnight, which is one reason I tend to avoid time periods that start then!

@jamespa Yes, it was on at midnight.

My confusion was when, at 22.00 and the system started to reheat, I tried to turn it off again for 2 hrs.  I set a new time period 22.20 to 23.50 with a temp at 8C and the heating didn’t turn off again.  I wasn’t “worried” expect that the controls didn’t do what I expected.

On a similar vein, I “boosted” the DHW yesterday when the temp was 43.5 with the hysteresis at 5C.  The app glowed orange but the system didn’t respond with an increased flow temperature to reheat to 48C. Not understanding, I call Vaillant Technical and was told boost means, as soon as there is demand from a tap, the system will kick in.  This seems weird to me and wondered if you had the same understanding?  

Posted by: @davidb

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On a similar vein, I “boosted” the DHW yesterday when the temp was 43.5 with the hysteresis at 5C.  The app glowed orange but the system didn’t respond with an increased flow temperature to reheat to 48C. Not understanding, I call Vaillant Technical and was told boost means, as soon as there is demand from a tap, the system will kick in.  This seems weird to me and wondered if you had the same understanding?  

No it's not and i think what yhey said is nonsense (how would ut work?)

Boost is, so far as i have been able to tell, a one off 'on'.  However I think it works only if the current temp is less than target -hysteresis.  In your case it wasn't so (in my understanding) it didn't reheat.

@jamespa Thanks James, that makes more sense.  Having said that, if it’s not actually heating why allow the orange glow to light up?  No need to respond, increasingly I just find it difficult to get my head round how Vaillant have set this up.  Thanks again.

Posted by: @davidb

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@jamespa Thanks James, that makes more sense.  Having said that, if it’s not actually heating why allow the orange glow to light up?  No need to respond, increasingly I just find it difficult to get my head round how Vaillant have set this up.  Thanks again.

Mostly I find Vaillant controls good, but occasionally not.  The legionella cycle warning (hot water) seems to come on before the cycle runs for example.

@jamespa Hi James,  I suspect you have got your head around the HP essential logic.  I am still going up a very steep curve.

interesting you mentioned the legionella cycle.  During my recent call to technical I was told that, even though I have the legionella cycle turned off, it will still operate.  We had made a private decision (my wife is medical) because of our cycle of usage to choose a small tank and turn off legionella heat.  So the Vaillant statement confused the heck out of me - assuming it’s true.  Apparently the only way the cycle is limited is when in absent mode.

Posted by: @davidb

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@jamespa Hi James,  I suspect you have got your head around the HP essential logic.  I am still going up a very steep curve.

interesting you mentioned the legionella cycle.  During my recent call to technical I was told that, even though I have the legionella cycle turned off, it will still operate.  We had made a private decision (my wife is medical) because of our cycle of usage to choose a small tank and turn off legionella heat.  So the Vaillant statement confused the heck out of me

Interesting and possible I suppose.  Of course this behaviour might be firmware-version dependent.  I have never tried turning it off, but I think I will now just to check! Suggest you do the same.

I am told by another user that in absent mode the legionella cycle turns off in absent mode but then runs when absent mode is cancelled/comes to an end.  This makes perfect sense, but frustrates another user who wanted to use absent mode to turn off the heat pump for a few hours per day!

The problem with heating is that, although simple in principle, there are any number of variations in practice.  This leads to quite complex controllers, providing loads of options for householders/installers so they can (hopefully) configure it to their house.   Even my previous Worcester Bosch boiler, dating from 2005 and with largely analog electronics, had a fair few obscure options buried somewhere.  Maybe AI will one day simplify this for us, but if the AI is trained by unvetted British installers (and in fairness unvetted installers from several other countries) it will just do the wrong thing whilst making the interface even more obscure.

Posted by: @davidb

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During my recent call to technical I was told that, even though I have the legionella cycle turned off, it will still operate

That is, I beleive,  the biggest pile of Bullsxxt ever (or at least, not the whole story).

I have an Arotherm+ and have no legionella cycle configured - we don't need it. Our DHW runs exactly as configured, heating to 50 and no higher, exactly as set by our schedule and hysteresis. There are no legionella cycles. It's all monitored on OEM so I can see exactly what happens.

@martinrobinson & @davidb confirmed turning legionnaires cycle off via installers menu on control box DOES work, we put it off when we have visitors since DHW will be up to 60C anyway, and in the summer solar thermal get up to 55+ on sunny days too.

So it definitely goes off and stays off, except for return from holiday setting which you can’t stop.