Vaillant Ecotec Exclusive Green IQ 835 combi modulation problem

[juliab]

Hello,

We got a Vaillant Ecotec Exclusive Green IQ 835 combi boiler in November 2021 as we wanted the most efficient boiler that modulated really well, especially for heating.

Two weeks after the installation, the heating engineer needed to recalibrate the boiler and after that we started having problems with modulation for hot water. Vaillant states that the modulation will kick in for a minimum flow of 1.8l, but even if we were running 3.5-4l, the boiler would just shoot up to maximum, overheat the water and turn off the burner, then start it up again and turn off, like before. This would usually happen if someone opens the tap on a lower flow from the beginning, so it looked like the boiler could not read the flow properly.

When we look at the live monitor, it looks like it starts cycling at the "measuring program", it never gets to the stage "Burner on".

Vaillant sent about 4 engineers our way who replaced multiple parts, from the CO2 sensor to the whole motherboard in the latest instance, and it was still doing that. We have been opening tap on maximum, waiting until it starts modulating and then turning the flow down to use it.

Vaillant agreed that the boiler should not be doing that, especially at 4litres. There was nothing else they could do, so they replaced the boiler.

The new boiler worked absolutely fine for over a month. Unfortunately it has now started doing the same thing. There have been no changes in use or any intervention. Has anybody seen anything like this before, could anyone suggest where to look, or what else we can try?

Many thanks
Julia

 

[John.g]

Is it modulating OK on CH?? All boilers including g Vaillant fire up at ~ 60% output, Vaillant used to and may still do maintain this output for 60 secs before allowing modulation. If true, in your case ~ 22kw which would require HW flow of 5.5/6 LPM for a minute or so to avoid exceeding 65C, is this what's happening I wonder?

 

[John.g]

Double posting.....double posting

 

[juliab]

Is it modulating OK on CH?? All boilers including g Vaillant fire up at ~ 60% output, Vaillant used to and may still do maintain this output for 60 secs before allowing modulation. If true, in your case ~ 22kw which would require HW flow of 5.5/6 LPM for a minute or so to avoid exceeding 65C, is this what's happening I wonder?

Hello, thank you for your reply,

CH is modulating as it should. For CH it fires up at about 20%, stays like that for a couple of minutes, probably preheats or something, then shoots to 60% stays like that for 5-10 mins then goes down to about 10%. We've got curve set to 1.4.

For HW the way it should work (and it was working like that for the first month) is as follows:
Temp set to 45, no comfort setting. I open the tap, flow about 4L, it goes to about 60% burner then pretty much immediately calms down to 30-40% and then keeps dropping to 15-20% and changes state from the "measuring program" on live monitor to "burner on". So you could see right away that modulation kicks in almost instantly.

Currently, i.e. after around one month, the problem with HW is that it shoots up to 60% burner straight away, then drops to about 40% (not immediately like before) still showing as "measuring program" on the live monitor, then it overheats, shuts down and goes into pump overrun. It then repeats this process - burner on abour 60%, then drops to 40% and shuts off. On the live monitor it never actually reaches the "burner on" status, always getting stuck in the "measuring program". It's as if modulation never kicks in. We've turned the boiler off for 10 minutes, and looks like that's resolved the problem for now, it's been fine for the last couple of days. Concern is that if it's started doing that again, it's probably gonna keep doing that until turning it off wouldn't solve the problem any more, which is exactly what happened with the previous boiler.

 

[John.g]

No comfort setting = preheat off??

How did it behave if you set the HW temp to 60C?.