Viessmann 111 short cycle / climbing to 100+ then F2?

[twixx]

This sporadically happens, and I have no idea why. Thinking it was air trapped, we just had a powerflush, but still the pressure gauge is bouncing like a nutter and the boiler is cycling…

I’m really hoping there’s a Viessmann pro around who might be able to advise. All the plumbers round here always say ‘I’ve never seen this boiler before’…!

 

[Johntheo5]

Is that a combi boiler, if so, is the water coming from a hot tap, hot or cold?.

 

[twixx]

It’s a combi with a 40l internal tank. It’s a cold water feed.

The boiler is ok on the radiator side, so it’s something specific to the hot water…

 

[Johntheo5]

Is the water cold coming from a HW tap?
Did you get the power flush because it was acting up in DHW mode only?.

 

[twixx]

The water is cold from the HW tap when in F2. After a reset you can occasionally nurse a bit of hot water through.

The dancing pressure gauge is what concerns me… (could it go boom?)

And yes, the power flush was in an attempt to resolve this. During annual clean down the water is often an oily black colour.

 

[Johntheo5]

The water is cold from the HW tap when in F2. After a reset you can occasionally nurse a bit of hot water through.

These boilers have a cylinder I think that stores the HW so may have another circ pump as well as the normal boiler circ pump and plate heat exchanger, I don't have a schematic arrangement but the plate heat exchanger may be blocked and require renewal as these are very difficult to clean or the secondary circ pump (if installed) may have failed or is failing, but far more likely to be the plate heat exchanger IMO especially with that oily black water. If you get the same black water from a rad vent then the whole system may require attention.

The dancing pressure gauge is what concerns me… (could it go boom?)

Don't think so as these have a pressure relief valve set to 3.0bar.

And yes, the power flush was in an attempt to resolve this. During annual clean down the water is often an oily black colour.

 

[dilalio]

Main rubber pipes to hex are known for catching crap and should be "squeezed" on annual service, flow one into bucket so it doesn't get into hex.

But you've said that CH is OK so that suggests plate hex.

Also, as said, storage combis usually have a shunt pump so worth investigating this.

A call to Viessmann would be next step but that can be hit and miss with them.

Check out Andrew Millward on YouTube, he knows all about these boilers and has lent me parts in the past to try and solve issues. It's usually blockages in the boiler due to poor maintenance.

Andrew Millward Watford

Heating, wood work and my other interests. My face book link. https://www.facebook.com/profile.php?id=100011041051437

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