Viessmann Vitodens 100 30kw combi

[LSP2020]

Help/advice greatly appreciated. The boiler is heating water on demand but not heating rads. I’ve tried resetting every thing even the external Siemens thermostat but this doesn’t help.
I’ve been in touch with Viessmann and they say it’s either the thermostat unit or the Flow Switch. I’m happy that the thermostat is working OK after resetting the connection in-line with the instruction manual.
Any ideas?

 

[Jackrae]

Flow switch ?

 

[LSP2020]

Yeah?
Flow Switch - Cold Water Inlet Elbow with Water Flow Switch??

 

[Jackrae]

Flow switch maybe stuck in "flow detected" position so boiler transfers heating facility to the domestic hot water mode. But if you are not drawing off water then presumably boiler flame extinguishes. Dare I suggest disconnecting the switch to see what happens.

 

[Phill Peck]

How old is it

 

[Phill Peck]

Does the boiler fire in ch
Are u saying it fires but rads don't get hot? Or it doesn't fire

 

[LSP2020]

The boiler is about 4 years old............
It fires to heat water but it doesn't click into CH mode so the rads dot heat?

 

[Agile]

Try selecting the CH and then turn the DHW temp control knob back to zero!!!

Tony

 

[LSP2020]

Hey Tony - tried what you suggested and the heating fires up.
When I turn the temp on the HW back up the heating cuts out??

 

[Jackrae]

In principal, isn't that what's supposed to happen. DHW takes priority over CH. However, if the cut-out of CH takes place with no DHW flowing it seems the boiler "thinks" that there is water flowing - which takes us back to the flow switch.

 

[Agile]

Hey Tony - tried what you suggested and the heating fires up.

So now following my help you can have some heating!

Have you at any time unplugged the DHW flow sensor?

Or tested it with a multimeter?

Tony

 

[LSP2020]

Thanks for the heating

Have you at any time unplugged the DHW flow sensor? No

Or tested it with a multimeter? No

Do you think it's the flow sensor that's failing?

 

[Jackrae]

You asked the manufacturers - they suggested the flow switch - why do you chose to ignore their suggestion ?

 

[Agile]

The first expectation would be the flow switch !

You asked and we have told you! So have Viess Tech apparently as well so I dont know why you seem so reluctant to accept that.

However, the last time I saw that fault it was not the flow switch but the PCB ( which are expensive for those boilers! ).

The test I asked you to do restores the CH but does not differentiate between flow SW or PCB faults!

Tony Glazier