Old Viessman vitodens 100 combi problems

[Jones82]

Hi guys,

Just replaced some radiators in a recently purchased student let. The boiler is a 12 year old vitodens 100, 7248589.

I tested the heating and hot water before draining down and it seemed to be working, maybe a little lack lustre on the heating.

After completing the rads, refilling and testing, the pump seems to be hardly pumping, I've bled it and thoroughly flushed through but the system barely circulates. Ok so maybe new pump.

Now for the unusual bit, I managed to get the heating working but the hot water is overheating the boiler every time then it goes into lock out at 99 degrees. Possibly air in the heat exchanger? The diverter and flow switch seem to be working as the boiler doesn't circulate round the heating and the hot tap fires the boiler... its almost as if the plate is blocked? The hot tap runs cold but the boiler overheats

The system water is clean and has a filter fitted on the return, also clean.

The manual just says to flush through the flow and return via the connections on the pipework under the boiler. Newer models of viessman I've worked on had a vent pipe from the top of the main heat exchanger to assist in the removal of air... this one does not.

Any tips?

Thanks

 

[Gasmk1]

could be air keep turning on and off but pump could be on way out that could cause o/heat on hot water

 

[townfanjon]

It sounds like a WB1A they didnt have them , probably air pal

 

[ShaunCorbs]

Can you stop the pump ?

 

[Jones82]

Managed to get it going again, the left hand isolator on the flow side was partially blocked / closed and once I got that flowing I got a fair bit of air through by flushing through with an external filling loop. The pump was actually fine, I couldn’t stop it thanks.

One thing I noticed, whilst running the hot tap the diverter switched over, only whilst running the tap at the same time as flushing did I manage to get that last bit of trouble some air / blockage out.
Not a big fan of viessman, I always seem to have trouble getting air out of these