Viessmann Vitodens 100 w wont start for radiators but starts fine for water or zone2

mgkelly

Hi I would be really grateful of any advice.

I have a Viessmann Vitodens 100 w connected to a Hive with 2 zones (zone1 Rads and water) zone2 (underfloor heating).
The central heating radiators are not coming on at all. I can see the Hive receiver gets the signal and lights up this goes on and off as it should when the temp on the thermostat is changed.
The hot water works fine - bolier fires every time and switches on off as it should (Hive lights as it should).

If I go to the airing cupboard and manually move the lever on the 3 port value the pump immediately kicks into action when the central heating should be heating - however, the bolier doesn't fire so the pump switchs off after a few minutes.

I can only get heat through the radiators by fixing the 3port valve lever to manual and then switching on the hot water at the same time as the radiators. I can feel hot water flowing on both A & B pipes coming from the 3 port valve. The pipes are getting hot. It is enough to warm the house a little but, as soon as the hot water cuts off then the bolier shuts down and the rads go cold.

Underfloor is a separate zone thermostat and also seems to work fine. I get similar effect with eat through rads if i switch the underfloor on at the same time as the rads. I think it is coincidence but I used the underfloor for the first time this year just before the problem started.

Thanks in advice for any advice or pointers
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