Vokera Combi Problem - Heating getting 'stuck' with hot water

[gw0udm]

Hi,

I've got a rather aged Vokera combi boiler (a 20-80 Flowmatic) which has recently developed a problem. Basically, whenever the hot water comes on in response to demand (and which works fine) the heating comes on as well, and after the demand has ceased the boiler stays on and the radiators heat up. The pressure dial goes crazy as well, flicking quite high accompanied by a loud clanking noise before settling down again.

It's almost impossible to stop it once started, as the control box is switched out completely (via the summer / winter switch), and the only way I have of definitely stopping it is by switching the boiler off at the main switch. Sometimes when I switch it back on the boiler fires up again!

It has the feel of a sticking relay or valve or something somewhere - is there anything obvious to check?

Thanks in advance,

James

[Ed Sirett]

It is normal for many combis to dump a little heat into the heating pipes after doing HW.

However this boiler sounds like it is having a problem switching off after HW. This is probably not the PCB but something to do with the water flow detector (almost certainly a diaphragm type).

My GUESS is that the diverter valve and/or diaphragm is sticking half way in the HW position, the gas stays on, some heat goes to the rads, the boiler is in HW mode so the gas is more or less full rate. Hence the knocking and clanking as the boiler over heats.

I'd check to see how the diverter valve works , especially w.r.t to the DHW microswitch.