vaillant turbomax plus 828E

Hope someone can help ! Am not a heating engineer but am a reasonably capable engineer & OK with checking & adjusting this sort of stuff if have relevant info or advice to hand!

Vaillant Turbomax plus 828E, approx 2002 vintage.
CH not working, HW OK.
System pressure 1.6 ~ 1.7 bar
Room thermostat checked electrically and switching voltage on/off as operated - all OK.
With CH demanded, CH system tries to work - burner status sequence looks OK and burner fires-up, remains lit for 12 ~ 15 seconds then extinguishes - temp gauge showing between 75 ~ 80 degrees typically. After about 5 minutes the burner lighted-extinguished cycle repeats, etc.
Suspect Diverter Valve may be no good; tried checking by following .... with CH demand 'on' (3 position switch vertical-constant & room thermostat 'on', unscrewed motor unit from end of diverter valve and motor & rod move in/out as HW tap turned on and off - HW flows OK for as long as tap open. Diagnostic D35 changes 0 - 1 & vice-versa as HW tap turned on/off.
Diverted valve is type with ~4.0mm diameter holes in body.
Could internals of diverter valve be stuck in HW position?
Any other ideas or suggestions much appreciated. Many thanks!

With that sort of temp rise I'd be thinking circulation problem, maybe a blocked filt in the return.

Thanks Mick - do you know if the Turbomax plus 828 has a filter in its system anywhere - can't see any reference in the manual.

Could a sticking diverter valve - stuck at HW maybe - cause the CH burner to cut off after 12 secs because the water heats quickly and just circulates around in the boiler until temp sensor causes burners to shut off ??

Sorry if this sounds a like b*&&!%s but I'm not a heating engineer, just an old engineer trying to sort the problem!

Thanks, Chris

can you not push or pull the rod that the motor attatches to and see if it moves?

I'll try moving the rod in the actual valve this evening when get home from work - should the actuator rod sticking out of the motor drive unit be attached to the actual valve part or does it just push against it and the actual valve be returned by an internal spring?

I noted the actuator rod on the motor drive unit is 'keyed' but due to location of boiler and diverter valve can't see clearly into the end of the diverter valve.

About how far should I be able to move the valve rod in/out?

Thanks for advice - will let you know if it moves.

Just an update following some more checks this evening after advice received on Forum .........

Removed electric motor -actuator from end of diverter valve and managed to use small pair of pointed nose pliers to pull main valve rod outwards about 7mm~10mm. Made a 'clonk' and hey-presto, CH working.

Have allowed house to get warm and manually pushed valve rod back again so everyone can have a shower in HW and will manually pull out again later to get CH working again.

have tried putting electrical actuator unit back onto valve but once HW has been activated the main valve rod doesn't return to CH position even if motor actuator rod has moved appropriately.

I guess valve should have an internal return spring that the electric motor valve and actuator push against for HW but it doesn't feel like any spring pressure at all in valve - spring broken?? (I don't have any picture of valve so am only guessing as to internals).

Anyway, have ordered new diverter valve assy to be delivered tomorrow and hopefully new valve fitted will sort problem.

Does anyone know if I can just isolate boiler water supply so just have to drain boiler to change valve without having to drain complete CH system - I presume boiler has a drain valve somewhere as haven't spotted one so far but not really been looking.

Many thanks for yesterdays hint to try manually operating valve rod.

Great web-site - hope I can help someone sometime!

Thanks