Vaillant Combi boiler VCW-T3W water heating snag

[Iaint77]

I have a Vaillant Combi boiler VCW-T3W which fails to heat hot water correctly. When the tap is turned on the water initially heats to the correct temperature however it then quickly cools to at best luke warm. If the cold water tap is run simultaneously the temperature increases but not to max. In the boiler the burners are running continiously from the moment the hot water tap is turned on. The central heating is working correctly. I suspect this is a valve problem but do not have the boiler manual and have not been able to find it at the Vaillant website. Does anyone have any information / thoughts?

 

[marios_kin]

Do you still get hot/warm water when the heating has been off for a while?
And does the boiler still light when the heating is switched off and you draw off hot water?

 

[Iaint77]

The answers yes to both questions. What are you thinking?

 

[galwag]

Heres a few thoughts

When running hot tap and ch turned off does any heat creep out to nearest rads if so suspect divertor valve letting by.

Check water valve is operating microswitches

Other fault I have come across is DC magnet (gas solonoid) I cant remember exactly but I think you need about 15V then you should get full gas rate to burner