I have an old Vaillant combo heating system (model unknown) which has been acting up. CH works fine but when I try to get hot water it stops working. The system tries to start and gas comes on for about 20 seconds. The temperature indicator reaches about 60 then shoots up to near 100 then the gas shuts off. I hear a few thumps (water pressure no doubt) and then bar pressure shoots up and down slightly and temp drops to around 40. About 30 seconds later gas fires up again for 20 seconds and it repeats the above. No hot water but when I switch to CH everything is ok as long as I don't try the hot water.
Is it possible I have a pump not working which would mean the water being heated is not flowing and thus the temp rises quickly and then gas shuts off as a safety feature? That would make sense as once the temp decreases again it repeats itself.
Have had the problem for a couple weeks now and have switched over to elec hot water heater but switching to elec CH would be expensive as hell.
Anyone had a similar problem? Have read threads about diverter switches etc but doesn't seem to apply here.
Any help would be appreciated.
Is it possible I have a pump not working which would mean the water being heated is not flowing and thus the temp rises quickly and then gas shuts off as a safety feature? That would make sense as once the temp decreases again it repeats itself.
Have had the problem for a couple weeks now and have switched over to elec hot water heater but switching to elec CH would be expensive as hell.
Anyone had a similar problem? Have read threads about diverter switches etc but doesn't seem to apply here.
Any help would be appreciated.