Hi Guys. Hope someone can help me. I have a Vaillant combi boiler that will suddenly turn itself on even when both the room stat and programer are not calling for heat. Any ideas what might cause this? I have to turn it off at the mains electrical supply for a few minutes, then it is ok for a few days.
Vaillant vcw sine18t3wfh
- Thread starter Adriantheplumber
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I suspect you are referring to what is called preheat. Boiler kicks in to heat water in the hot water heat exchanger so when you run a hot tap, you get hot water right away and boiler gets time to reach working temperature by the time you have used up the stored hot water.
Don't remember that old beastie with a pre heat. There are some Vaillant experts on here who may advise on such an old combi.
DP I stand corrected. 
You need to push the bottom part of the button I think.
I thought it was the right hand button. But my memory is not always perfect.
In fact I went to my doctor and he told me that I had something beginning with "A" but I forgot what that he said !
I thought it was the right hand button. But my memory is not always perfect.
In fact I went to my doctor and he told me that I had something beginning with "A" but I forgot what that he said !
Hi Guys. Thanks for the suggestions but I dont think it is preheat. The boiler fires up and the pump runs. The roomstat is turned right down and programer is set to off. The rooms get hotter and hotter. Any ideas? Thanks
This boiler is very fault tolerant.
If the diverter valve is jammed then the preheat will also heat the rads!
But for a start turn off the preheat! Then see what happens!
I still fix a few of these boilers but I dont remember the control button positions as I see them when I am at the boiler!
If the diverter valve is jammed then the preheat will also heat the rads!
But for a start turn off the preheat! Then see what happens!
I still fix a few of these boilers but I dont remember the control button positions as I see them when I am at the boiler!
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Common fault on an ageing VCW18. The gland on the water section will have leaked slightly, and the spindle is sticking up making the dhw microswitches. This will lead to the heating coming on as the flow switch lifts due to the temperature increase in the boiler.
This may be intermittent, and using the hat tap may make the water section spindle drop back ok sometimes.
Overhaul the water section with a new diaphragm and gland. Alternatively replace the water section complete.
This may be intermittent, and using the hat tap may make the water section spindle drop back ok sometimes.
Overhaul the water section with a new diaphragm and gland. Alternatively replace the water section complete.
Rrrr a trip down memory lane mr 831 water valve is 011156 if still available Vaillant say oblate but still available on line
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Plenty still available from my stores 
I get some odd replies from the off the street they employ at PC.
A few months ago they said the single diaphragms were obsolete, BUT the packs of five WERE available.
The glands are in short supply but not listed as obsolete, I had the one remaining and they said that was the first they had sold for six months. BUT its used in later boilers as well!
I dont know about the entire hydraulic unit, but the prices of those are usually prohibitive.
Tony
A few months ago they said the single diaphragms were obsolete, BUT the packs of five WERE available.
The glands are in short supply but not listed as obsolete, I had the one remaining and they said that was the first they had sold for six months. BUT its used in later boilers as well!
I dont know about the entire hydraulic unit, but the prices of those are usually prohibitive.
Tony
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