vaillant vcw sine 18 boiler trouble

23 Mar 2009
Reaction score
West Midlands
United Kingdom
Hi,i have an Vaillant vcw sine 18t3 combi,and when i turn on the central heating fires up and when temp reaches 60 boilers cut out temp drops to about 55 then fires back up and just keeps on doin this all the time rads dont get hot can any one help please
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Kevin I would be looking at the was capsule in the diverter.
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Check you haven't got a hot tap dripping. This symptom can also be a leak on your hot water pipework somewhere.

Does the second pipe in from the left of the five pipes under the boiler become hot when you run your heating? This indicates one of the above.


If all you have said is true then wax capsule it is. Also check for stiff switch mechanism on top of the Flow Switch (Diverter Valve). Spray some WD40 on the spring/cage.

For reference the LHS microswitch should make when the flow temperature is 60C. When you turn on a hot tap the RHS microswitch switch plate should sit flush with, and just touch the red microswitch. Adjust the 7mm nut on the spindle to achieve this when you have replaced the wax capsule.

Good Luck

dont know what a "was capsule" is or diverter ill mention it to my plumber but sofar he dont know what problem is thinks it might be a board prob.
plumber changed flow switch 2 days ago for a new one wasnt getting constant domestic hot water,now its changed water fine i will tell my plumber what you ve all told me looks like it must be wax capsule, many thanks people
If the flow switch complete was changed then it would have come with a new wax capsule. Get you plumber back to check/clean the pins of the multi-plug connector at the top of the flow switch. They have a tendency to not contact properly if not pushed in really firmly.

will do,. also he pulled off purple wire from ntc and burner dont cut out so he changed the ntc to but still cuts out with purple wire on, and he did have trouble getting plug back on but i pushed it on till clicked but dont know if all conntacts are touchin will get him to test and i checked flow thermostat with multimeter and is not getting full reading do you think this might have something to do with problem,water pressure is fine to.
With Hot water or Heating on - The burner will light if the wire is pulled off of the NTC sensor. The burner will turn off if it is shorted to the boiler frame. This proves the continuity of the sensor wire and usually the pcb. To further check the NTC & pcb:

Replace the purple wire on the sensor, draw hot water at a slow rate. After a minute or so the main burner flame should modulate up and down to regulate the hot water temperature. If this is the case then the NTC sensor (a very reliable component) and PCB are fine .

If the burner does not modulate then turn the flow down till it does. If after this the burner is still on full rate the NTC needs to be checked for resistance using a multimeter (2.1K Ohms @ 30C). You should have no problems with this though as you said it was new.

If there is still no modulation, check/change the microswitch on the DC Solenoid (note colours of wires and replace as found!). If still no joy, replace the 'Electronic Regulator' PCB.

Hope this helps. These are really reliable boilers, but sometimes a poor connection on any of the microswitches or multi-plug connectors can produce strange results!!



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