Boiler not ignighting after sytem drain

16 Jun 2004
Reaction score
United Kingdom
I have just removed 2 radiators and drained my whole system. I refilled the system with the radiators off to the required pressure levels on the boiler. I still have 4 rads connected to the system.
The Vaillant Combi boiler now does not ignight in both Hot Water or Central Heating mode. Is this something to do with the Rads being off or do I need to repressurize the expansion vessel?

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1) Is the cover off the boiler? If so, replace it - some wont ignite without it!

2) I did have something similar with a Glow Worm, and it was only after much draining down, refillling and then finally in desperation stripping the boiler I found that the piezo ignitor was duff thru disuse.........
it sounds like youve got the vcw combicompact series (about 9-10 year old) Prone to not working straight away after a drain down/refill. Top right back of the boiler is the auto air vent. make sure the dust cap is unscrewed about a turn. Inside the combustion case there is a bleed point attached to the heat exchanger on the left. attach a 10mm spanner and open it with a bucket in front of it until all of the air stops.

If your boiler doesn't look like as described below disregard everything written.

the combicompact has 2 square dials on the right hand bottom of the control panel, one is a pressure guage, one is a temperature indicator (that probably doesn't work anymore). it has two switches on the left one is black, one is red (the neon may or may not still work) beside these switches is a ch thermostat numbered 1-9.
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Description was bang on I will try this next time it happens. I put rads on again and hey presto.

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