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Ravenheat condensing combi boiler CSI 85AT fault?

My 1.5 year old boiler has developed a random & intermittent tripping fault, in both modes of heating (i.e. CH & DHW). With the remote programmer calling for heat or the hot taps turned on, it often runs for a few minutes OK and then trips out and restarts after about 5 seconds the starting purge/fire cycle. Sometimes it will trip out before ignition occurs, but it usually processes through the cycle and the flames are good and NG blue. It sometimes continues running for maybe 5 minutes prior to tripping, so the intervals vary. Occasionally on DHW mode it will not fire for quite a few minutes so one gets a good supply of cold water!

I have checked the air pressure switch tubing to see if there was a blockage or leak, and also that the switch operates by blowing the tube/sucking and it seems to be free & operable. I have checked that both the CH & DHW thermistors are working and are giving sensible readings. The fan is free to rotate and runs OK. The overheat thermostat does not seem to be operating when the boiler eventually gets the system up to temperature & the red LED has not been illuminated. The boiler gas pressure was set by the CORGI commissioning engineer and has not been altered. The water pressure is set to 1 bar + and the low water pressure stat does not appear to be incorrect.

Any ideas what to check next? I suppose it could be a weak connection on the PCB but tapping it did not cause a trip!

Brian Mallalieu
January 2007
Buy a worcester!

the gas man!
January 2010
my boiler will not switch on at all only the green light comes on nothing else pls help

February 2009
I believe I had a similar type of problem with the same boiler type but mine would not light without continually resetting the switch over and over again. The process would commence on either cycle but the boiled would trip out and the red light would come on. I tried all the usual suggestions (no small expense !!) until I finally decided that I seriously needed to understand how the thing works. What I discovered was that the fan, although it was working, it wasn't rotating fast enough when cold to increase the air pressure within the chamber, this acts like a sealed unit with the steel cover in place, and trips the air pressure switch (top right) which turns on the gas and lights the boiler. I found I could demonstrate this by removing the rubber tube from the flue ring (inside top centre), turning on the boiler, blowing gently into the tube and the boiler would light every time, as soon as you stop blowing the flame ignition stops. The cure was simple and cheap, I got my trusty tin of 3-in-one oil from the garage, turned off the power to the boiler and oiled the shaft of the fan in-situ, I found that a bendy straw fitted snuggly over the spout of the oil can and gave me good access to the fan shaft, do not flood it though, a little oil goes along way. I then ran the boiler without lighting it to let the oil do its work. Fingers crossed, the fan now whizzes round and the boiler lights when it should. I can only conclude that initially, by continually resetting the boiler the fan shaft must have warmed up sufficiently to spin fast enough to operate the pressure switch. Might sound crazy, but it worked for me and the tin of oil is cheaper than all those uneccessary replacement parts I ended up buying. Hope this helps.

David from Watford
April 2007
Had a similar problem with a new CSI 85T. Rotating the ring at the flue junction so that the venturi was at 90 degrees to the flue (on the right hand side viewed from the front). Cured the problem after trying all the old favourites such as changing thermistors which did no good at all. Make sure there is not even the slightest kink in the venturi tube

January 2007


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