After having problems with my boiler not supplying any DHW or CH the last couple of days i am of the opinion it is a faulty fan that is the cause and would just like a second "experts" view before forking out for a new one.
The boiler is a Vokera 20-80 R.S. flowmatic. The pump appears to be working fine as does the three port mechanical diverter v/v (thought it might be sticking!). Without any demand on it the fan runs fine with no abnoraml noises or obvious changes in speed. When i turn on the hot water tap or the central heating the fan then comes to a stop, not immediately but after a couple of seconds. Is this normal?? I thought maybe when there is a demand for hot water that the fan should go to high speed rather than stopping. Also by disconnecting the silicone pressure tube to the diff. press. switch and physically blowing into it (and with hot water tap turned on) the main burner then ignites. This is the only time i can get this to ignite, the pilot light is there all the time and i have had no problems at all with that. Any ideas??
Thanks in advance.

Liam, September 2007

I dont understand why this should be the differential air pressure switch. I would have thought the microswitch that is actuated by the dhw flow switch/ch demand could be faulty. I do not understand how blowing through the flue venturi tube would permanently fix a problem. I would have guessed that the microswitch that switches the fan from the low speed scavenging mode to operating mode is faulty, or not being properly actuated.

The manual describes the chain of events leading to burner ignition: the dhw/ch demand turns on the fan that operates the differential air pressure switch that turns on the pump that operates the boiler flow switch that turns on the main burner. The air pressure switch can not cause the fan not to work.

My guess is this problem will recur, but WTFDIK?

c, September 2007

thanks to hi-spec plumbing who gave me the answer.

andy, September 2007

ok, have blown through the tube. Also disconnected the pressure switch and cleaned up the connections. Boxed back up and hey presto! The boiler is firing!! cheers andy, was about to rush out and buy a new fan!!!

Liam, September 2007

Okay, i'll have a look. Although there is only one tube that goes to the pressure switch!?! cheers.

Liam, September 2007

sounds more like a pressure switch problem than fan. check the tubes that go to the pressure switch as they can get blocked with dust

andy, September 2007