This unit has the Modairflow 2 control which runs the fan from very slow to full speed using infinitely-variable speed control. When in the 'normal' state, where a constant but small amount of heat is required, the burner lights, runs for about 40 seconds, then goes out. The circulation fan starts running at a constant speed rather than ramping up gradually (same speed as the ventilation speed selectable by a switch on the electronics module), usually a few seconds before the burner goes out. This runs for about a minute and a half, and then stops. The cycle then repeats, with varying time gaps. Sometimes the fan restarts without the Burner having started. What used to happen was that the fan ran almost constantly, at very low speed, with the burner coming on and off regularly. This gave a very comfortable feeling and seemed to heat the house very well.
The Thermistastat seems to be giving about 6.3V to the controller when up to temperature, this falls when the temp cools or the thermostat is manually increased. The burner lights in response to these thermostat movements.
The Air flow detector reads about 3.5K ohms when disconnected. The fan will not start at all if it is not connected. Shorting it out completely runs the fan at top speed.
This all seems to point to the R005 electronic control module, but as these are about £170 I am a bit hesitant to just order one without better diagnostics. This fault does not seem to match those in the J&S Installation, Servicing and Commissioning instructions ZZ384/1.

Alan Duerden, November 2006

Fixed this at last. Sent original fan to CET Ltd and had the bearings replaced. I then had two fans: the original, with no capacitor start, now with new bearings; and the 'new one' I had purchased in 2006, which came in a J&S box and was marked as being suitable for my WAU, which had a 4 microfarad start capacitor. I put in the original fan, and it all works just as it is supposed to. When the thermostat calls for heat, the gas starts and then after 30 seconds or so the fan starts at slow speed. If only a small amount of heat is required, the fan continues to run very slowly, with the gas coming on for short periods until the required temp is reached. If the thermostat is increased a large amount, the fan gradually speeds up to top speed, and the gas stays on, giving a fast warm up. If the thermostat is lowered back down the fan slows gradually and the gas comes on and off as required. Now I will contact J&S, whose technical department said that the capacitor on the fan "should make no difference". Incidentally, I did try the R005 controller module and all the other bits, so I have two R005 units which will not modulate properly with a capacitor start fan. Sorry I did not say about replacing the fan in my original submission, but to be honest I did not notice the fault until several months after fitting it.

Alan Duerden, October 2010

Hi,

I had a similar problem after my unit was 10 years old. When I tried to run on Ventilation setting (no Heating )the fan would not run and it would not modulate when running on heating. The basic problem was the fan was knackered and bearings were close to seized. hence it would run on max speed but as soon as it tried to modulate the fan couldnt overcome the resistance of the bearings. The boiler then shut down until it cooled down when everything switched on at full speed againand everything came on at full blast. A good test is to try on summer ventilation setting, if the fan doesnt run thats probably the problem

A new fan is a doddle to fix but will cost you about £200, alternatively get the motor bearings replaced which could be cheaper.

Glyn Jones, December 2008

Try the thermocouple see if it has burnt away

richard, November 2007

Try the thermocouple see if it has burnt away

richard, November 2007