stubertio said:Right. I took the hood, fan and top plate off (including the flu elbow and venturi) and ran the fan in 'standalone' mode (wired into the mains by itself). Connected the old manometer and it pulled a steady 4mbar on full whack.
Cleaned the doo-dahs on the fan (the venty bits) and put it back in the boiler housing. I then wired it up again to the separate mains supply and ran it in situ. It pulled a steady 3mbar. I then put the air chamber cover back on and ran it again. 2mbar! Going down slowly!!! I then wired it all up again as should be and fired up the boiler as per norm. It still cuts out for a fraction of a second between modulation. I hooked up the manometer while it was running and watched what happens when the burner switches from high to low flame - there is a very short loss of vacuum to the pressure switch (manometer went down from about 2 to 0 and then flicked straight back up again) and this is what kicks the pressure switch out.
I guess the way to proceed would be:
* Slightly stronger/quicker fan motor
* Less sensitive pressure switch (not a fan of this idea as I'm sure it's at a certain rating for a reason)
* some kind of 'smoothing device' between the venturi and the pressure switch (is there an equivalent of a capacitor for air?!?!?!) to stop the sudden fluctuation in pressure kicking out the switch
* Give up (no chance!)
On a 'why not, let's try anything, it's worth a shot' whim, I connected a length of tubing about 50cm to the positive pressure hole on the pressure switch and ran the boiler again. It now runs perfectly..... I have no idea why. Some theory like.... when the burner downsizes and the pressure switch loses pressure, it has to pull the air through the dangling length of tube in order to switch 'out', but by the time it's pulled the air through the pipe, the vacuum pressure is back up on the negative pressure hole.
I have a headache.
Suggestions on a £5 note. I'm not going to jinx anything, it works beautifully! Time for a corgi inspection and then to get the little git insured so I never have to do all of this again!
You lot are great. I'm going to see if I can offer anyone any help on the computer section of this site.
Cheers!
Stu
stu, not read though everything on the thread but have you tried taking out cleaning the venturi, also check for moisture in the aps tubes or small crakcs or splits !
this may have already been mentioned or checked though ?