Vokera 20/80 RS Turbo - hot, cold, hot, do the dance with me

[mmalone]

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 ?

 

[stubertio]

If I read you right, you've introduced a resistance in the aps tube...? That's standard on some boilers.
Bit like the switch is a switching voltage source, but the system has capacitance. By introducing a resistance you've lengthened the time constant the gas valve sees.

not really. The original tube from the venturi to the vacuum pressure hole is still there and is not modified in any way - I've just added a length of tube to the positive pressure hole. The other end of the tube doesn't go anywhere, just dangles down the side of the boiler. Weird. I should have paid more attention in my physics class.

 

[stubertio]

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 ?

Yes, some clever people pointed to the venturi at the beginning of the thread. I took it apart and it appears to be clean. At least, I can blow through it and there is no resistance. Is there any other way I should try cleaning it, or is it simply a case of it being ok if I can blow through it?
The rubber/plastic tube has been replaced, just in case pressure was being lost due to a minute crack or split etc. Thanks for those suggestions though!

 

[mmalone]

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 ?

Yes, some clever people pointed to the venturi at the beginning of the thread. I took it apart and it appears to be clean. At least, I can blow through it and there is no resistance. Is there any other way I should try cleaning it, or is it simply a case of it being ok if I can blow through it?
The rubber/plastic tube has been replaced, just in case pressure was being lost due to a minute crack or split etc. Thanks for those suggestions though!

normally ok if you can blow through them, just make sure there no debri stuck inside it, i normally clean with a small screwdriver through every part of it...

sounds a very strange fault if not the venturi, not really worked much on these boilers though

 

[stubertio]

yeah, baffling indeed! I did notice that the fan was last replaced in '99 (it's written in pencil on the motor/fan). Perhaps it's on its way out, or perhaps it wasn't quite right/the correct one in the first place? So many possibilities. Just tried everywhere to get insurance for it, but as it's over 15 years old nobody will touch it Looks like this forum may hear more from me when it breaks down next! lol (touches every piece of wood in sight)

 

[DP]

I can only suggest that you do NOT modify anything on the boiler. If the boiler does not work, then there is a fault present. To mess with air pressure switch may well result in gas valve opening when in fact the fan is not runing.

This boiler is not prone to defect like what you have posted.

I would start with cleaning the main heat exchanger matrix, fan inpellers, venturi and check the flue components.

To recap, do not experiment. You could bring the house down.

 

[ChrisR]

I've just added a length of tube to the positive pressure hole

That would damp the response of the aps a bit, which makes sense. WHy it should need it I'm not sure. I've seen other aps's flicker though.

 

[DP]

The venturi is inside the flue elbow. Being able to blow through it is not enough, it needs to be examined and bore cleaned (I use a long woodscrew that removes the deposits without scratching the surface). The pressure switch air tube connection- only one spigot (upper me thinks) is connected silicon tubed to venturi. Other spigot samples room pressure.

The fan impellars must be clean otherwise reduced airflow is likely to effect APS operation. (best way to do this is wash the rodent cage, NOT THE MOTOR, under fast running water). Similarly, reduced fan speed can also have detrimental effect on boiler operation.

 

[mmalone]

The venturi is inside the flue elbow. Being able to blow through it is not enough, it needs to be examined and bore cleaned (I use a long woodscrew that removes the deposits without scratching the surface). The pressure switch air tube connection- only one spigot (upper me thinks) is connected silicon tubed to venturi. Other spigot samples room pressure.

The fan impellars must be clean otherwise reduced airflow is likely to effect APS operation. (best way to do this is wash the rodent cage, NOT THE MOTOR, under fast running water). Similarly, reduced fan speed can also have detrimental effect on boiler operation.

as DP says, i'd try this before anything else especially the venturi, take it out and clean thouroughly, blowing is not enough...