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

[stubertio]

Just a quick update:

My father and I replaced the thermostat this afternoon. I was hoping for a miracle, but unfortunately it's still much the same. Only a shower in the morning will tell for sure . One thing I did notice with preliminary testing is that the hot water no longer comes out hot enough to burn. It still runs cold every few minutes, however.

One question: Will I need to bleed and re-pressurise the system for the hot water to work satisfactorily, or is that just for the heating? We re-pressurised it by opening the 'filling point' stopcocks until the pressure meter reached 1.5 bar, but could we have introduced air into the system by removing the DHW thermostat and by draining the boiler (through the safety valve)? If so, would this affect the DHW?

Many thanks
Stu

 

[Dan Robinson]

Yes but it should vent out through the AAV or settle in rads once you run hte system for a while. Bleed and top up as needed until the sound of rushing air is gone and the boiler is operating quietly. - chances are the AAV is shagged anyway.

 

[ChrisR]

Flames should go hi-lo-hi-lo, where lo is full flames just shorter, ie not just the pilot. Do they ever seem to be low?

What's the water flow rate from the hot tap? Measure it in litres/minute. Limescale unlikely, but possible.

Temperature measuring can't be working properly, 60 degrees isn't exactly scalding. ??

Could be sludge/scale in primary side of flame>boiler-water or boiler-water>tap-water heat exchangers. Running a dose of desludger for a week and draining/flushing could help that.

Add to above suggestions...
Could be poor circulation , due to tired/blocked pump, or some other blockage.
Primary boiler thermostat could be faulty, turning off the flames prematurely.
Could be faulty microswitch leading to boiler looking at the wrong thermostat
Gas valve could be set wrongly, so it goes out on low setting,
or faulty.
Circuit board could be faulty - bad relay contacts or similar.
Could also be - something else, or , very likely , a combination of some of the above. It is 20+ years old or so after all.
Fascinating for someone who wants to find out about the problems of old boilers but not, to be honest, my idea of a nice day out.

As for

The temperature control system on this boiler is a little simple and needs some skill to set it up properly.

There's nothing obvious to set up. (Apart from the GV)

 

[DP]

Turn off the power so NO POWER GOES to boiler. Take 11mil spanner and loosen nut on top of white vessel located right of boiler behind the facia.

Run hot tap (boiler will not fire with power off) and draw water for 15 seconds (timed). You should have just under 2.5 liters. If quantity incorrect, post and will offer advice on how to set correct flow rate.

Repower boiler. Run hot water at a tricle with HW stat at max. While watching the burner (not the pilot flame) turn HW control to minimum. You should hear a click and flame should reduce in size. If flame goes out (I suspect this is happening post results.

In fact, email me on address in my profile.

 

[stubertio]

Turn off the power so NO POWER GOES to boiler. Take 11mil spanner and loosen nut on top of white vessel located right of boiler behind the facia.

done, some air rushed out and then some water, at which point I tightened the nut back up.

Run hot tap (boiler will not fire with power off) and draw water for 15 seconds (timed). You should have just under 2.5 liters. If quantity incorrect, post and will offer advice on how to set correct flow rate.

perfect 2.5 litres.

Repower boiler. Run hot water at a tricle with HW stat at max. While watching the burner (not the pilot flame) turn HW control to minimum. You should hear a click and flame should reduce in size. If flame goes out (I suspect this is happening post results.

With hot tap running on a 'medium' setting (a trickle wasn't sufficient to make the boiler fire), and with the stat on 9 (max), the burners were at high flame and the water output was hot. I then turned the DHW stat down slowly until it clicked and the burner went out completely (back to pilot). Then, after a couple of seconds it fired back up onto a low flame, but then after about 10 seconds it went out completely (back to pilot).

I tried the above a few times and every time yielded the same results.

Cheers!

Stu

 

[stubertio]

so, a month on and I'm a lot more clued up about this baby. I realised I was working to the wrong manual - I had the manual for the flomatic and not the RS Turbo. Once I rectified that, I was able to follow the flow charts and circuit diagrams a bit better!
I ended up doing some continuity testing and removing and testing most of the switches. Yesterday I removed the fan and flu pressure switch to check them both. I did this because I noticed that when the burner went out completely (instead of going to a low flame), there was also no voltage at the gas valve (no low voltage on the coil nor 240 volts at the operator). I worked out that the pressure switch is responsible for supplying the voltage to the gas valve, so surely it had to be something up with that.
I removed it and blew in and out of it while testing the switching with a multimeter. It seemed to be fine so I put the entire thing back together. Low and behold, things are 1000x better than before. The boiler now modulates to a low flame EVERY TIME it needs to, but there is a tiny cut out of a second or two in between high flame and low flame. Again, during this cut out, there is no voltage at the gas valve, so there is still something not quite right with the pressure switch, or at least the pressure switch sensing the vacuum.
What I find strange is that the burner cuts out IMMEDIATELY after the DHW stat has clicked in. It's as if the sudden reduction of flame strength causes the pressure switch to switch 'off' and then after a second the pressure builds back up and it switches 'on'. Could this be happening? I thought it was simply the fan operating at full speed that should cause the pressure switch to activate.

Thanks to all that have helped out so far! I have learnt so much over the past couple of months. We're so close!

Stu

 

[ChrisR]

Check the aps contacts go zero and infinite ohms, and not bounce. Only blow veery gently or you'll break it.
Really you need the high and low burner pressures measuring - corgi territory. I suppose you could measure the gas usage at the meter on high, then on low (by pulling mod coil wire off gv)
When the volts go completely from the gv main coil it could be the main thermostat doing it I expect, so its worth checking.

 

[stubertio]

Yep, solid contact, no bounce. Gas pressures have been checked and adjusted.
I just tried something else! I may have success! I got the boiler to fire (DHW) and then clamped the tube which runs from the flue elbow to the pressure switch (therefore keeping the pressure switch 'on'). I then adjusted the DHW thermostat and the bloody thing modulated like a beauty! No cut out or anything like that!
I then had a crazy thought. I released the clamp from the tube and then blocked off the other (positive pressure) hole on the pressure switch (one hole is for negative pressure and one hole is for positive — it is and should be connected to the negative, vacuum hole). I then tried adjusting the thermostat and it bloody worked perfectly!!!!!!!!!! I now have a boiler that's modulating away quite happily and there's no intermittent burner cut out.
Is this 'other' connection on the pressure switch supposed to be blocked off?
Surely all my problems couldn't have been caused by this little git being left open?!?!?!?! Could an end cap have fallen off at some point or something?


SOOOO CLOSE!

Cheers

Stu

 

[stubertio]

hmmmm, maybe it wasn't as simple as that! Blocking the positive pressure hole prevents the switch from switching in the first place, so the boiler fails to fire. It works fine after it's fired, but not before. Cap removed. Ho hum.

The thing that I want to do next is to measure the pressure that is being created at the venturi (think that's the correct term). As the pressure is so low, would a normal manometer do the trick? I want to see if the pressure drops when the burner switches to low flame and if so by how much. If it drops a bit but is still within the pressure range which is supposed to activate the switch, that will prove that the switch is at fault, no? Maybe I should just bite the bullet and order a switch for £40.
Sorry for my rambles.
Stu

 

[ChrisR]

Well you have the manual so you know as much as most of us (with known exceptions!)
It's easy to test an aps when you've got a digital manometer which reads to 0.001mbar!

The pressure the aps is supposed to switch by is probably printed on it. It's usually between about 0.25mbar and 2 mbar, Wide range. If you rig some plastic U tube, 1mbar is 10mm water, near enough.
Poke something like a wire/paper bag tie up the venturi tubes. Can make a huge difference. Check the fan vanes aren't dirty.

Is it a black plastic Dungs APS? Vokera 3112, obsolete

 

[stubertio]

just connected my manometer (non digital, old skool) and with the fan on full blast, it's sucking about 0.5 mbar. The switch is an LGW3A1, which, after some hunting, I found on ebay and a couple of suppliers via Google. It's spec sheet (on Dungs's website) states that it should switch at between 0.4 and 3 mbar. It doesn't state what the tolerance is, but it states that there is a tolerance. I shall whip the hood off this evening and have another look at the fan blades and entire assembly.
Thanks!
Stu

oooh, before I do that I'm going to poke some stuff in the venturi tubes. Now, where did I see that bag tie the other day.....

 

[Nixt]

When you consider its 22 years since the first and almost 19 years since the last 20/80RS Turbo rolled off the production line it might be worth saving up for a replacement. At the time the manufacturers claimed a working life of 10-15 years which means this one has given almost cast iron longevity from a copper HE'd combi. Good luck anyway.

 

[ChrisR]

0.5mbar doesn't sound much. Might be more with the case on.
That is the difference between the two levels?

0.4 to 3 is likely to be the range over which the aps will adjust, not what it's set to. COuld always call Vokera, though they may not know

 

[stubertio]

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

 

[ChrisR]

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.