Vaillant Sine 18 - not quite right.

[Mike1501]

Happy New Year to all.

I need advice re: my Vaillant Sine 18.

Hopefully there are still some of the old Bunny brigade reading this forum.

Yes, the boiler is donkeys’ years old but it has served me well and I appreciate its superb engineering. Having said that, I will be replacing it in the summer, simply because it is difficult to get it serviced and spares are becoming more rare than hens’ teeth.

It was last properly serviced three years ago, although by a young man who I suspect had never seen one before. Initially no problems but then it started to develop a problem with the DHW, in that it would not get very hot and was simultaneously warming up the heating circuit. Referring to this forum, I concluded that the wax-stat in the diverter needed replacing. Luckily, I had sourced a new wax-stat as a precaution a few years ago, before they completely disappeared. I did not replace it immediately for fear of causing leaks etc; and the boiler soldiered on. Then last month, intermittently it started to fail to fire-up when requested. Following some investigation, I found the differential pressure valve was failing so the gas valve was not opening. I also noticed that the boiler pressure was dropping more quickly than previously and then found that the AAV was leaking like a sieve.

I replaced the differential pressure valve, the wax-stat and the AAV. I evacuated and re-pressurised the expansion vessel to the recommended pressure and the boiler returned to what I initially thought was normal working: central heating fine, DHW better than ever. But then I noticed that in central heating mode, the boiler was constantly firing in 15 second bursts and then going out completely rather than modulating (if that is the correct term) as I am sure it used to. Checking the solenoid terminals showed a constant 14.9v whether firing or not (which is surely not right?). Switching the DHW preheat switch on or off makes no difference. Detaching the NTC wire makes no difference. Adjusting the thermostat setting makes no difference. I should mention that my system has no room stat and all radiators have TRVs.

I suppose the problem could be the electronic PCB, although that was replaced about six years ago. Could it be the thermostat? Does the new wax-stat mean that the micro-switches on top of the diverter need resetting? i.e. could it be operating the central heating as a sequence of DHW preheat cycles? This is all guesswork on my part but I am hoping that someone can help me keep the old girl running properly for the rest of the winter until she is pensioned off in the summer.

Thanks in advance.
Mike

 

[The831Bunny]

Could be the Flow Switch (diverter valve) microswitches requiring adjustment. The calibration is quite straight forward. With the hot tap RUNNING (and the electric to the boiler OFF), adjust the 7mm nut on top of the switch cage so that the RIGHT hand microswitch metal switchplate is about 0.5mm away from the top of the microswitch red body. This should reset the distances correctly.

Run the heating, set the thermostat white knob to 8 and watch the spindle and switch cage rise as the boiler warms up. With a new wax capsule the LEFT hand switch plate should operate the left microswitch when the flow temperature (check temperature gauge) reaches approx 60C. If the spindle doesn't rise up enough to make the left hand microswitch check you don't have any hot taps dripping, if all OK then the wax capsule is duff.

The numbers 1-8 on the white knob relate to 60C-90C flow temperature. The boiler will only modulate when the flow temperature is above 60C. Let the boiler heat to 65C-70C, turn down the white knob slowly down from 8 and the flame should decrease around 3 and go out at 1. Note: The boiler will not fire again until the left switch plate drops off of the microswitch, or HW is used.

Do these tests and report back. Under NO circumstances remove the inner combustion chamber, or touch the gas valve. The gas control solenoid, gas valve, FFD, PD valve and Electronic PCBs require you to be Gas Safe registered to test, adjust and prove safe operation. Do not touch them unless you have the relevant qualifications.

 

[Mike1501]

Thanks to “The831Bunny” for the expert advice.

Report back:-

With the hot tap RUNNING and the electric to the boiler OFF, I checked the clearance on the RIGHT hand microswitch (above the Flow Switch). There was no clearance - the switchplate was pressing down hard against the microswitch body. I adjusted the 7mm nut to achieve a clearance of ≈ 0.5mm between the switchplate and the microswitch BODY.

Starting from cold, I then ran the heating with the thermostat knob set to 8. Over several minutes the spindle rose and made hard against the LEFT microswitch at about 60C according to the gauge. When the gauge read ≈ 70C, I slowly turned down the thermostat knob and the flame did indeed decrease at ≈ 4 and extinguished at 1.

So at that point the boiler appeared to be performing to spec however, later in the day (6pm - ambient temp lower), when the boiler came back on, I noticed that (with the thermostat still set at 1) the switchplate was dropping off the LEFT hand microswitch quite quickly, causing the boiler to fire, and then raising and remaking the microswitch after about 15 seconds, causing the boiler to extinguish. It continued to cycle in this way: 15 seconds firing, 30 seconds off, 15 seconds firing and so on; in other words, on and off like a yo-yo. The temp gauge never exceeded ≈ 60C. Turning the thermostat knob up to 8 made no difference at all as the temp gauge remained at ≈ 60C. I have monitored the boiler now for a few days and it has continued to behave in this way ever since, never exceeding ≈ 60C, irrespective of the setting on the thermostat knob.

In summary, at switch-on it burns continuously until it reaches ≈ 60C irrespective of the thermostat setting, then reverts to the 15 secs on- 30 secs off firing cycle mentioned above and never exceeds ≈ 60C.

So it seems that something is still amiss but at least I am thankful that I have some CH and decent DHW for a change.
How it will perform though if we have a really cold spell is anybody's guess.

 

[The831Bunny]

Ok, that’s an easy fix. Wax capsules are not an exact science, so it looks like yours is a bit sluggish and the flow temperature is rising above 60C before it contacts the left hand switch. So in doing so it’s turning off. Try bending up the left hand plate so it contacts earlier and see what happens.

 

[Mike1501]

Thanks for the reply.
I bent the plate up a bit but it has made no difference; still the 15 secs on - 30 secs off firing cycle and does not exceed ≈ 60C.


Read more: https://www.diynot.com/diy/threads/vaillant-sine-18-not-quite-right.515581/#ixzz5cEIbo8Z9

 

[The831Bunny]

OK, in that case it may be open circuit on that from Grey to Blue on the microswitch. Try spraying with contact cleaner with the power off and pilot out, or replace the microswitch with any standard changeover microswitch. Check the normally closed/normally open routing through the switch and make a note of the colours when you take the wires off. Unmade or rest in DHW, means the made circuit is Grey (top spade/common) to Pink (middle spade of the three).

Here you go, the page you need from my Germany training folder. Useless info for nowadays, apart from who needs it! The left hand switch is known as S2. The text near the bottom pins the fault down for you.

 

[Mike1501]

Much appreciated. That page is gold dust. I'll clean and check the switch tomorrow.
Many thanks.
Mike

 

[Mike1501]

I checked the continuity of the microswitch S2 and it seemed ok however, to be certain, I replaced it with a spare that was definitely ok. But still no change to the boiler's behaviour. It still fires in bursts, does not exceed 60C and the thermostat knob has no effect whatsoever. It surely can't be a break in the wiring loom between the S2 blue and the CH pot (or the CH pot kaput) can it, because when I first reset the switchplate clearance the boiler reacted to the thermostat correctly but not since.
Puzzled.
Any further advice welcomed.
Mike

 

[The831Bunny]

Next would be the multiplug on the front of the flowswitch, the pins can be a poor contact, and the edge connectors on the PCBs. You’re getting there lol! I’ve also known the microswitch on the DC solinoid to cause strange faults too

 

[Mike1501]

Thanks for that.

I have checked continuity from the terminals on S1 and S2 (and earth) through to the back of the multiplug with the aid of a paperclip poked into the back of each cell of the multiplug and all are good. I have also checked continuity and function of the solenoid microswitch (S3?) and that is also good. With regard to checking the edge connectors on the PCBs, am I right in assuming that I need to check microswitch S2 terminals through to E5, E9, E10 on the electronic PCB and from microswitch S1 through to A3, A7 on the main switch board?

Mike

 

[Mike1501]

I have a further question concerning my Vaillant VCW Sine 18.
Should the differential pressure valve diaphragm pin remain energised (i.e. pushing open the gas valve) all the time that the pump is running?
I assume that it should but would appreciate confirmation.
Thanks
Mike

 

[The831Bunny]

Yes, all the time the pump runs, the PD valve should be operating the gas valve. If not suspect pump capacitor is failing influencing pump speed, the Venturi (above the pump where small tubes connect) is dirty, PD valve tubes are partially blocked, PD valve spindle sticking, or main heat exchanger partially scaled. Basically anything influencing the PD valve operation.

However, this area concerns the gas valve operation so a Gas Safe registered engineer is required .

 

[Mike1501]

Thanks to The831Bunny for the useful guidance.

Your first suggestion seems to be favourite: the pump capacitor (or maybe the pump itself?).
I think the other suggestions are less likely because the PD valve is brand new, the heat exchanger was replaced about six years ago and the venturi setup cleaned and checked at that time. We have no scaling issues here being in a very soft water area.

The capacitor on my pump seems to be sealed inside a black plastic canister which clips into the base of the wiring box on the pump and is held in place by the cover. I can't seem to find a direct replacement for this so could it be replaced with a freestanding generic capacitor of 2.5 µF, suitably insulated?

 

[The831Bunny]

Any 2.5μf capacitor that handles mains voltage is fine. Some multimeters can measure capacitance. If not it’s certainly worth snipping the wires to the original and trying a new one. You may have to break the plastic holder to get to the capacitor. Easy to cut the wires close to the capacitor and join another on.

There is a low hydraulic resistance around the DHW heat exchanger, so as soon as the wax capsule opens the heating circuit the pump sounds like it hasn’t got enough head to keep the PD valve open.

 

[Mike1501]

The 831Bunny, you were spot-on .
It was due to the capacitor.
You certainly know your way around these Vaillant boilers.
Hopefully this thread will be of use to others.
Many, many thanks for all your help and advice.

Mike