Vaillant VCW GB 221 H Water valve question

[mdskids]

We still have one of these boilers, which is still going, despite now being about 35 years old. It can however be a bit temperamental, and since recently changing a leaking servo valve, I've noticed the CH doesn't always fire up after using hot water. I've tried adjusting the push rod that activates DHW, but the problem seems to be that the rocker/push rod is not returning all the way down when DHW stops. I can easily push it down manually with a finger, and then CH fires up. I don't think the servo valve/rocker is sticking, since its new, so I assume the issue is in the water valve. I did recently check and clean the water valve when I replaced the diaphragm and there didn't seem to be any issues with the push rod - it was moving in & out freely. However - there's another sprung rod inside the water valve (below the diaphragm) which is not moving as freely or smoothly. According to the parts diagram, this lower rod is the 'water quantity regulator' At one point it looks like it was possible to remove this regulator by unscrewing it using long nose pliers, but it's basically seized in now so I can't take it apart to clean it up. Is it possible this regulator/lower rod is sticking, and stopping the upper push rod from lowering properly when the pressure changes inside the water valve? On a related note, I seem to go through diaphragms fairly quickly, and its always due to a small hole where its pushing against the end of this lower rod - which gives me more reason to suspect its sticking and giving more resistance than it should.

So the question is - should I just get a new complete water valve, or could it be something else? (I see in the installation manual for this boiler that I can also adjust the microswitch position using a couple of screws - worth trying?). If I do get another water valve, I think its part no 0011299 (which no longer has a point to connect the pressure vessel) - is that right?

Many thanks for your help - I'm determined to get a few more years out of this boiler, until its no longer possible to buy spares.

 

[cross thread]

You can buy a full service kit ,personally if a complete new one is available I would replace the full valve.

 

[Gasguru]

The following (in no particular order) will sort most issues....


Most of the water sections I came across had the temperature and flow regulators but they were often seized and made little if any difference to the basic operation of the pin. However, unscrew the venturi and clean off any scale (this device "amplifies" the pressure differential across the diaphragm).

The servo valve applies the differential pressure developed across the pump to the diaphragm within the diverter. The servo valve just reversed that pressure depending on a DHW or HTG demand. Anything that affects that pressure should be checked.

Check the expansion vessel air pressure (the schrader valve is either top left or at the base...a release of a 1/2 cup of water is normal), set o say 0.8 bar.

Check the auto air vent is fully working (it's a standard 1/2 inch Caleffi). If you need to change it disconnect the pipe on the pump top and remove pipe & aav.

Check the pump torque...insert a 4mm flat blade screwdriver through the bleed plug into the impeller shaft and run the pump, it should take an effort to stall it.

Check the adjustment pin (under the servo valve) is free to move in the bush on the metal plate above the water section. The bushing diameter can reduce over the years...open it out with fine emery wrapped around a drill bit. Then apply silicon grease to the pin. Adjust the pin to servo valve clearance as per the manual. For access pull the control box forward and be very careful of the plastic elbows on the air pump behind...they can split (and the gas valve will no longer modulate)....assuming you have the metal plate DHW heat exchanger.

Check the stuffing box (the gland seal) on the water section is not leaking or scaled. The pin should easily move in/out.

Clean out all the servo control pipes (use cycle brake bowden cables). Check the banjo bolt connection on the diverter (sometimes they have a restrictor - if so pull out the bits, see the image).

There's probably a few others but I've long since forgotten.

Untitled

• Gasguru
• 16 Feb 2013
• 2