Vailiant VCW-Sine 18 T3WFH Water Pressure

Hi, The I last reference to this boiler on here seems to be a couple of years ago so I hope I'm not going over old ground. I recently bought a house with this boiler fitted and although the boiler operates OK, I've noticed the water pressure rises from about 0.75 bar to 2.5 bar in about 4 or 5 minutes from a cold start. If left to continue it seems to auto release pressure around 3 bar but if then allowed to cool, the water pressure falls to below 0.5 bar. The system can be topped up and the whole thing repeated. Hot water is available without any increase in water pressure. The pressure increases only when the central heating is calling and all radiators warm up nicely. I had originally hoped to obtain a maintenance manual for this boiler and would still like to obtain one. However, with the wealth of knowledge available here it would be plain stupid not to ask for help with this. I have served a full engineering apprenticeship and consider myself a reasonably competent engineer.
Thanks for your help.

Check your boilers expansion vessel. This is covered in the FAQ's.
//www.diynot.com/forums/viewtopic.php?t=37170

It will hopefully be possible to repressurise it.

But contrary to what the FAQ say, start by pressing the valve pin in and let ALL the air out. Hopefully it will be air and not mostly water.

If it needs to be replaced there may be some complications!!!

Tony

Hi Guys,
Thanks for the links and the advice. I feel much more positive about this now as it's the 'not knowing' that bugs me. Even if it turns out to be a major problem, I need to know.
I have the front panel off the boiler and using an extension light I've tried to identify all the 'bits a pieces'. It seems as though there are two expansion vessels tucked away at the back of the boiler casing behind the gas boiler. I don't know if these are seperate vessels or are connected together to provide the required volume. If seperate, what is the job of the second one? I've found one valve on a small cylinder (smaller than a tennis ball) near the right hand side of the boiler, close to the casing, which seems to be connected to the 'domestic hot water' system rather than the central heating circuit. Is this the valve I'm looking for? My main difficulty is identifying where and how to get to the valve I'm after if this isn't it. Will it be connected directly to an expansion vessel? or is it the one I've already identified on the 'tennis ball'? There is no obvious way to remove the whole boiler casing. Will that be necessay in order to find the valve and re-pressurise? I could do with a diagram of the boiler layout unless you can tell me where the valve is?
I seem to be asking a lot here but until I find the valve, I'm stuck.
Thanks very much..........Alan

Its two vessels joined & I think the valve is on the top, if its not possible to re- pressurise just tee another exp vessel into the return pipework as close as poss to the boiler.

Alan

The two vessels are connected in parallel!

The air filling valve is about 300 mm up on the left hand side.

Do get all the water out ( if any ) and if very much you will need to keep the boiler pressurised as you do that to force it out.

Then with the system depressurised pump up with air to about 0.8 Bar or 9 psi ensuring as you pump you kep the system pressure at zero.

I actually have a used one but not worth the work required to fit them.

Good luck!

Tony

Thanks for the help guys. I now have a starting point and will get to it asap. Unfortunately the next few days are going to be spent with a gent from 'Chemfree' who is knocking plaster off all my internal ground floor walls to a height of a meter before injecting a new damp course. Rising damp throughout as well as a dicky boiler!
Once more, thank you for all your help. I'll let you know how I get on with the boiler, but it will be a few days now before I can get to it.

I quickly discovered how to remove the left side casing and the expansion vessel valve was easy to see. After pressing the valve, nothing happened, no water, no air. I pumped air into the vessel using a bicycle pump then noticed the water pressure gauge had increased slightly. There was also a slight hissing sound as the valve was leaking air. The valve plunger was tight and sticking down. I pumped more air into the expansion vessel then pulled the valve plunger out a little which appears to have stopped the valve leak. From a starting pressure of about 0.5 bar the water pressure gauge rose to display about 1.6 bar. I started the boiler and ran it for several minutes without any discernible increase in water pressure. After shutting the boiler down I released some water to bring the pressure down to just over 1 bar with the boiler cold. Starting the boiler again showed no increase in pressure as the temperature rose.

My rising damp should be corrected by Wednesday night so things are looking a little brighter now.

Thank you very much for all your help with this.

Best regards
Alan

Agile said:

Then with the system depressurised pump up with air to about 0.8 Bar or 9 psi ensuring as you pump you kep the system pressure at zero.

Good luck!

Tony

Click to expand...

Its part of the procedure to pump up the air with the system pressure at ZERO all the time!

This particularly boiler seems better run at a pressure of 1.2 Bar when cold. All others I recommend 1.5 Bar.

Tony

Hi, Yes, thanks for reminding me of that, I had forgotten that bit of advice. I haven't started running the boiler yet but will look at it again when the 'damp' guy has finished his work.

However, the water pressure has remained constant at 1 bar over night with the boiler shut down so it's looking good. I'll re-check everything using your advice before firing it up.
Thank you very much for your patience.
Alan