Without any apparent leakage from the system I'm having to re-pressurise my system to 1.5 bar after every use otherwise it will not operate. After switching off it takes around 1 hour for all the pressure to go and the warning light come on.
I spoke to Vokera Helpdesk, they think the expansion vessel may need re-pressuring/ replacing and maybe the pressure release valve too.
Heating Engineer calls today, listens to the boiler but does not even take the cover off and says I need a new one (it's less than 7 years old).
As he did not have Clark Kent written on his van(x ray vision), would I be wise to get a second opinion?
Email: Kbarclay@lineone.net

Keith Barclay, January 2008

Great stuff this was brill info ,,,, ch working great now

jb, March 2010

Eventually got to the bottom of this so thought I'd do an update.
Boiler started cycling not on just hot water but on heating too so had the main circuit board replaced( this is the second time one has gone). The combi specialist also replaced the heat exchanger ( this was well bunged up). Surprisingly he did not replace the Pressure Release valve which started to leak quite badly, especially after he emptied the system through it to do the Heat Exchanger. However bought this myself, (a word of warning whilst it is possible just to change the internal springs and seals, if it has been leaking some time the surfaces can be quite corroded) so I replaced the whole thing, details how to are in the manual. Result all is working as it should, system holds pressure so now putting in cleanser for a few weeks then draining and then leak suppressant/additive as all the topping up adds air to the system which aids corrosion and helps sludge form.
The expansion vessel is fine, does not leak, so all in all a result although it would almost have been cheaper to buy a new boiler, the combi specialist charged £350 but when your heating goes on Christmas Eve...!
Thanks for all the suggestions and advice

Keith Barclay, January 2009

Hi Mate I have a vokera combi boiler and this happened to me the fitter I called out said common problem thier is a small flexible pipe in the boiler which has a bend in it and after a couple of years use the muck from the radiators and pipes builds up in the bend and the pressure drops . He took the flexhose off at one end cleaned out the very mucky sludge built up and put the pipe back on . everything ok that was 2 years ago still fine . He said common problem hope this helps

John, February 2008

Hi Mate I have a vokera combi boiler and this happened to me the fitter I called out said common problem thier is a small flexible pipe in the boiler which has a bend in it and after a couple of years use the muck from the radiators and pipes builds up in the bend and the pressure drops . He took the flexhose off at one end cleaned out the very mucky sludge built up and put the pipe back on . everything ok that was 2 years ago still fine . He said common problem hope this helps

John, February 2008

Thanks for your replies, I'll try and get a Vokera expert in this time!

Keith Barclay, February 2008

Yes, the external overflow dripping is not a good thing, as the Vokera man said, could be you need a new pressure relief valve (you cant change the washers AFAIK). As an alternative you could try squirting some Fernox leak sealing concentrate into a radiator bleed point, no guarantee it will work though.

The temperature cycling thing is more likely due to the air pressure switch cutting out, if it is then the flue venturi needs cleaning. Try googling 'vokera flue venturi', job for a gas man because of new safety legislation. Another less likely possible cause is scale/muck in the DHW heat exchanger causing the primary water to overheat & overheat stat to cut out.

If your system pressure does not shoot up when the CH heats up then the expansion vessel is fine, it is just there as a cushion so that the expansion of the hot CH water has somewhere to go (other than through the pressure relief valve when it gets to 3 bar). You would not be able to get 12psi into the expansion vessel with a bleed point open if the vessel was broken as the air would just leak out into the CH water circuit.

It sounds as if you boiler has not been properly serviced for a while, they do need servicing every year or two by someone familiar with the make and model, you could ask Vokera for a local man.

c, February 2008

Thanks C, I did what you suggested.
When I pressed the expansion vessel valve a little air came out ( which I believe is a good thing) and I re-pressured it as recommended with a pump and then added more water too. I have not had to top up with water to the system since via the water valve but I notice my external drain on the system is still dripping slightly which I guess is a bad thing. Also the hot water runs for a few minutes , then goes cold for 10 seconds, then runs hot again , then goes cold. Do you think a new expansion vessel is the answer as it is not easy to get to?

Keith, February 2008

Yes, the Vokera help line man's suggestion is reasonable. If the pressure goes up to 3 bar when the CH heats up then your expansion vessel probably needs recharging.

With no pressure registering on the front you need around 12 PSI air pressure in the expansion vessel.

Get a car tyre pump and gauge, and locate a car tyre type valve on the top of the large red pressure vessel (the expansion vessel) behind your boiler. First push the central valve pin down , a little moisture/air may come out but not a jet of water. If water squirts out the vessel needs replacing.

Next let the pressure off on the radiator side. The least messy way to do this is to lift the overpressure valve on your boiler. You will need to take off the outer casing to do this. Alternatively open a radiator bleed point somewhere where you dont mind 6 litres of water squirting out over the floor.

Next connect the pump and pump for longer than you expected whilst you first displace 6 or so litres of water, and then build up 12 psi of pressure. Check it with the tyre pressure gauge.

Close the bleed point. Let water in via the filling loop till you get to 1 bar on the front panel pressure gauge. Replace valve cover on expansion vessel valve. Job done.

No more need for frequent top ups. The water was probably escaping from your system via the pressure relief valve & through the vent pipe when the CH was hot. If allowed to do this for some time it can cause a permanent drip from the PRV necessitating replacement of the valve.

There's a uk-diy FAQ on this, and other descriptions on the web if you google.

I wouldn't trouble that Gas man again with boiler problems, find another.

C, January 2008