Vaillant boiler problems

[harrysmum]

Hi Can anybody help me.
I have a vaillant boiler ( VCW something ) and it is not working.
The problem began with the heating working OK, although quite hot, but no hot water unless the heating was switched on.
I called an engineer who sent his lad around. He spent 1.5 hours looking at the boiler, said he couldn't find the fault and left. After he left we had no hot water or heating!!
The next day the engineer himself arrived and tried to diagnose the problem and get the boiler working again. He said he thought it could be one of 2 things but wasn't sure which and needed to phone Vaillant to check. This all sounded very reasonable but since he left when the boiler is switched on and the hot water is running the radiators are cold and the pressure on the boiler low. When the water is not running the radiators heat up but the pressure on the boiler rapidly increases.
Can anybody diagnose the problem from this description. Shall I wait for the engineer to come up with his diagnosis or phone someone else?
Would really appreciate any help.
Thanks

 

[kevplumb]

when the boiler is switched on and the hot water is running the radiators are cold and the pressure on the boiler low.

normal

When the water is not running the radiators heat up but the pressure on the boiler rapidly increases.

from what to what over how long

 

[harrysmum]

The radiators normally heat up while the water is running.
The pressure goes from less than 1 to 2.5 in about 15 minutes.

 

[corgiman]

thats not too bad, is there a leak from a pipe outside when the pressure goes up?

 

[kevplumb]

rads shouldnt heat while water is running diverter not closing properly

if the pressure is rising that much the expansion vessel may need recharging

 

[harrysmum]

There was a leak from a pipe the one time we let the pressure increase past 2.5. Switch the bolier off before it gets there now.

 

[kevplumb]

that will be the pressure relief valve opening

 

[corgiman]

what do you reckon kev expansion vessel??

 

[harrysmum]

So,
Could be the diverter or expansion vessel?
Anyway to check which and is it an easy job?
Thanks
For all your help.

 

[kevplumb]

its looking promising it's the ex vessel

might only need re charging

 

[harrysmum]

Thanks,
It sounds like the engineer I called may be arring at a similar answer slowly!!
I pressume it is not a good idea to run the boiler until the excess pressure is released too often?

 

[keego]

Drain a radiator off so it's half full of air (preferably the one nearest the boiler), that will act as an expansion vessel, if the pressure stays within the normal operating range then that will confirm it's the PV.

 

[harveywarbanger]

Sounds like the diverter valve. I had a problem with a valliant whereby my heating was working but I was getting no hot water when turned on. This was the DWH Microswitch.

Hope this helps.

Harvey

 

[harrysmum]

Thanks Keego and Harvey.
I've bled the radiator closest to the boiler and the pressure seems to be staying around 1.2.
The pressure problem only occured after the engineer had been so maybe the original problem i.e no hot water unless bolier on was the diverter valve.
I'm not really into DIY but have been fascinated by excellent help provided here