loss of pressure on central heating

[mrminto]

I have an old vaillant bolier (combi about 12 years old) and we keep loosing pressure when the central heating is on, it seems fine when the Central heating is off.

We had a engineer to have a look at it a couple of times. He enjected the system with a leak sealer but after 4 days we still have the same issue.

It is really puzzling as we do not see any sign of water on the ceilings.
and there is no water coming from the safety valve on the outside wall.

When cold the pressure is 1.2 when hot 1.8. it is constant when hot but when it cools down it loses 0.4 each time.

any suggestions?

 

[chrishutt]

Get a competent heating engineer in. He will know what to look for and how to determine if the problem is in the boiler or not.

 

[mrminto]

we using british gas engineers, we have homecare plan. But they don't seem to bother about finding out what it is, just carry out small repairs

 

[tryitandsee]

Put a glass jar under the safety valve discharge pipe, you will then see if the valve has activated or not. I suspect it will have!

 

[mrminto]

i noticed that the higher the temperature you run the more it loses pressure, any other suggestions?

 

[mark/uk]

Isn't it normal for the pressure to rise when the water gets hot then drops when it's cooled down again, water expanding etc.

 

[STEPH-58]

Could be a leak on the heat exchanger, I had to change one about 3 months ago for the same reason, ie no visible leaks on the rads or pipework. Just an idea mind you.

steph