Expansion Tank overflowing

[davidstanton]

Advice please...

I have an old (about 30 years) Glow-worm balanced flue gas boiler. It provides hot water to a tank upstairs and pumped heating downstairs and gravity feed upstairs.

There is a stat on the hot water tank and a zone valve.

Recently the hot water tank has, every now and then, started making noises similar, I imagine, to a volcanic eruption, with hot water being shot up to the expansion tank in the loft, raising the level so fast as to shoot water out of the expansion tank overflow.

Recent work undertaken includes a new pump in August 2004 and a new thermocouple in February 2005.

Any help as to why this is happening would be appreciated.

 

[kevplumb]

it's boiling

 

[davidstanton]

Thank you but....

Why is it boiling?

The boiler stat seems to be working ok in that it cuts in and out and never seems to be in a position where it doesn't cut out.

Any thoughts are appreciated.

Thank you

 

[felix]

Do these eruptions coincide with the zone valve opening?

 

[davidstanton]

The zone valve has been both open and closed during the "eruptions", although I have lowered the temperature on the strap-on stat on the cylinder.

 

[doitall]

Nothing to do with the stat, or the cylinder, it's the boiler circulation or lack off it.

Is this problem in the hot water mode only

Next you have to look at the gravity pipes for scale at the boiler end and cylinder connections, air will also cause the same symptoms

 

[davidstanton]

I have no idea if it's in hot water only mode, as the problem is recent when I've had the heating on all the time.

Thanks for the advice, I will check the pipes for scale build-up.

 

[davidstanton]

I've read on a couple of other threads that if the pump is fitted in the wrong direction a similar problem can arise.

How do I know which direction it should be going? The arrow on the pump is pointing FROM an external pipe TOWARDS to boiler inlet.

Thanks

 

[kevplumb]

the "boiler inlet " is at the top or bottom

 

[davidstanton]

The boiler inlet is at the bottom.

???

Thank you

 

[ChrisR]

If the water KEEPS coming out of the overflow (to outside) then the level in the f&e tank is too high, it will be taking in new water to replace what's lost. This is a Bad Thing - lower the level by setting the ball lower.

Scale in the boiler causes local boiling, erratic flow through the boiler causes water to get above the average temperature which the boiler stat sees because it's slow to respond. Does lowering the boiler stat temp reduce eruptions?

Suggest you pour a bottle of Fernox heavy Duty Restorer in the f&e tank and leave it a week or so then drain and refill system.

It would be better if you could do some flushing by connecting the mains to suitable places, with routes out organised, to clear sections of system pipework/rads. For that you'd have to understand all the pipework and do a bit of cutting and capping of the pipes.

 

[davidstanton]

The problem is not continuous, and has only happened 2 or 3 times in the last couple of months. But I do know the boiler is scaled up, given its age, as it bangs, gurgles and kettles all day!

I have used Fernox decalers numerous times to the point where the water comes out clean, but the boiler still kettles. I have inhibitors in the boiler but after 30 years I can't really complain!