central heating overflowing

[sPoNiX]

My sister has just powered up her central heating for the first time and is having a problem.

Apparently, when it is first switched on the system there is a surge of water into "a tank in the loft" which promptly overflows down the overflow pipe. When the initial surge has subsided then the system works as normal.

From memory it is a conventional boiler with cylinder and expansion tank.

Any ideas why this sudden surge? Trapped air maybe?

Unfortunately I can't get to look at it for a few weeks.

sPoNiX

[Des Higgins]

There are lots of plumbing experts who read this newsgroup who can give a proper answer but I think this is normal or at least is what happens me in my house. I think it is just expansion from heating. In my case the surge is modest and I get drips every time it goes on.

[EricP]

The float valve is set too high. The system heats from cold and the expansion of water exceeds the tanks capacity to contain it, so it goes out the overflow.

[Des Higgins]

I just learned something. Ta!!

[logized]

It is not normal to have a regular overflow - the rust inhibiter will gradually become too diluted, causing system corrosion.

Dave

[sPONiX]

That was my first reaction but she seems to think it isn't...however she is of the female variety so who knows? Looks like I'll have to check!

sPoNiX

[Set Square]

Well, it fits the description. When it heats up, the water expands to above the overflow level - so it overflows. when it cools, it contracts to below the float level, so the ball valve lets more cold water in. Ad nauseam . . . and very corrosive!

Get her to tie the ball valve up for a day or two, and see whether it stops.

[EricP]

Only other thing I can think of is that she has a self-priming hot water tank. When they prime, there is a lot of air going up the expansion pipe and it takes all the water in front of it along. It is fairly obvious though, because it makes a terrible racket and sounds like the loft is being washed away.(Usually at 3am on a quiet night)