Hot-water tank overflow outlet query

may want to read some of the answers given here

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http://www.pistonheads.com/gassing/topic.asp?t=997849

Could it be that because the temperature was so low last night it created more steam   and condensation around the pipe making it drip, and the reason  you have no  leak in the daytime is because the outside temperature isn't so cold, also you're using water in the daytime.

I think, although I am quite as likely to be wrong, that the overflow kicks in when the tank is overfilling Had the same thing with my boiler a while back, don't know though about immersion heaters. Mine turned out to be faulty switch which was constantly feeding water into boiler, albeit v. slowly.

Is it a cylinder tank immersion heater fed by a header tank?
Because it'll be the feeder tank that will be overflowing, the ball-valve may need a new a washer to stop the water reaching a level in the tank.
Or it maybe that the washer wasn't working (to seal the inlet of water) under a certain temperature and is working again now it's warmer.

Many thanks for answers.

Condensation alone unlikely to be the cause because the overflow pipe is cold and empty unless there's an overflow of hot water.

I'm not 100% sure what the arrangement is here, as the tank is encased in a solid insulating jacket. It's a very old house, in which there appears to be no separate cold water tank, so it's possible there's a feeder tank above it hidden under the insulation. Bobtoe's theory about the inlet seal not working because of the cold is possible, as I wasn't expecting such low temps last night and didn't switch on the storage heater in the bedroom where the tank is situated. I just relied on the woodburner downstairs and ordered the dog onto the bed when I started feeling cold.

Whatever, if it happens again, then I'll assume that, as Quvega and bobtoe surmise, there's some fault with the feed-in and will get that fixed. Many thanks for offering plausible theories which don't imply the tank is overheating dangerously.

May just need a minor adjustment to the nut on the thread on the orange ball arm so the water shuts off at a slightly lower level in the tank.
Almost certainly related to the unusual outdoor temperature and pressure. Could also run the taps a little extra at bedtime to lower the level, but not so much as to cause refilling.

I've a similar system to you ie. no cold water tank.You'll have a header tank for your hot water tank.Change the washer in the header tank.As A quick fix bend the arm on the ball valve down slightly(if it's a metal rod) so as to put more pressure on the washer.If it's a plastic arm,which I doubt if it's an old system then do as a previous poster has said & put more pressure on the washer that way.As I've said these are only quick fixes until you get time to change the washer.

Thanks for the advice, Ifor and allpoints. But having investigated I can't see any evidence of a feeder tank. The hot water cylinder seems to take its feed directly from the mains supply and is heated by the element within the tank. So there isn't any need for a valve or ball. Just an outlet near the top to allow hot water to escape in case the thermostat fails and the heated water expands dangerously.

Which brings me back to wondering what caused last night's apparent overflow and why it wasn't repeated when I turned it on again this afternoon.

Oh well. As long as it continues not broke, I won't try fixing it.

"She cannae take much more Captain she's gonna blow!"