Overflow from expansion tank overflowing

24 Jul 2010
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United Kingdom
Help ! The overflow pipe that runs from the expansion tank in my loft to outside the house is constantly overflowing (2 bucket fulls a day).

The float has been replaced twice in the expansion tank AND had the emersion water cylinder replaced as my plumber initially thought the coil may be broken in it.

However, the overflow is still dripping constantly even when the heating, hot water and emersion tank are switched off.

I have emptied a bucket full of water out of the expansion tank but within a couple of hours the water level had risen above the overflow outlet and the dripping started again.

Any ideas ? My plumber is flummoxed !
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Check everything really carefully - including your plumber!

Ball taps can leak very slowly. Is the overflow combined with another?

How high is the static water in the cold feed tank with the heating system stone cold? The level should be low, to allow for expansion when hot - that's why it's called a feed and expansion tank.

Your system could be sucking air in [down the vent pipe] when the pump is running, but if so, you'd need to be venting radiators all the time.
Which tank is overflowing? The large one or the small one?

Can you give us a bit more detail about your system/boiler? Also where are the tanks in relation to each other especially height wise.

If it's doing it even with the heating/hot water/immersion heater OFF, And the ball valve has been changed, AND the cylinder has been changed....

I'd be trying it with with your cold mains stop-cock off. It could be a mixer tap somewhere in the house sending water back up the hot pipe.
I'd be trying it with with your cold mains stop-cock off. It could be a mixer tap somewhere in the house sending water back up the hot pipe

Or perhaps a mixer shower
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There is just one overflow pipe coming from the small expansion tank.

Yesterday I emptied water out of the tank so the water level was approx 1.5 inches below the overflow. I had no heating or hot water on and didn't have the emersion on. Within a couple of hours the water level have risen sufficiently to start overflowing again.

I have an old Wickes wall mounted boiler which is probably about 20 years old. The pump is next to it. My plumber says that the pump should actually have been put next to the emersion tank upstairs. Boiler was there when I bought the house.

Your help is much appreciated :)
I dont think we can come to any conclusion without visiting you.

Your plumber made a guess and replaced a cylinder for which we charge about £460 but that has not solved the problem.

He clearly does not have a clue?

Shoule he have charged for replacing something that was not at fault ???

Trouble is, with these symptoms, when you have exhausted all the cheap, smple possibilities, its the cylinder that's in the frame. I'm sure the OP's plumber isn't the first to fit a cylinder that doesn't solve the problem.

A very difficult situation for the OP and their plumber.
If I was faced with that situation all I would need to do is to drain the heating system and disconnect the heating coil connections and see if HW ccomes out !

Isn't that a very simple test that would take no more than 45 minutes?

For anyone to change a cylinder at about £460 without doing that simple test they are either very thick or a cowboy.
I'm a bit puzzled. If I haven't had any heating or hot water on why would this overflow problem suddenly happen ?

Not sure if it matters but the water coming out of the overflow is cold (and always has been even when i had the hot water on).

Since the water cylinder has been replaced the problem has actually gotten worse.

My plumber, (who came on recommendation), is calling me tomorrow to try and figure out the next course of action. Do you think he will take offence if I say about draining the heating system ?
The old cylinder was apparently not faulty and the new one is not expected to be either so that test is now irrelevant.

Its obviously a simple cause but without seeing your system I obviously cannot say what it is. There is no point in listing 20 possible guesses !


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