I have 2 overflow pipes at gutter height, one for the CH header tank and the
other for the hot water cylinder feed tank. The one to the HW drips on a
morning. I have been up and had a look at the tank, suspecting a new washer
etc would be required and the water level was a good 3" below the overflow
outlet pipe! I asked her indoors to run some hot water to drop the level
and then I waited for the tank to refill. It did and the flow stopped as
expected. There are no other tanks etc connected to this pipe so why does
it drip, my first thought was condensation in the pipe but why doesn't the
other one drip. Both tanks are side by side and the pipes are similar in
length and run the same way so why one and not the other?
What happens is that when you use the hot water during the daytime, the ball
valve shuts off and you think it is good. But it might need a few hours to
leak and reach the overflow. At night when you are asleep the valve has all
that time to leak and reach the overflow. Just change the valve and problem
I also had the same thought as you. So this morning before I went to work
knowing the house was going to be empty for 8 to 9 hours. I rigged up a
pice of wood across the top of the tank and hung a piece of toilet paper
over it so the ends were about 1/2" above the water level and also attached
a pencil to it so the end was just touching the surface of the water. When
I had a look when I got home, the toilet paper was bone dry and the pencil
was still just breaking the surface. I have changed the washer whilst up
there so I will wait till morning to see if it has helped but I am not
anticipating it will have stopped, I think there is something else that is
Is the CH feed and expansion tank mounted higher than the cold water
If so there is another possible failure mode: a small pinhole in the
heat exchanger coil in the hot water cylinder would allow water from the
central heating circuit to get into the domestic hot water. This would
have the effect of slowly raising the level in the cold water cistern
when no water has been drawn off for a period of time.
(the CH F & E tank should be installed such that it has a lower "head"
than the main cistern to eliminate this possibility)
They are both on the same platform in the loft but with a 3" to 4" gap
between them. Both bases are on the same level but as the CH tank is
smaller the top of it is obviously lower than the hot water feed tank.
I share a supply pipe with my neighbour and find strange things can happen
at night when they are home; for instance we can hear every time they have a
shower as there is a corresponding water hammer. I also know that the
water pressure is quite a bit higher during the night and this has caused me
to have a similar problem to you. The higher pressure could be affecting
one of the ball valves more than the other.
Stewart could well be right. At night when most people are asleep the
pressure will be higher allowing the leak to occure. Another thing you
should check is the expansion pipe from the hot water cylinder which should
be over the same cistern. This is the safety device for when the hot water
gets too hot and it then discharges. Check this over night, assuming you
leave your hot water on all night.
The expansion pipe was cold this morning and the water level was as it was
last night. I checked the levels etc. before anybody in the house ran any
hot water. After reading your post Ken I cable tied an old sock on the end
of the expansion pipe before going to bed and this morning it was bone dry,
where it would have been wet if the water was coming up this pipe. Many
thanks for all your advice/suggestions chaps but I really think it is
something other than the water level rising in the tank. It's not a
constant drip,drip,drip... like you would expect if it was a water level
rising problem but a drip every minute or so. The annoying part is that the
overflow pipes are directly above the back door, so if you get the timing
wrong you get a drip of cold water on your head when entering or leaving the
house. One last thing I am going to try, is to cut the pipe in a convenient
place and put a bucket under the end from the tank and see which end drips
then. I will of course reconnect the pipe after my experiment is over.
Another thing to check for John, is condensation. Is the leaking overflow
pipe in copper and perhaps the other pipe is plastic. The cold exterior air
blowing up the copper pipe could be condensing once it hits the warmer pipe
in the loft.
A house near to me has had a hose attached to a WC overflow pipe for around
20 years. Could have wasted an enormous amount of water. Odd that ball
valves mystify so many people - at this house the owner had to go up a
ladder - connect the hose - clip the hose to the wall and lead it to a
rainwater hopper. However, he saved about 5p on a ball valve washer.
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