Overflowing header tank

2 Jan 2006
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United Kingdom

I have a Potterton Prima which is fed from the header tank in the loft. Recently the header tank has started to overfill and then flows out of the overflow pipe.

I have managed to work out that the ball cock isnt leaking, nor is the overflow pipe which goes in at the top. Therefore it has to be the pipe at the bottom which feeds the system... but why does it fill up the tank with water?

I understand from a post by someone earlier today that it is normal for warm water to come back up through this pipe?...but surly not at the rate mine does (at the moment I am having to siphon out around 1 washing up bowl every 2 days to stop it reaching the overflow level)

Any help / suggestions would be appreciated.


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there is a good chance u have a pin hole in the coil in your cylinder.& if your domestic cold water tank in the is higher which it often is water will allways find its own level. To confirm this turn off your cold water main & lower the water level to the same as in the htg F/E tank & see if it stops overflowing. If so you need a new cylinder mate.
Try adjusting the balltap so that the level in the tank is no higher than 6 inches from the bottom.

Is it possible that someone has run the overflow pipe from the DHW storage tank into the CH feed tank?

If you have an indirect cylinder and the DHW cistern is above the height of the CH tank it is possible that there is a leak in the coil in your cylinder.

Has this started since you had some other plumbing work done. I have seen some things in my time and it wouldnt be the first time that someone has ran hot water from the heating system into a bidet or washing machine or something.
With regards to the HW cylinder overflow pipe this defiantly does not go into the smaller header tank as there is a much larger tank next to it which is also at the same level. The larger tank which feeds the HW cylinder seems to operate fine with no increase in water level above the norm.

I did have a new radiator installed several months ago to replace an old one which had gone rusty and the air vent pin had snapped off. I have checked this and the rad warms up fine and all the valves are on etc as should be.

To be honest the whole house works fine except every couple of days the header tank has filled up again and started to drip at the overflow. I have been advised that it may be the water pressure as on a number of occasions the dripping overflow has started even when the boiler has not been on…like over xmas I was away for 5 days and left with the tank at a low level…when I returned it was about ¼” off overflowing!!.
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I dont think u understand the point i'm makin. The big tak fills the cylinder & the little tank fills the boiler,radiators & the coil in the cylinder. inside the copper hot water cylinder is a long pipe bent round into a coil that is full of water & gets hot by the boiler. if u have a pin hole in this coil then water out of the cylinder will enter the coil & lift the water level in the htg header tank.
To corroborate what bab has explained, I see this symptom all the time, and once the float valve has been eliminated as the cause it always turns out to be the cylinder.

You will find out for certain if you follow bab's guidance - he knows what he's talking about.
Thanks softus. Do u get WRX. Haveu a subaru. If so you can afford a new cylinder. Don't by any petrol 4 a week
Cheers for the info guys, I will get the cylinder checked out as advised.

WRX def means 'Impreza' and your not wrong about the petrol...28mpg isnt good!!.


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