Help please, another Header Tank overflow

5 May 2011
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United Kingdom
About a year ago, after a weekend away we came back home to see one our overflows from the loft dripping. Closer inspection revealed that it was the header tank which was being filled with warm water from the expansion pipe when the boiler was running. In the end we replaced the thermostat on the hot water cylinder and this cured the problem, and the dripping completely stopped.
However the dripping has started again and it is continuous, so I took a look myself at the header tank in the loft. Quite confusing as there was a slow drip from the ball valve joint, which I will replace, but the water in the tank was warm. Once the heating was switched on, hot water once again started flowing from the expansion pipe, and the drip from the overflow became a gush.
The thermostat on the cylinder appears to be working fine as I hear it click as its adjusted and I hear the boiler kicking in when I turn it up.
The boiler is a Baxi Solo about 17 years old, the dial in the boiler goes from 1 to 5 low to high and I have it set to 2. Could this therefore be a boiler problem with the control thermostat or the overheat thermostat?
Any advice, greatly appreciated.
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You have a hydraulic imbalance in the circuit.

You could turn the circ pump speed down, but this is unlikely to stop the problem completely.

Pumping over will kill all the rads and the boiler by enriching the rusting process in short order.

Blocked cold feeds, sludge etc are all likely culprits.
The pump settings go from one to three, it was set to two and now turned down to one, can I rule out the thermostat on the boiler then?
One of the most common causes of pumping over is as said before, blockages around the cold feed and open vent pipes where they are connected to your system pipework.These connections are usually on the flow pipe from boiler just before the pump. The use of a magnet on the copper pipe is a good way of detecting areas where magnatite has built up and blocking your pipework. Once detected, the best solution is to cut out and clean or replace the offending pipework.
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