If the leaking overflow pipe is at loft level or protrudes from the eaves of the roof then the source is either the cold water storage cistern or the smaller feed and expansion cistern that supplies the boiler if you have one.
Overflow pipe dripping from roof.
How to diagnose and repair a dripping overflow.
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Or it could be the smaller feed and expansion cistern which puts water into the central heating circuit.
However if it s at loft level and protrudes from the eaves of the roof then it could be coming from your cold water or central heating feed and expansion tank.
In all cases if you do not know which is the problem cistern then a visual inspection will tell you.
If the leaking pipe is at ground or first floor level then it s likely to be your bathroom overflow pipe dripping which will be coming from the toilet cistern.
A constant drip in your tpr valve is an indication we must know about.
Is it from the eaves of the roof near a bathroom toilet or near a boiler hot water cylinder.
Look where the overflowing pipe comes out of your house.
2 look at the type of pipe the water is coming from what is the material is it metal of plastic.
It s designed to release the pressure when there is too much of a load built up inside the water heater.