In all cases if you do not know which is the problem cistern then a visual inspection will tell you.
Overflow pipe dripping from roof.
A constant drip in your tpr valve is an indication we must know about.
If there s water dripping or pouring from a pipe which points out of the house at loft level or from the eaves of the roof then one of your cold water tanks is overflowing.
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Is it from the eaves of the roof near a bathroom toilet or near a boiler hot water cylinder.
How to diagnose and repair a dripping overflow.
Or it could be the smaller feed and expansion cistern which puts water into the central heating circuit.
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.
Look where the overflowing pipe comes out of your house.
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.
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.
This could be either the cold water storage cistern which supplies the hot water cylinder and possibly the cold bathroom taps.