Unless your pressurised hw system is in the attic too then it could be a blowoff safety valve no idea about heating system the heating is run from a stanley oil fired cooker don t think it s pressurised though.
Overflow pipe from tank in attic.
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I think you might be right about the tank on the landing.
If the water in the tank or cistern fills up to a higher level than it should the excess water flows down the overflow pipe and runs outside rather than leaking into the room.
The tank in the attic has been trickling out of the overflow for a few weeks now.
If not then u will still have a expansion tank in the attic with an overflow.
How to diagnose and repair a dripping overflow.
With this type of valve the brass stem is bent downwards carefully to adjust the water level.
If its the toilet it will be directly connected to the white overflow pipe so should be easy to trace.
It could be that tank or it could be a toilet overflowing.
Looking for some advice.
Tested it fine know it shuts off the feed that way.
Set it very low but water is above the float.
The tank will either be your cold water header tank or the flow and expansion tank for the heating system and the valve is a type 1.
I have an open vented system with a hot water pump stuart turner monsoon twin 3 fitted.
They re a vital part of your home s plumbing system.
Have already changed the valve and float in the attic.