Rising main: plumbing question

[Fnaar]

I live in a house which is about 120 years old. The rising main runs up through the kitchen, through a bedroom and up to the top floor (3 floor house). Even when the heating and hot water (run off 2 boilers and a tank) are turned off, I can hear a hissing sound that indicates water is running through it. There are no leaks that I'm aware of... any ideas folks?

 

[Night Train]

First thing is turn off the main valve where the main comes in the house and see if the hissing sound stops. If it does then you have definate flow from your main.
Then go into the loft to have a look at the tank. The tank should be full and the float valve should be floating and shutting off the water. It could be that the valve is leaking and despite the float being up there is still water seeping through. You can pull the float upwards to further close the valve to see if this is the case.

If you are still not sure then there should be an isolation valve just outside the tank that shuts off the supply to the float valve. Close that and see if the hissing stops.
If it does then the float valve needs a washer, or replacing depending on its condition.

You can also check the toilet cysterns too as they could be leaking a little water.

Leaking float valves are often 'hidden' leaks as through regular use the water in the tank or cystern doesn't get to overflow level. However, you can also check the overflow pipes outside to see if there is any sign of water dripping or havign been dripping from them. Assuming you have overflow pipes.

 

[mickle]

By law modern installations have overflow pipes which are placed so that any overflow is noticeable enough to be a bother. I know of someone who had a similar 'running water in a pipe' noise coming from the roof space of their flat for a decade. A chance visit by an electrician alerted them to the fact that the tanks for every flat had been overflowing at close to mains flow. Straight down the drain so no-one ever noticed.

 

[Fnaar]

Thanks for the answers folks, i shall investigate further.