Why is my domestic (non main feed) cold water getting hot?

[amvs]

I have a problem with my domestic heating system. what happens is that the cold water supply from the header tank in the loft is also being heated when the boiler system is running - what we have effectively got is two hot taps!

I don't know much about the system other than it's got a std three way valve and thermostat control system. The header tank is a shared one for the radiator heating and water system and is in the loft. It's gas fuelled and was installed when the house was built in 1987. I don't want to get a heating engineer in straight away as I'd like to have an idea of what the problem is first - typical engineer!!

Thanks for reading this and for any advice you can provide.

 

[KSherlock]

It sounds as though you have hot water migrating into the cold water due to thermal expansion. This condition is prohibited in most codes these days, but it wasn't addressed until recently. What we do here is install a check valve in the cold water supply to the heater and we add an expansion tank to accept the volume of expanded water. Amtrol makes a good one.

 

[hydrae]

The old fashion way this was solved was to make sure the cold water supply to the hot water tank has a heat trap, the supply pipe is routed in an upsidedown U shape to prevent convection of the hot into the cold water, the 'U' needs to be tall enough so the amount of thermal expansion does not overcome the height of the 'U'. Most locations do not have this problem because the hot water heater is mounted at an elevation greater than the cold water supply.

Hydrae