Hydronic Open-Vented Central Heating System

[Denny]

Hi

Last weekend the overflow to the expansion tank started to drip. I took a look at the tank and manually reduced the water level in the tank. I was expecting to find the ball-valve feed dripping and needing to be replaced - but it wasn't.

Over the last few days the water level in the tank has risen around 1", so I presume that water is returning via the expansion pipe. Could someone expand on what the possible causes are?

Thanks

[Roger Mills]

I assume that you have 2 tanks in your attic - a large cold water header tank for the HW system, and a smaller fill and expansion tank for the primary circuit? I assume that it's this smaller tank with which you have the problem? I assume further that the water level in the large cold header tank is higher than that in the small F&E tank?

If these assumptions are correct, I fear that you need a new hot water cylinder. The HW cylinder has an internal coil/heat exchanger through which water heated by the boiler passes, heating the domestic hot water without mixing with it. BUT, if this coil springs a leak, the primary and secondary circuits *do* mix - causing water from the higher tank to flow into the lower tank.

[Denny]

All your assumptions are correct - Thanks for the clear explanation Roger