Shower / CW Tank problem

[alexco]

Hi, First post so be gentle.
The system i have is new. Boiler with hot tank in the loft and a big cold water tank. Its not a combi boiler and the system is not pressurised ( as much as i know sorry!!)I have a salamander pump feeding a mixer shower. Its been fine for ages (until the new bolier).
However after investigating the reason for a constantly dripping overflow i have noticed that the cold supply to the shower seems to be pushing hot water back up into the cold water tank via the pump. Enough to make the whole tank luke warm
Any ideas why this would happen. Can i knock it on the head by installing a non return valve to the cold feed for the pump? The shower mixer used to have NRV's in it but as they kept sticking i junked them with no problem till now!
Sorry for the serious lack of info, i know i could get the installer back, but if i can sort it myself in half an hour i will
Any help appreciated

Alex

 

[neiltheplumber]

Is the shower above the level of the water tank and pump. If it is then you may have a negative head of pressure. A non return valve would stop the header tank warming up but it may cause damage somewhere else, like the pump. Also does the hot feed from your cylinder come straight out of the top of the cylinder via a 'S' flange. The 'S' flange reduces air going into the pump so if there isn't one its possible that it's causing a problem!