I'm really hoping someone can offer some advice. I recently had installed a new oil heating installation. Its a large installation with 3 zones and water. All the zones have a shared return due to lack of space for individual returns for each zone, therefore a shared return for all zones has been installed. It now seems that when an individual zone is on in isolation, the water returning from the heated zone migrates through the return to the other zones which are off, causing the radiators in the zones that are switched off to heat up. My question is how can this problem be resolved?