Hi, We installed a brand new Brivis HX30 two days ago. It has been set up with 2 zones. However, downstairs gets really hot but upstairs remains cold. Our return air is on the wall downstairs. Would that make a difference? With our previous heater, we had a switch on wall which we would flick to work the heater either up or down. Had the electrician checking the zoning today and he says everything has been installed correctly. I don\t understand why we have this issue? Our previous heater worked so well without all the complications of zoning and so on.