Expansion Tank Location on Viessmann Boiler Installations

Alan (California Radiant) Forbes

I was having problems with the diverting valve on a new Viessmann Vitodens 100-W (B1HE-150) boiler. It would freeze in the heating position and the boiler would stop making DHW. I would get the call and would have to exercise the diverting valve, either in the service program or by hand. Speaking with technical support, he asked me if the x-tank was on the supply or return. Mine was on the return and he told me that when the tank is on the supply side, a vacuum can form because the diverter is also on the supply side and can cut off the function of the x-tank. Kinda' makes sense, kinda' doesn't. Nevertheless, something to keep in mind during installation.