The instructions in my NG boiler say you should not exhaust the boiler into a chimney with ANY other appliance (such as hot water tank). My boiler has a power vent. I'm assuming one reason a power vent could be dangerous with another appliance is that the power vent could 'push' flew gasses back through the other appliance and into the house. Modern boilers do not have the high temperature exhaust to create a proper draft, so the power vent does the drafting. If there is a down-draft in the chimney, the gases could end up in the house via the other appliance. This is the theory as I understand it, anyway.