Stephen, Sounds like you might have a cross connection. There are several possiblities, 1/Turn off the hot and cold water to the washing machines at their shut off valves, then see if you still have the same problem. 2/check if any has a washing machine in their apt. shut it off at the hot & cold water valves. They might have connected it to a sink faucet with some sort of quick connection. 3/Check if any shower heads or tub spout have been removed and capped off and the hot and cold water have been left on as this can cause a cross connection. 4/as far as the boiler system its always good to have proper maintenance on this system and a quilified mechanic will be best to advise you of its proper working statis. If your not sure how it works, your doing the right thing by calling them even if its not related to this problem. They can walk you through the system and advise you properly on it operation. I doubt that it is related to your boiler if it a constent problem. Unless it is cross connected with the hot and cold water piping. 5/As water is heated a certain amount of pressure is created as the heated water expands it needs a place to go so in a boiler system their is a expantion tank plumbed in line just for this purpose. This expantion tank needs draining down from time to time. Your boiler man can advise you on this. However even if it were not working properly the hot water would back up into the cold water system but you would be able to draw the hot water out of the cold by simply running the cold water. 6/ Try looking for a cross connection first.