Hi I am at my daughter's. She's got a Worcestershire Bosch gas condensing boiler. Her washing machine is immediately underneath. The condensate drain is tapped into the same plastic pipe that the washing machine drain is connected to. She's just had the boiler maintenance engineer visit and he's left her with a warning notice: "Siphon inside boiler is draining and filling whilst washing machine is on. There is a potential for P.O.C to come(? can't really read this word) back into the boiler." He had the cover off the boiler when he was here and the washing machine was on / pumping out. He showed my daughter the level in the condensate tank(?) rising and dropping whilst the washing machine was on. Is the water from the washing machine backing up into the condensate pipe? Can't see how it can as the condensate tank(?) is four inches above the open end of the pipe where the washing machine drain pipe hooks inside. If it was back flowing that high wouldn't it also back up over the top of the open drain pipe? Maybe the level in the tank is rising and dropping due to air pressure as the washing machine pumps out? Thanks in anticipation.