Abstract: A condensing boiler includes a heat exchanger with a burner adjacent to the top of the heat exchanger. There is an exhaust for condensate adjacent to the bottom of the heat exchanger. There is a intake system including an air conduit, a fuel inlet and a fan which supplies a fuel/air mixture to the burner during a heating mode and which supplies air to the heat exchanger during an idle mode. There are controls which modulate the fan so that the air flow to the heat exchanger during the idle mode is sufficient to inhibit upward migration of condensate through the heat exchanger, but insufficient to cause significant outflow of air to the exhaust.