Abstract: A furnace is adapted for operation in downflow, left horizontal flow and right horizontal flow positions. The furnace includes plural burners, a two-stage heat exchanger, a header box at the heat exchanger outlet for collecting products of combustion from the heat exchanger, and an inducer for inducing a flow of products of combustion through the heat exchanger. A condensate drain and trap are also provided for draining condensation from the header box and from an exhaust vent through which products of combustion are exhausted from the furnace. In accordance with one feature of the invention, a manifold is located inside the header box to divide the header box into first and second chambers. A pressure sensor senses differential fluid pressure between the first and second chambers for detecting a blocked condensate drain condition.