A new oil fired furnace has a flapper valve installed in the exhaust line between the furnace and the wall. I can see the valve operates as a check valve, calibrated to swing in on a certain negative pressure in the exhaust.
The old furnace didn't have such a thing, and the new valve blows by, releasing exhaust gas into the space ... it appears to be designed to a "what's a little exhaust release among friends" standard, with a 1+ mm gap around the flapper plate! Why is the system set up this way, and what bad things might happen if this damn valve were capped?
(The furnace is on the ground floor of a three story house).