This document discusses, among other things, a valve assembly for controlling an exhaust pressure of a gas combustion system. The valve assembly can include a valve housing, a flapper, and a biasing device. The valve housing can be coupleable to an exhaust pipe. The flapper can be pivotably couplable to the valve housing to pivot between a full open position and a restricted position in response to exhaust pressure. The biasing device can be couplable to the housing and extendable into the exhaust pipe to engage the flapper, where the biasing device can be configured to bias the flapper toward the restricted position and can be configured to resist movement of the flapper between the restricted position and the full open position.