A forced air furnace may include a pneumatically modulated gas valve that is configured to provide gas to a burner. A pneumatic sampling device may be disposed proximate a combustion blower and may be configured to provide the pneumatically modulated gas valve with a first pneumatic signal and a second pneumatic signal that are representative of fluid flow through the pneumatic sampling device. The pneumatically modulated gas valve may regulate gas flow in accordance with the first and second pneumatic signals. In some cases, the pneumatic sampling device may include a restriction, a first pressure port disposed upstream of the restriction and a second pressure port disposed downstream of the restriction. The first and second pressure ports may provide the first and second pneumatic signals to the pneumatically modulated gas valve.