Bluetick
'N dik boom met 'n klien byltjie kap.
OK, with the garage door wide open, I lit the furnace. No smoke, really, when lighting. Throw some wood in and let it burn, leaving the garage door wide open. Forty five minutes later, I'm smelling smoke upstairs. So that furnace is getting all of outdoors to use for fuel-air and it's still back puffing. So I think it's looking more like the chimney is too short and partially blocked by the masonry chimney. What think you?