Complete 2 heating maintenance in Lilburn. Change both gas furnace filter 16x25x1. Find upstairs Lennox system operating with weak flame sensor and calibration off. Find downstairs Goodman gas furnace operating with in manufacturer spec. No concern at this time. Suggest to replace flame sensor and do combustion calibration for furnace. Suggest to seal duct in crawlspace. Replace upstair Lennox flame sensor and complete combustion calibration service. System operating with in manufacturer spec.