Investigate furnace and hot water exhausting into the mechanical room Conclusions of the investigation are; Not enough makeup air in the building & the entire building is in a negative vacuum drawing air in any place it can including down the flue pipes. Remove water heater flu pipe from the combined 644 vent Y designed for the two 100k/120k 80% furnaces and rebuild sealed 4-in 26 gauge vent pipes Replace original 6-in b vent flu cap on roof Remove obstructions from 10-in combustion air in mechanical room that was installed on the roof Clean and shop vac both D&E furnaces Completely remove April Air 600 bypass style humidifier with manual control and all low voltage control wiring including drain and copper water line supply. Patch all holes in supply and return ducts Investigate no high voltage on left side furnace D Run test both systems Both furnaces run in fairly good condition with new air filters installed today Mechanical room with 4-in combustion air at ceiling height