An associate and I are working to resolve a problem identified by a home inspector as "inadequate combustion air in mechanical room" where 2 gas furnaces and 2 water heaters are located. According to the inspector, the problem can be fixed by installing a vent into the room from the finished area, in this case a large finished basement, at a height of 1' foot above the floor. We have also been advised by a HVAC contractor that we could also fix this by installing a vent in the cold air plenum just before it enters the furnace. The second approach seems like an easier way to go since we wouldn't have to cut through any walls to the finished part of the basement. However, we are concerned, that since the cold air plenum is at negative pressure compared to the room, this approach could actually reduce the amount of combustion air. To make things more interesting, there are currently 4" vents from the warm air plenums of both furnaces which blow air into the mechanical room. Seems like these vents already are providing ventilation for combustion because of the positive pressure in the warm air plenum. Appreciate any comments as soon as possible since the house is under contract for sale and this issue needs to be resolved before closing. TIA