First things first, I am not an HVAC contractor, I have a handyman business in Phoenix AZ. I have little experience with HVAC, so please bear with me. I have been researching the EqualizAir and the Skuttle devices for make-up air. I have a customer who has a home that we will soon be installing insulation in his attic. He has an intake in the furnace closet that goes directly into the attic. It is appx. 10"x16". I have been told by other HVAC contractors that this is for combustible air and it has to remain open. There seems to be a lot of unconditioned air coming through this vent, so I am wondering if it would be a good idea to install a make-up air device in the opening? The reason I am looking at these devices is because they seem to be user friendly, and don't require any work from the homeowner after they are installed. Also, if it makes any difference, the house has a fireplace. Thank you for any and all of your help.