I've had my new American Standard 2-Stage, 95% furnace installed since May and this is my 1st heating season.
The unit is installed in the attic which has pleanty of ventilation and is sealed from the living space below. Its a downflow furnace configuration and it's working well. For December here in NY my gas usage is significantly less than December of last year even though its been much colder this year. I upgraded from an 80% single stage furnace.
The installation looks fine based on all of the information on this site on how the unit should be installed except for one item.
My HVAC guy didn't extend the Air Intake beyond how it is set up from the factory meaning when you open the cover of the furnace you see the air intake PVC with it's plastic screening cap. No PVC was added to extend the air intake out the side of my unit.
When i asked my HVAC guy about this he said that it's ok to have it configured this way since the attic is open to the outside air. Also forgot to note that i also have a attic fan vent within 10 ft of the furnace so besides the normal attic vents there is this 14 inch vent to the outside air near the furnace.
I also saw that he removed the "metal knock-out" where the air intake would enter the furnace cabinet so i can see that there are no obstructions between the furnace air intake and the attic air.
My novice estimate is that there is enough air from the attic to satisify the furnace and i don't believe it's sucking any air from the inside of my house. I don't feel any drafts near the attic stairs or anywhere else when the furnace is running.
I'm thinking that this configuration must be ok since the furnace is working great and very efficiently and i know i'm going to have to make sure that the existing air intake grill is vacumed and kept clean each season but is this common practice for units in the attic? Again this is a down flow furnace so its standing vertically.