I'm adding a catwalk to my attic and opening up a small door between two portions of the attic. I also have a pull-down stair which decreases the width of opening up to the attic. Is there a standard residential furnace width that i need to prep for? I can temporarily remove the pull-down stair to get a full 22.5", but I understand that technically I would not be able to put the stair back in following furnace installation since the code requires an access big enough for a furnace. I've heard that there are narrow furnaces (17 or so inches), but would this limit the selection available to my HVAC contractor?