AboutJim Barnhart Expertise Fifty + years in Heating, Ventilating, Air Conditioning, Sheet Metal Manufacturing.
Semi retired since 1995,
Experience Answer questions about , residential and commercial.
Answer questions about sheet metal fabrication.
Fifty years plus experience.
No answers for oil equipment, No answers for kitchen appliances, No answers for laundry appliances.
Question QUESTION: I am extending the return air drop under my furnace so I can add a second return air vent. I see that the existing box is constructed of 2x4s with 5/8 sheetrock on the sides and 2 pieces of 7/16 plywood on top. What so I need to use in the extension? Can I use 1/2 plywood on top if it will not support any weight? I plan to built a workbench which will partially sit on top of this extension.
ANSWER: Gary,
I don't know what your set-up is? Why is your "furnace" sitting up on a constructed "box"
The word furnace to me indicates a "gas fired furnace" is that what you have? You plan to extend the box which way, side, top, back? where will the added return grill be? Is your furnace in a basement?
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QUESTION: The return air box under the furnace is what I think is sometimes called the "return air drop". Yes, it's a gas furnace, in the garage. The furnace and water heater both sit atop this box. This kind of set-up is very common around here. The existing RA duct goes out of the front entry area of the house into this RA box (on the left side). This duct is too small for the house (4 tons of heating and 3 tons or cooling) so I am adding a second RA duct. This will go from the living room, through the garage wall and then into the right side of the RA box. The RA box is simple 2x4 frame construction, with 5/8 drywall on the sides and what looks like 2 pieces of 7/16 sheathing on top.
ANSWER: Gary,
You can cover the top with 1/2" plywood.
My main concern is that the box/platform is sealed tight on the outside from the furnace blower and the water heater so that the blower doesn't suck gas fumes or raw gas for that matter into the return air flow.
You say the furnace is in the garage? I assume that the furnace is not in the garage but maybe in a furnace type room that is in the garage and that the open flame from the furnace and water heater are sealed off from the garage area.
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QUESTION: The existing box/platform isn't very well sealed but I plan to seal it up real tight. This should have been sealed anyway, as I'm sure with the undersized RA duct, the furnace was sucking some dirty air in from the garage. Yes, the furnace really is in the garage. There are two vents through the garage wall to the outside which I am sure are there to meet code for the gas water heater and furnace. The open flame is not sealed off from the garage area. Both the water heater and furnace are also vented through the roof. I know this isn't a great configuration, but like I said, it is very common around here (Albuquerque NM). All of my neighbors have this set up and I've seen many like this all around town. I've also seen some with the furnace in a closet in the house, and some in a small room separated from the house and garage, but these are less common.
Answer Gary,
There is an International Building Code, IBC that covers the United States of America (Formerly known as the Uniform Building Code, UBC). The says (not exact words) that there can't be any type of an appliance with an open flame in a garage unless the flame is at least six feet above the floor.
Also there can't be any access to an appliance with an open flame through a garage.
However your local building department and every city, town, or county has one, has the authority to adapt the IBC or change it for there jurisdiction.
You might just want to call your building department and ask what there code is, with out giving your name or address.
