What is the Free Area of Register Covers?

I’ve been trying to find a chart somewhere on the free area of register covers but have been unsuccessful.

I did an inspection at a condo where the furnace was draft induced 88,000 BTU/Hr input sharing a utility closet with a 52,500 BTU/Hr input water heater. Both appliances were behind a fire door with no air input on the door. I saw 6 x 9 vent in the ceiling in the closet.

The furnace mfg recommends 1 sq in of free area of outside air for every 3,000 BTU of combined appliances. In this case it would be 46.8 sq inches.

52,500 + 88,000 = 140,500
140,500/3,000 = 46.8 Sq In

If there wasn’t a register cover, the area would be 54 sq inches. This is really close and being technical but it’s one of those DLR’s. (Doesn’t Look Right) And I don’t want to Refer a HVAC Tech if it’s OK.







Great find…We don’t have gas here in SW Florida. But it appears that it is WRONG. Wish I could help more. But according what you have shown, its wrong to me and I sure would call it out.

According to HVAC standards 25% of the opening.:smiley:
So the two openings are too small.

Metal grilles are rated for 75% wooden louvers are rated for 25% free area.

Remember according to code there must be two openings. One within 12 inch of ceiling and one within 12 inch of floor if this is a passive combustion air opening.

Where passive combustion air proves impractical mechanical combustion air is an alternative.

304.9 Mechanical combustion air supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from the outdoors at a rate not less than 0.35 cubic feet per minute per 1,000 Btu/h (0.034 m3/min
per kW) of total input rating of all appliances located within the space.

According to the standards today.
Do not mention “CODE” unless you are a “Code Official”:smiley:

I write up code violations on most every inspections just don’t use the word code Gary is correct should have the two pipe vent system if there is a vented attic