Carbon Monoxide (CO) – Part 2

Posted on:

March 5, 2021

How much air is required for complete combustion?

Every cubic foot of natural gas requires 10 cubic feet of air for complete combustion. That’s a pretty high ratio at 10:1. Why so much air? The problem with air is that it is mostly nitrogen (78.084 %). We don’t need nitrogen to sustain combustion. The nitrogen is inert. We need oxygen. But only about 1/5 of the air contains oxygen (20.9476 %).

That begs the question, how much space is needed in a confined area to ensure that the proper amount of air is present for complete combustion? The answer is, according to all of the mechanical codes, 50 ft3 per 1000 Btuh input of all appliances in the confined space. For example, if we had a 120,000 Btuh input natural gas furnace and a 50,000 Btuh input natural gas hot water heater in the same utility room, that would mean:

120,000 Btuh gas furnace

50,000 Btuh hot water heater

170,000 total Btuh input

170 x 50 = 8500 ft3 required.

And, if the ceiling height is 8 feet, then

8500 / 8 ft. ceiling = 1062.5 ft2 required.

Now, if we had the typical 8’ x 5’ utility room in a finished basement. That would amount to only 40 ft2. We would need more space/air.

Where will you get the air from?

Confined spaces like furnace rooms, boiler rooms, or utility rooms are of most concern. If you were to take the air from an adjacent area, like a finished basement, as in Figure 1, then, you would need two openings, one within 12” of the ceiling and one within 12” of the floor. Each opening must have a minimum free area of one square inch per 1000 Btuh of the total input of all the appliances in the enclosure.

So, in our example utility room, with 170,000 Btuh total input, we would need two openings each with a free area of 170 in². If we wanted to take advantage of the open space between studs (14”), then, 170 / 14 = 12.14”. A 14” x 12” opening would be a little too small. Two 14” x 14” openings would be an excellent choice. Remember, the codes we work under establish minimum standards. Also, when choosing a grille to cover those openings, pay attention to the free area (Ak) of the grille.

You can opt to take the air from a ventilated attic and a ventilated crawl space, as in Figure 2. Now, you will require two openings, one within 12” of the ceiling and one within 12” of the floor. Each opening must have a minimum free area of one square inch per 4000 Btuh of the total input of all the appliances in the enclosure.

Another possibility is to take all of the necessary air from a ventilated attic as in Figure 3, and the opening requirements will be the same (one square inch per 4000 Btuh of the total input).

You can also take all of the air from outside via ducts, as in Figure 4. Now, your requirements are, two openings, one within 12” of the ceiling and one within 12” of the floor. Each opening must have a minimum free area of one square inch per 2000 Btuh of the total input of all the appliances in the enclosure.

What if you can’t get large air ducts or openings to the outside?

What if your boiler room is an interior room and there is no good way to get air ducts to the outside or to a vented attic or crawl space? Not to worry. There are mechanical devices that can bring the air into that interior utility room. For instance, Field Controls makes a device they call a Fan In A Can. Using this product (Model CAS-4) you would be able to supply enough combustion air for our furnace and hot water heater (170,000 Btuh total input) with one single 4” round duct up to 152 feet away from an outside wall. You can electrically interface this product with the appliances in the utility room so that when any one or all of them fire up, the necessary outside air will be introduced into the utility room.

It is the installing and/or servicing contractor’s responsibility to make sure that the proper amount of oxygen is present for complete combustion to occur. Make sure that you and everyone on your team understands these principles. Get a good handheld CO detector and learn how to use it properly. The safety of your customers should always be your first priority.

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Jack Rise

Owner at Jack Rise HVAC Technical Trains

Jack Rise, CMS, is a Certificate Member Specialist of the Refrigeration Service Engineers Society.He has extensive industry experience having worked as a contractor for 18 years, as a distributor of HVAC products for 13 years and for a major HVAC manufacturer for 5 years.In 2004 Jack formed his own company, Jack Rise HVAC Technical Training, and works out of his office in Tampa, Florida.

Jack is an ACCA/EPIC certified instructor for all of the ACCA residential and commercial design manuals including the 8th Edition of Manual J (Residential Load Calculations), the 3rd Edition of Manual D (Residential Duct System Design), the 5th Edition of Manual N (Commercial Load Calculations) and Manual Q (Commercial Low Pressure, Low Velocity Duct System Design).Jack travels extensively around the country conducting seminars on residential and commercial load calculations, system design and on the use of HVAC computer based software.He was an RSES instructor for 10 years, currently runs training sessions for NATE and EPA/CFC Section 608 Certifications and ACCA/ANSI Standard 5 (HVAC Quality Installation Standard).

Jack has authored two books as companion manuals for both Manual J and Manual D.He is the author and presenter of the ACCA HVAC Essentials series and the NATE Essentials series of training CDs.He is a 35+ year member of RSES, a Life Member of the North Jersey Chapter of ACCA, an associate member of ACCA-Florida and FRACCA and a Life Member of the NATE Technical Committee.