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Combustion and Confined Spaces

All fuel-burning appliances require oxygen to burn. They require sufficient oxygen to burn clean and safely, without soot and CO (carbon monoxide).

I live and work in Florida, where most of our fuel-burning appliances are 80% efficient with open combustion. (You can learn more about the basics of combustion on a podcast with Benoit Mongeau.) The systems utilize air and oxygen from the space for combustion.

With these low-efficiency appliances, whether the appliance is forced air or natural draft, combustion air leaves the space and exits the flue.

This phenomenon causes negative pressure that must be allowed to equalize, and it consumes oxygen from the space. So, these open-combustion appliances must be in a sufficiently large space. Alternatively, they may need to communicate with (be open to) a larger space or the outdoors.

When you consider that other gas appliances also need to use oxygen and need to vent outside, you can see that negative drafts can occur on natural draft appliances like water heaters that lack sufficient communication to the outdoors.

That's why all open combustion appliances that utilize combustion air from inside the space must be in an “unconfined space” or connected to an unconfined space or the outdoors using an approved method.

I see many furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.

The Code

The NFPA contains three standards that apply to combustion heating appliances: 31, 54 & 58 (for oil-burning, natural gas, and LP equipment, respectively). According to the NFPA, an unconfined space is a space that has at least 50 cubic feet of open area for every 1,000 BTU of input. That means that a 100,000 BTU furnace must be in a 5,000 cubic ft space to be considered unconfined.

If the appliance is not unconfined, then additional combustion air must be made available to the space. We usually do this with one opening at the ceiling level and another one near the floor.

If the air is coming from another unconfined space, then the openings should be at least 1 square inch per 1,000 BTU and 1 square inch per 5,000 BTU if it is connected to the outdoors.

While these openings are needed to allow for proper combustion and venting in many cases, it helps illustrate why modern sealed combustion “direct vent” appliances that take all of their combustion air from outdoors make so much sense.

Not only are direct vent appliances more efficient on the fuel utilization side, but they also prevent the negative home pressures or thermal losses associated with having vents in walls and ceilings.

So, either make sure you have an unconfined space, bring air in from an unconfined space or the outdoors, or have a direct vented appliance.

—Bryan

Comments

David Kelley
David Kelley @bryanorr

So we put a 4x12x6 boot in the floor and ceiling of the mecanical room and that is acceptable by our inspector. My problem is that in the winter time there is now cold outside air being introduced into the house and that brings up condensation and possible drafts. We used to put a return air grille in door both top and bottom but that brings in sound and a pair of grilles in a door doesn’t really look so good. Any suggestions?

10/18/17 at 07:41 PM

So we put a 4x12x6 boot in the floor and ceiling of the mecanical room and that is acceptable by our inspector. My problem is that in the winter time there is now cold outside air being introduced into the house and that brings up condensation and possible drafts. We used to put a return air grille in door both top and bottom but that brings in sound and a pair of grilles in a door doesn’t really look so good. Any suggestions?

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Anonymous
Anonymous @bryanorr

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4/25/25 at 01:50 AM

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Anonymous
Anonymous @bryanorr

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4/18/25 at 10:04 PM

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To provide a hint to the males that you want to meet in individual soon, you’ll be able to write in your profile that you’d like to satisfy a month (or no matter you’re feeling snug with) after you speak to someone on-line. Rushing to a decision can sometimes do more harm than good. While you have possible heard “be your self” many occasions earlier than, the deeper, more helpful message inside is one of the crucial, if not probably the most, essential tip with regards to dating. 9. What’s one thing you would like individuals asked you more about when online courting? Things equivalent to it will get them questioning extra. Most people have about 10-15 and they don’t seem to be things like how tall someone is or how previous or what automobile they drive. 21. What are your pursuits in a relationship? 13. What is the most important alternative in a relationship? 14. What do you think is the biggest reward in a relationship? 4. How do you handle conflicts in your previous relationship? 6. What do you suppose is the least vital factor in a relationship? 11. What’s the least necessary high quality in a associate? 4. What do you assume is least important in life?

Uncover the passions and pastimes that gentle up your match’s life. 20. What are your passions in a relationship? 15. What are your priorities in a relationship? 19. What are your weaknesses in a relationship? But remember, you are wooing potential employers, in a matter of speaking. Visiting shady web sites akin to these that offer free software, deal in on-line lottery, promote dirt low-cost offers on otherwise premium products, might pose as potential threats to your company safety. Though some people may discover online relationship sites as one thing superficial and virtual, however the major success that it has bought in uniting different souls all over the world, speaks of a very different story. Price aggregation and procuring websites access multiple listings of a product and allow a consumer to rapidly examine the prices. It’s a method to know their social circle and values in interpersonal relationships. It’s turn out to be the norm. Attempt to be genuine and see what wonders you are able to do.

Files:
Jerry Springer
Jerry Springer @bryanorr

Depends where the location of your equipment is. I have installed communication grills between the attic and mechanical room if located on the 1st floor. You can also install louvre doors if your in a basement type dwelling. Hell even installed foe dryer vents as a combustion air inlet.

10/20/17 at 11:13 AM

Depends where the location of your equipment is. I have installed communication grills between the attic and mechanical room if located on the 1st floor. You can also install louvre doors if your in a basement type dwelling. Hell even installed foe dryer vents as a combustion air inlet.

Files:
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