Remember those two furnaces I showed you photos of last week? You know, the ones that had ducts placed—or taped, in one case—right in front of them to bring them combustion air. I told you it wasn’t a good way to deal with the combustion air issue, but let’s go a little further today. Let’s look at what building codes say is the right way to do it.
The need for air
The photo below shows the burners in a particular type of furnace. When you pull the cover off of this furnace, you could stick your fingers right there in the flames. (I said you could do it. I certainly don’t recommend it, though.)
What that means is that the flames are open to the air inside the furnace cabinet. Since we know combustion is the chemical reaction of a fuel (natural gas in this case) with oxygen, there’s got to be a supply of air to keep it going. The air in the furnace cabinet itself would be exhausted quickly so more air’s got to come in somewhere, right?
That’s why the furnace shown below has louvers in the cover. It pulls air from the surrounding space. This is what we call the standard efficiency furnace. If you buy a new furnace these days and want to get the cheapest one you can, this is probably what it’ll look like.
Some older homes, though, have a type of furnace that isn’t made anymore: the natural draft furnace. The photo below shows what they look like. Notice there are two places where they pull in air, and these guys pull in a lot of air. They’re also easier to backdraft, but that’s not our focus here.
Where does the air come from?
So if you’ve got any type of atmospheric combustion appliance, you need to make sure it gets air for combustion. The two options are:
- Indoor air
- Outdoor air
Let’s take a look at them separately.
Using indoor air for combustion
If you’re going to use indoor air, there’s got to be enough of it inside the house. Two building codes that address this issue are the National Fire Protection Association standard 54 (NFPA-54) and the International Residential Code (IRC).
Both say the same thing about using indoor air for combustion. The space that the atmospheric combustion air can draw from has to have at least 50 cubic feet of volume for each 1000 BTU/hour of appliance capacity.
Let’s say you bring the attic inside the building enclosure by putting spray foam insulation at the roofline (the case with the two furnaces in my previous article on this topic). Now, let’s put some numbers to it. (Yes, we’re going to do math, but it’s just simple arithmetic. I know some of you were hoping for partial differential equations, but you’ll just have to console yourself today by memorizing some more digits of Pi, I guess.)
If the attic has a floor area of 800 square feet and an average height of 4 feet, for example, the volume would be 3200 cubic feet. Divide that number by 50 to find the maximum capacity appliance you could put in the attic, and you get 64,000 BTU/hr.
That would be a medium-sized furnace. But you could put only one in this attic. Put two of them up there, and the codes say you don’t have enough air for them. And an 80k or 100k furnace, neither of which is uncommon in homes, would be right out.
One way to get more volume of indoor air for your atmospheric combustion appliances would be to connect the space they’re in to other spaces in the home. You could put a couple of grilles in the wall separating a mechanical room from the conditioned space, for example. The codes specify how you should do that and how big the openings have to be.
Another way to get more air is to count infiltration. Again, you should check the codes if you’re planning to do that.
Using outdoor air for combustion
The other way to provide air for your combustion appliances is to bring it in from outdoors. The standard way to do this is with “high-low vents.” The photo below shows an example. Two ducts are open to this mechanical room in a basement. One of them terminates near the ceiling, the other near the floor.
The diagrams below are from the 2012 IRC and show two ways to do the high-low vents with vertical ducts. It’s OK for the ducts to pull combustion air from the crawl space or attic as long as those spaces communicate directly with the outdoors. In the first diagram, the low vent is connected to the vented crawl space, and the high vent is connected to the attic.
In the diagram below, both vents are shown connected to the attic.
Another method specified in the code is to use a single opening. The photo below shows a basement mechanical room attempting to satisfy the code requirement this way.
With both the high-low vents and single opening methods, the building codes specify the amount of vent area required. As with calculating volume when you use indoor air, the vent area required is based on the total capacity of the combustion appliances.
Here are the rules for the two-permanent-openings method:
- 1 square inch per 4000 BTU/hr – vertical ducts
- 1 square inch per 2000 BTU/hr – horizontal ducts
When using the one-permanent-opening method, you need:
- 1 square inch per 3000 BTU/hr – vertical or horizontal duct
But will it work?
Whether any of the methods above will work is debatable and depends on what criteria you use to gauge acceptability. Here are a few pitfalls:
- Using indoor air still means using outdoor air. Each cubic foot (or cubic meter) of indoor air that gets pulled into a combustion appliance will leave the house with the exhaust gases. When that happens, another cubic foot gets pulled in from leaks through the building enclosure.
- Air doesn’t always follow the arrows we draw on diagrams. You can put all the vents you want into a mechanical room, but whether air moves in the direction we’d like it to depends on the which way the pressure difference pushes it. Wind, stack effect, and other mechanical systems could cause air to flow the “wrong way.”
- Bringing combustion air in from outdoors can waste energy and lead to comfort problems. We know the old saw about how “a house needs to breathe” is a myth. Houses need to be able to dry out and they need good indoor air quality. They don’t need extra leaks. It’s the people who need to breathe.
- Occupants sometimes seal them up because they don’t understand why there’s a hole in their house. It happens, just like they sometimes caulk the weep holes in a brick wall.
As I’ve said before, it’s time to put atmospheric combustion appliances on the pile of obsolete technologies. Let them disappear into the dustbin of history along with steam engines, typewriters, and slide rules.
Photos by Energy Vanguard. Line drawings from the 2012 International Residential Code from the International Code Council, included under the Fair Use exemption.
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