Combustion air

I’m sure that we’ve all seen the elementary school experiment where there’s a lighted candle inside a jar, and a lid is put on.  After just a few moments the candle flame is extinguished.  Without oxygen the candle can’t burn.

Your appliances that burn natural gas (usually just called “gas”) are just the same.  They need oxygen to burn, and they get that oxygen from the air.  If there’s not enough air, then there’s not enough oxygen – and bad things will happen.

The air that your gas furnace (and water heater, and boiler) needs in order to burn safely is called combustion air.  In this post we’ll discuss combustion air, including what happens if we don’t have enough and what the requirements are for “enough.”

There are two basic ways for your furnace to get its combustion air.  Most high efficiency furnaces get their combustion air directly from the outside.  This is the best and most efficient way to do it.  If your furnace has two PVC pipes going to the outside, then one of those pipes is taking the flue gases out and one is bringing fresh air in for combustion.  This is called a direct vent furnace.  You have nothing to worry about in terms of combustion air for this furnace, and this post won’t deal with direct vent furnaces.

On the other hand, standard efficiency furnaces typically have a metal flue, and they get their combustion air from inside the house.  This post deals with this type of furnace.
When your furnace is running it’s pulling combustion air out of the room and through the flame, and then exhausting it out of the house through the flue.  These flue gases contain lots of bad things including carbon monoxide, which is deadly.  So of course it’s important that the flue gases be sent to the outside.

So while the furnace is running air is constantly being sucked out of the house and sent up the flue to the outside.  This is basically a big exhaust fan, pulling air out of the house.  Figure 1.

Figure 1: The gas appliances are pulling air out of the house for combustion, and sending those flue gases out of the house.

If the gas appliance is in a small closed space – a confined space – then the air pressure in that space can drop when air from the room is sucked out for combustion and sent up the flue.  Figure 2.

Figure 2: As the appliances suck air out of the house, the house comes under negative pressure with respect to the outside.

Imagine putting a straw into an empty plastic soda bottle.  If you simultaneously seal up the top around the straw then you can suck the air out and the bottle will even collapse because the air pressure inside is so much lower.  Same idea with your furnace.  It’s sucking air out of the room and lowering the air pressure in that room.
With standard efficiency appliances, flue gases rise up the flue and out of the house because they’re warmer than the surrounding air.  Warm air rises.  But if the air pressure in the furnace room is lower than the air pressure outside then those flue gases can be sucked back into the house instead.  This is called back drafting.  Remember, those flue gases are unsafe and are likely to contain carbon monoxide.  Figure 3.

Figure 3: When the room with the furnace comes under negative pressure the flue gases are sucked back into the house instead of going out.

So the flue gases can’t rise up and out of the house, because they are being pulled back into the house by the drop in pressure.  And the drop in pressure is caused by the furnace pulling combustion air out of the room and sending it up the flue.

So how do we prevent this problem?  That’s easy: we provide lots of combustion air to the gas appliances.  If the furnace is in a small room, then we need ventilation openings in the walls or door of that room to let in plenty of air to make up for the air that the furnace is using.  If we provide plenty of air to the room then the air pressure in the room won’t drop and the furnace will be able to send the flue gases out of the house.  It’s just like the plastic soda bottle with a straw.  If you don’t seal up the top around the straw then plenty of air can get back into the bottle to make up for all the air you’re sucking out.  In that case there is no drop in air pressure and you can’t get the bottle to collapse.

The size of the ventilation openings we need is a fairly straight forward problem, because requirements for combustion air are specific, and they are uniform across all the codes and manufacturer’s requirements that I’ve ever seen.

The first thing to determine is if the room with the gas appliance is big enough to supply all the combustion air.  If it’s not big enough then it’s called a confined space and we have to provide ventilation openings for more combustion air.

To determine if a room is a confined space you add up the gas input ratings of all the gas appliances in the space, measured in BTU’s per hour (BTU/hr).  (A BTU is a British Thermal Unit, and is a standard measure of the energy contained in natural gas.)  Then you calculate the volume of the appliance space.  If the volume of the space is less than 50 cubic feet per 1,000 BTU/hr of total input for the gas appliances then it’s a confined space and it’s too small for safe operation of the gas appliances.

For example, say your furnace and water heater are in a room together.  In this example we’ll say that the furnace has an input rating of 80,000 BTU/hr and the water heater has an input rating of 40,000 BTU/hr.  So the combined input rating is 120 when measured in 1,000 BTU/hr.  So the room needs to have a volume of 120 x 50, or at least 6,000 ft³.  If the room is smaller than this then it’s a confined space and is too small to be safe.  To put this size in perspective, with 9 feet ceilings this room would have to be 667 square feet, or almost 26 feet by 26 feet.  That’s a pretty big room, and there are an awful lot of gas appliances stuffed into small spaces that just don’t meet this requirement.

So what to do if the appliance space is too small?  We need to get more combustion air from somewhere, and there are plenty of options for doing that.

A louvered door

The most common solution is to get combustion air from the rest of the house.  This is done simply by putting vent openings in the furnace room to connect it to other rooms in the rest of the house and allow air to get in.  This will work as long as the rooms combine to meet the 50 ft³ per 1,000 BTU/hr input requirement.

The most common way to add vents to the room is to install a louvered door.  (A louvered door has horizontal slats for venting.)  If the door is fully louvered over its entire surface then this will work as long as the combined input rating of the gas appliances is less than 175,000 BTU/hr.

If you don’t want to or can’t use a louvered door, here are your other options.

You can connect the furnace room to the rest of the house with two permanent vent openings.  Each of the two openings needs to be at least 1 in² for each 1,000 BTU/hr of input of all the gas appliances combined, but each opening must be at least 100 in².  One of these openings must be located within the top 12 inches of the appliance space, and one opening must be located within the bottom 12 inches of the appliance space.  The minimum dimension of each opening is 3 inches.  Figure 4.

Figure 4: Two permanent openings must be sized and located correctly.

Note that this requirement is for the free area of the opening, so if you put some sort of vent cover or grill over the openings you have to take that into account.  Usually a metal grill reduces the free area by 25%, and a wood grill reduces the free area by 75%.  So a 10 inch by 10 inch opening with a metal grill has a net free area of 75 in², while a 10×10 opening with a wood grill has a net free area of only 25 in².

For example, let’s look again at our situation above where we have a furnace with an input rating of 80,000 BTU/hr and a water heater with 40,000 BTU/hr.  And let’s say that we know the appliance room is too small.  So we can put openings in the wall of this room, so long as the openings lead directly into other rooms with a combined volume of 6,000 ft³.  We have to put two openings in the wall, one starting within 12 inches of the ceiling and one starting within 12 inches of the floor.  Each opening has to have a net free area of 120 in², which meets the requirement of 1 in² for each 1,000 BTU/hr.  So if the openings will have a metal grill we need to allow for that, so each opening needs to have a total area of 120/0.75, or 160 in².  So each opening can be 13 inches by 13 inches square.  Or 18×9, or even 53.3×3.  But 3 inches is our minimum dimension, so the opening couldn’t be any more squished than that.

There are several other options for getting combustion air, including air from a crawl space, an attic, and from the outside.

A single opening in the room is OK if it connects to the exterior or a properly ventilated attic.  The opening must be in the upper 12 inches of the appliance space and it must have a net free area of at least 1 in² for each 3,000 BTU/hr of appliance input.

If there are two permanent openings in the appliance space then one must start in the top 12 inches of the space and one must start in the bottom 12 inches of the space.  If the openings connect directly to the outside or connect to the outside through vertical ducts then each opening must have a net free area of at least 1 in² for each 4,000 BTU/hr of appliance input.  If the openings connect to the outside through horizontal ducts then each opening must have a net free area of at least 1 in² for each 2,000 BTU/hr of appliance input.

In general, ventilated attics and ventilated crawl spaces are considered equivalent to the outdoors.  Although ventilated crawl spaces are a bad idea (but that’s a topic for another post).
There are some other details that must be satisfied for combustion air.  Probably the most important detail to keep in mind is that it’s never acceptable to get combustion air from a bedroom.  If a gas appliance is in a bedroom closet then the closet door must be solid, self-closing, must be weather stripped to make it air tight, and the gas appliance must get all its combustion air from outside.

There are other details as well, but this gives the basic information.  Be sure to contact a qualified contractor when providing combustion air.  And of course if you have any questions feel free to contact Nations Home Inspections.