How to Calculate Combustion Air
Note: This calculation is not required for sealed combustion/direct vent
The amount of combustion air today is extremely limited due to our attempt to reduce our fuel consumption. We are replacing our windows, doors, weather-stripping, caulking, and adding insulation. When I started in this trade there were no double pane windows in the basements much less steel insulated doors in the basement leading to the outside. There were old single pane glass windows and old wooden doors which, a lot of times you could see daylight around the door frame.
Today we have added thermopane windows in the basement, new steel doors complete with magnetic seals. We even insulate the exterior perimeter between the floor joists. All these ideas are good, but they steal much needed combustion air required for a safe combustion process. If your heating appliance is using indoor air for combustion you must be aware of the amount of combustion air that is needed for safe combustion.
Take a look at this picture of a home. I believe all heating appliances should have combustion air brought into the appliance. You may wonder why. When air inside the home is used for combustion as the air is used in the combustion process the pressure inside the home is reduced. When this happens, cold air is pulled into the home through all cracks and crevasses cooling down the home and must be recovered. Outdoor air to the appliance will stop this from happening. When there is not enough combustion air the appliance may pull down the chimney thus pulling products of combustion which includes CO.
A home loses combustion air through kitchen and bathroom fans, clothes dryer vents, woodstove and fireplaces that does not have outdoor air connected to it will reduce the amount of oxygen in the home. Other things that affect the amount of combustion air available is outdoor air temperature. The colder it get outside the more heat loss the structure has. Remember heat loss is air leaving the home. The height of the home will set up more thermal circulation which increases heat loss.
The combustion process requires the air being burned to contains 20.9% oxygen. If it does not, the flame will start producing CO (carbon monoxide). This is an odor free gas which gets inhaled and stops the body from absorbing oxygen. CO poisoning deaths happens every year in the USA. We must be more concerned about this when we tighten up our homes and have appliances which use indoor air for combustion.
Below is a chart which shows concentration values and the adverse effects of CO in the air.
As you can see it does not take much CO to start being a problem. When you think of this as parts per million....wow. Let's see how much air we need. The federal code requires 50 cu ft. of space per 1000 btu's input from your heating appliance. Your state or local code may exceed that. If we know what the input of the appliance is and divide it by the total cu. Ft. of the mechanical room the answer would have to be at least 50 cu ft. per. If we do not have 50 cu. ft. per thousand btu's, we must bring combustion air in from the outside. If there are hallways and other rooms available due to no door being on the mechanical room we can also calculate these other areas.
We need to measure all the areas we can get combustion air from, until we come to a doorway with a door. The code states even if it is a louvered door, we still stop when we come to a door. If the utility room has a door mounted, even if it is open all the time you stop at the door. Since the formula uses btu's as MBH (thousand btu/h) we do not use the last three (3) zeros. For example, 130,000 would be just 130. Let's look at a mechanical room which would have a door.
If we did the math it would look like this;
32 feet long x 25 feet wide x 7 feet high
Boiler = 130,000 Btuh input and a 40,000 Btuh Water Heater
Volume = Length x Width x Height
' Volume = 32' x 25' x 7' = 5600 Cubic Feet
Total Btuh Input = 130 MBH + 40 MBH = 170 MBH
5600 Cubic Feet / 170 MBH = 32.95 Cubic Feet per 1000 BTU
Combustion Air will be Required
When we look at the differences of free inches from metal grilles to wooden grilles and some of these decorative grilles may vary a lot. The grilles could lose 15% to 70% of free area.
Direct communication with outdoors (grille through wall) - 1 sq in per 4000 btu's input
*Vertical duct - 1 sq in free area per 4000 btu's input
*Horizontal duct - 1 sq in per 2000 btu input
*Cross section of duct must equal opening free area free area
If you want to use round duct this chart will give you square inches of round duct.
Let's look at an actual job site I was on and how it was calculated.
Boiler 130,000 btu/h
Water Heater 45,000 btu/h
15 x 15 x 7 = 1575 cu ft
Appliances 130 + 45 = 175 mbh
1575 cu.ft. / 175 mbh = 9 cu.ft. per 1000 btu's (should be a minimum of 50)
9 cu. Ft. / 50 cu. ft. required = 18% of required
175,000 /4000* = 43.75 free sq. in. needed
43.75 x 0.82 = 35.87 free sq. in.
* Direct communication with exterior of home
There are some states that have increased the 50 cubic foot per 100 btu's per 1000 btu's. This is a safer number as the homes are tighter today and the air required for combustion will come in slower. You can change the cu. ft. per thousand btu's in your formulas if you have tightened up the home or the home was built since 1970.
Here is the same formula above using 100 cu. in. per thousand btu's
|
Applications |
BTU's Per Square Inch |
|
Direct Communication to Outside (Grilles) |
4000 BTUs per 1 sq. in. |
|
Vertical Duct |
4000 BTUs per 1 sq. in. |
|
Horizontal Duct |
2000 BTUs per 1 sq. in. |
|
Grilles to interior rooms |
1000 btu's per 1 sq. in. |