Combustion Air Calculation

jhewings

Since combustion air (make up air) was discussed recently on another thread I thought it would be interesting to calculate the combustion air requirement for a boiler room. This post references a document

"Combustion Air and Inspection National Fuel Gas Code ANSI Z223.1/NFPA 54"

which can be found here:

https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/aga_combustion_safety.pdf

Interested readers may want to download the document and read it with the following calculation. Comments welcome!

What do you think regarding the question at the end?

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The boiler room has a WM EGH-105 and an atmospheric water heater


Boiler WM EGH-105 405,000 Input BTU /hour (Natural gas, Steam)
Water Heater 76,000 Input BTU /hour
Total 481,000 Input BTU/hour

Question 1: Is the boiler room itself big enough to provide combustion air?

Section 9.3.2.1 Minimum required volume shall be 50 cubic feet per 1000 BTU/hour
Boiler room is 30.5ft x 9ft x 10ft = 2,745 cubic feet

Minimum required volume shall be (481,000/1000) x 50cuft = 24.050 cubic feet

Section 9.3.2.2 Air infiltration and ACH (air changes per hour) are unknown. There are 3 old windows permanently closed and probably leaky. However, disregard Section 9.3.2.2

Section 9.3.2.3 Conditions for high and low openings to adjoining rooms are not met. Disregard section 9.3.2.3

Answer to Question 1: No the boiler room itself is not big enough to provide combustion air.

Outdoor combustion air: There currently is a 12" round duct with a metal grill outside and a motorized damper, installed up high near the ceiling.

Question 2: Is the outdoor combustion air sufficient for this boiler room?

Section 9.3.3.1 does not apply at this time. There is only one outdoor opening.

Section 9.3.3.2 applies in this boiler room. One opening is just below the ceiling. (Note there is no provision for one outdoor opening ducted to near the floor.)

The duct is 12 inch round. The opening is 3.14 x 6 x 6 equals 113 square inches

Section 9.3.7.1 Since there is a metal grill outside, it is assumed the free area is 75%.

113 x 75% = 85 square inches is the size of the outside opening

Section 9.3.3.2 The opening should be sized for 3,000 BTU/hour per square inch

481,000/3,000 = 160 square inches which is more than 85 square inches existing

Answer to Question 2: No the outdoor combustion air is not sufficient.

Question 3: Is the combined indoor volume plus the outdoor opening sufficient for this boiler room?

Section 9.3.4 Combination Indoor and outdoor Combustion Air

Section 9.3.4(3)(a) The ratio of interior spaces equals 2,745/24,050 = 0.114

Section 9.3.4(3)(b) The outdoor size reduction factor is 1-0.114 = .886

So the outdoor size requirement is 160 square inches x .886 = 142 square inches which is more than 85 square inches existing

Answer to Question 3: No the combined indoor volume plus the outdoor opening is not sufficient

Conclusion: There currently is not sufficient combustion air according to the referenced document.

Proposed Solution: Add another outside air intake and duct it to the floor vertically according to Section 9.3.3.1

Assume the second opening is the same size 85 square inches

Question 4: Will the two openings provide sufficient combustion air?

Section 9.3.3.1(1) Each opening shall have area equal to 1 square in/ 4000 Btu/hour

(481,000/4000) x .886 = 106 square inches required which is more than 85 square inches

Answer to question 4: No the two openings 12" round do not provide sufficient combustion air.

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RayWohlfarth

@jhewings I use the sizing that is in the International Fuel Gas Code because that is the code for here. That is also the code the boiler inspectors will use when they visit. See link below. I would be sure the openings meet your local codes or it could be an expensive list by the boiler inspector
https://codes.iccsafe.org/content/IFGC2015/chapter-3-general-regulations#IFGC2015_Ch03_Sec304

Zman

CFM per BTU of CA would be an interesting number to know. These codes that refer to cubic feet of space without addressing the tightness of the space don't make much sense to me.

jhewings

Thanks Ray. I will check which code applies to Chicago.

Going by the code I have used, if there are two openings, they both have to meet code, so adding a second bigger one won't work. I think the options are:

1. Change the existing outside opening to the correct size (14") and add a second.
2. Add a fan with adequate CFM. There does not seem to be a provision for an outdoor opening and a fan
3. Change the existing outside opening to a fan.

jhewings

@Zman The document I referenced does give that number in Section 9.3.6

0.35 CFM per 1,000 Btu/hour. This is for Mechanical Combustion Air Supply (a fan I suppose).