Residential Combustion Air Calculation Method

(for Furnace, Boiler, and/or Water Heater in the Same Space)

Complete vented combution appliace information:

Furnace/Boiler:

Draft Hood Fan Assisted Direct Vent Input: Btu/hr

(Not fan Assisted) & Power Vent

Water Heater:

Draft Hood Fan Assisted Direct Vent Input: Btu/hr

( Not fan Assisted) & Power Vent

Calculate the volume of the Combustion Appliance Space (CAS) containing combustion appliances.

The CAS includes all spaces connected to one another by code compliant openings. CAS volume: ft

3

Determine air Changes per Hour (ACH)

1

Default ACH values have been incorporated into Table E-1 for use with Method 4b (KAIR Method). If the year of construction

or ACH is not known, use method 4a (Standard Method).

Determine Required Volume for Combustion Air.

Standard Method

Total Btu/hr input of all combustion appliances (DO NOT COUNT DIRECT VENT APPLIANCES) Input: Btu/hr

Use Standard Method column in Table E-1 to find Total Required Volume (TRV) TRV: ft

3

If CAS Volume (from Step 2) TRV then no outdoor openings are needed.

If CAS Volume (from Step 2) TRV then go to

Known Air Infiltration Rate (KAIR) Method

Total Btu/hr input of all fan-assisted and power vent appliances

(DO NOT COUNT DIRECT VENT APPLIANCES) Input: Btu/hr

Use Fan-Assisted Appliances column in Table E-1 to find

Required Volume Fan Assisted (RVFA) RVFA: ft

3

Total Btu/hr input of all non-fan-assisted appliances Input: Btu/hr

Use Non-Fan-Assisted Appliances column in Table E-1 to find

Required Volume Non-Fan-Assisted (RVNFA) RVNFA: ft

3

Total Required Volume (TRV) = RVFA + RVNFA TRV = + = ft

3

If CAS Volume (from Step 2) TRV then no outdoor openings are needed.

If CAS Volume (from Step 2) TRV then go to

Calculate the ratio of available interior volume to the total required volume.

Ratio = CAS Volume (from Step 2) TRV (from Step 4a or Step 4b) Ratio =

Calculate Reduction Factor (RF).

RF = 1 Ratio RF = - =

Calculate single outdoor opening as if all combustion air is from outside.

Total Btu/hr input of all Combustion Appliances in the same CAS (EXCEPT DIRECT VENT) Input: Btu/hr

Combustion Air Opening Area (CAOA):

Total Btu/hr by 3000 Btu/hr per in

2

/ Btu/hr per in

2

= in

2

Calculate Minimum CAOA.

Minimum CAOA = CAOA RF Minimum CAOA = x = in

2

Calculate Combustion Air Opening Diameter (CAOD)

CAOD = 1.13 Minimum CAOA CAOD = 1.13 x Minimum CAOA = in

1

If desired, ACH can be determined using ASHRAE calculation or blower door test. Follow procedures in Section 304.