Required Volume Needed for Rinnai 199iN

The following is publicly available information for the Rinnai RU199iN tankless water heater and analysis to determine the volume of air required for venting

References:  

• Henrico County Building Inspections (publically available plans and communications with building inspectors)
• Rinnai:  https://www.rinnai.us/residential/product-detail/ru199in
• 2012 International Residential Code:  https://codes.iccsafe.org/content/IRC2012P13/chapter-24-fuel-gas
• Virginia Uniform Statewide Building Code: https://law.lis.virginia.gov/admincode/title13/agency5/chapter63/section120/

1. My home's Garage Area = 398 square feet * 9.2 ft height = 3,650 cubic feet (including trash bin area).  The garage is below the minimum volume to pull air directly from the space, therefore, air needs to be pulled from outside, unless the confined space has two combustion air openings or is not tight construction.  An analysis follows:

• Per 2012 International Residential Code (reference G2407.5 (304.5) Indoor Combustion Air), the minimum required volume of air required for a confined space = 50 cubic feet per 1,000 BTU/hr
• Per the Virginia Uniform Statewide Building Code (VUSBC), 13VAC5-63-120. Section 112 Workmanship, materials and equipment:
• Section 112.3.1 Conditions of listings. Where conflicts between this code and conditions of the listing or the manufacturer's installation instructions occur, the provisions of this code shall apply. 
• Exception: Where a code provision is less restrictive than the conditions of the listing of the equipment or appliance or the manufacturer's installation instructions, the conditions of the listing and the manufacturer's installation instructions shall apply.
• Confined Space:  A confined space is defined in the National Fuel Gas Code, ANSI Z223.1/NFPA 54 as “a space whose volume is less than 50 cubic feet per 1000 Btu/hr (4.8 m3 per kW per hour) of the aggregate input rating of all appliances installed in that space.” Examples include a small room, closet, alcove, utility room, etc. A confined space must have two combustion air openings. Size the combustion air openings based on the BTU input for all gas utilization equipment in the space and the method by which combustion air is supplied. (reference)
• The manufacturer warns that failure to provide adequate combustion air can result in unit failure, fire, explosion, serious bodily injury or death.

2. My Rinnai RU199iN maximum input = 199,000 BTU/hr

• maximum input at 96% efficiency = 191,040 BTU/hr
• Minimum required volume for combustible air intake = (191,040 BTU/hr) * [50 cf / (1,000 BTU/hr)] = 9,552 cubic feet or 2.6 times the current volume in the garage.  A plan of my garage area follows (publically available through Henrico County Building Inspections).  Highlighted in green is the area (square feet).

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