Calculating Combustion Air Needed for Bosch Gas Tankless Water Heaters

The amount of combustion air today is very limited due to our attempt to reduce our fuel consumption. We are replacing our windows, doors, weather-stripping, caulking and adding insulation. Years ago 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 thermo pane 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 needed for a gas appliance. If your heating appliance is using indoor air for combustion you must be aware of the amount of combustion air that is required.

The combustion process requires the air being burned to contain 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 happens every year. We must be more concerned about this when we tighten up our homes and have appliances which use indoor air for combustion.  Gas appliances should never be installed in bathrooms or bedrooms where doors are normally kept closed.  This is stated in the National Fuel Gas Code (NFGC) manual.  Please also consult local codes for further clarification.

The federal code requires 50 cu ft. of space per 1000 Btu's input from your gas 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 say where the appliance is installed 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) zero's. For example 130,000 would be just 130.

As an Example:

If we do the math it would look like this; 

Room = 32 feet long x 25 feet wide x 7 feet high 

Boiler = 130,000 Btu input and a 40,000 Btu 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!!

Any additional combustion air needed beyond what is already available must be taken from the outdoors.  When bringing air from the outside you only need to bring in the balance of what you need. When in direct communication with the outdoors you need 1 sq in of free space for every 4000 Btu's.