# Combustion air ?



## rktect 1 (Feb 5, 2010)

I was going over the fuel gas code.  I don't know a lot about this code and our mechanical inspector looks through the plans for compliance but I started to get to wondering.  At any rate, the standard method for all air from indoors is 50 cubic feet per 1000 btu.  So I started thinking of my home.  Its a split level so my sub-basement is half underground and then my basement is a full basement.  Would these two floor levels be regarded as a single floor.  I ask because I have my mechanical room which is in the full basement.  I have an 80,000 btu furnace, a 50,000 btu water heater and a 35,000 btu dryer = 165,000 btu.  The full basement area is 800 square feet x 8 feet high = 6400 cubic feet.  but 165x50=8250 cubic feet of area for combustion air required.  I'm short almost 2000 cubic feet but the sub basement, which is located another 4-5 feet higher up the stairs with an 3/4" undercut door at the top, has another 4800 cubic feet.  So was this designed to include that 3/4" undercut door for the sub basement or am I really short by 2000 cubic feet?

Really just curious as to combustion air and how this gets calculated.


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## TJacobs (Feb 5, 2010)

Re: Combustion air ?

You can add openings connecting the basement to the sub basement to give you the extra area.  G2407.5.3.1.

With 165K BTU you would need openings of 165 sq. in. each with return grille free areas of about 124 sq. in. each.  The free area is usually stamped on the grille.  165K x .75 (metal grilles assumed).  G2407.10.

The undercut only gives you 27 sq. in. of area assuming a 36" door.


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## rktect 1 (Feb 5, 2010)

Re: Combustion air ?



			
				TJacobs said:
			
		

> You can add openings connecting the basement to the sub basement to give you the extra area.  G2407.5.3.1.With 165K BTU you would need openings of 165 sq. in. each with return grille free areas of about 124 sq. in. each.  The free area is usually stamped on the grille.  165K x .75 (metal grilles assumed).  G2407.10.
> 
> The undercut only gives you 27 sq. in. of area assuming a 36" door.


See, that doesn't make sense to me.  If I have a room that is 6400 cubic feet already and I am short 2000 cubic feet, I should only have to make up the 2000 cubic feet.  Instead the room that the mechanical appliances are in, whether 400 cubic feet or 6400 cubic feet still needs 165 sq. in. of holes strategically placed and calculated     (high/lo) in a wall into another room.   :?


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## TJacobs (Feb 5, 2010)

Re: Combustion air ?



			
				rktect 1 said:
			
		

> TJacobs said:
> 
> 
> 
> ...


See, that doesn't make sense to me.  If I have a room that is 6400 cubic feet already and I am short 2000 cubic feet, I should only have to make up the 2000 cubic feet.  Instead the room that the mechanical appliances are in, whether 400 cubic feet or 6400 cubic feet still needs 165 sq. in. of holes strategically placed and calculated     (high/lo) in a wall into another room.   :?

G2407.5 (304.5) Indoor combustion air.

_The required volume of indoor air shall be determined in accordance with Section G2407.5.1 or G2407.5.2, except that where the air infiltration rate is known to be less than 0.40 air changes per hour (ACH), Section G2407.5.2 shall be used._ The total required volume shall be the sum of the required volume calculated for all appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section G2407.5.3, are considered to be part of the required volume.

G2407.4 (304.4) Makeup air provisions.

Makeup air requirements for the operation of exhaust fans, kitchen ventilation systems, clothes dryers and fireplaces shall be considered in determining the adequacy of a space to provide combustion air requirements.

If you can determine that your air infiltration is 0.40 or greater ACH, you don't have to follow G2407.5.2.  Since you are not using any outdoor air the code takes that into account with larger openings to the connected spaces.

Be glad it's not a full story because you would need 2 sq. in. per 1,000 BTU...


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## Pcinspector1 (Feb 11, 2010)

Re: Combustion air ?

rktect 1,

Is the ceiling covered in your basement?

Would you be leaving some air out of your calculation from 2x10 (9.25") floor joist space, minus the cavities you can't use or is this space never used for doing the calculation?

It's a reach I know, but just wondering.


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