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Old 10-27-2011, 01:57 PM   #1
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Combustion air calculation


I understand the 50 cubic feet per 1000 btuh rating and to calculate that, but it's unclear to me how to take into account the doors enclosing the aclove'd space the gas appliances are in. If the doors were air tight, is that where the 1 square inch per 1000 btuh per opening comes into play. I've found it in the code but it reads as if they assume the aclove is open with no doors. Any typicall gas furnace and/or water heater is usually enclosed in an area with doors. Anyone have a better read on that part of the code?

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Old 10-27-2011, 02:08 PM   #2
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Combustion air calculation


I don't have any numbers for you but here the code specs louvered doors for rooms enclosing furnaces and water heaters that draw combustion air from inside so the space communicates with the rest of the house. My 10'x10'x9' util room, that had a 87K BTUH furnace (since replaced with a direct vent model) and a 40K BTUH water heater, has such a door.

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I understand the 50 cubic feet per 1000 btuh rating and to calculate that, but it's unclear to me how to take into account the doors enclosing the aclove'd space the gas appliances are in. If the doors were air tight, is that where the 1 square inch per 1000 btuh per opening comes into play. I've found it in the code but it reads as if they assume the aclove is open with no doors. Any typicall gas furnace and/or water heater is usually enclosed in an area with doors. Anyone have a better read on that part of the code?

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Old 10-27-2011, 03:56 PM   #3
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Combustion air calculation


My 30+ year old townhouse has a utility room with a newer 80-85% gas gas furnace and a gas water heater. It was required to have a fresh air intake from the attic space that dumps into a 5 gallon pail as a prevention of syphoning the warm air out. - Solid door, but is usually open for convenience. No problems at all. It would have been nice to put in a high efficiency furnace with a ducted supply, but the extra efficiency would not be justified (even at -20F winters) because of the installation and cost.

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Old 10-27-2011, 04:50 PM   #4
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Combustion air calculation


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Originally Posted by kevsprojects View Post
I understand the 50 cubic feet per 1000 btuh rating and to calculate that, but it's unclear to me how to take into account the doors enclosing the aclove'd space the gas appliances are in. If the doors were air tight, is that where the 1 square inch per 1000 btuh per opening comes into play. I've found it in the code but it reads as if they assume the aclove is open with no doors. Any typicall gas furnace and/or water heater is usually enclosed in an area with doors. Anyone have a better read on that part of the code?

If the room with the furnace is attached to a room that has solid doors, then only the volume of the room with the furnace is used. If the door is a louvered, then the volume of the other area can be used.
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Old 10-31-2011, 06:47 AM   #5
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Combustion air calculation


Thanks for the input!

I have recent learned from a local HVAC contractor that the prefered method is to use two 6 inch pipes (PVC or metal duct), one near the floor & one near the ceiling terminating in an upside down candy cane configuration. The two pipes exit the ceiling/wall and terminate into an 8 inch pipe and exit the roof. I really wish I could get a pic of this! The code reads that the two pipes (air sources) must be independent of each other though. We operate off of 2003 code where I live but its hard to tell if anyone is really following it
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