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An associate and I are working to resolve a problem identified by a home inspector as "inadequate combustion air in mechanical room" where 2 gas furnaces and 2 water heaters are located. According to the inspector, the problem can be fixed by installing a vent into the room from the finished area, in this case a large finished basement, at a height of 1' foot above the floor. We have also been advised by a HVAC contractor that we could also fix this by installing a vent in the cold air plenum just before it enters the furnace. The second approach seems like an easier way to go since we wouldn't have to cut through any walls to the finished part of the basement. However, we are concerned, that since the cold air plenum is at negative pressure compared to the room, this approach could actually reduce the amount of combustion air. To make things more interesting, there are currently 4" vents from the warm air plenums of both furnaces which blow air into the mechanical room. Seems like these vents already are providing ventilation for combustion because of the positive pressure in the warm air plenum. Appreciate any comments as soon as possible since the house is under contract for sale and this issue needs to be resolved before closing. TIA
 

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DGR,IABD
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There is already code in the IRC and identical language in the Fuel Gas code that addresses this. The inspector's suggestion is the only one that will meet these codes. The other guys are just suggesting a rigging. IRC §M1702.2 states that for a confined space that contains heating appliances, you need TWO vents. One vent one foot off the floor, and another vent one foot from the ceiling. Each vent needs to be one square inch per 1,000 BTU of heating appliances in the room. For instance, if you have two 50,000 btu furnaces and two 10,000 btu water heaters, each vent would need to have at least 120 square inches of free area.
 

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Cutting into the return is a big no no, as it will cause a neg pressure in the room of the applainces and in trun pull the flue gas back into the return.
In short words, it will use the flue as a air inlet.

BJD
 

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Mdshunk and BJD are correct. You do not use your household air from a supply or return to give combustion air. Do as your home inspector recommended and follow the codes.
 

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your best bet is to pull a 6" flex pipe from out side for your combi air and drop it right beside the furnace.. 12 " off the floor with 6 " hard pipe inside end of the flex pipe. or. at lest thats the way the codes are out here in michigan. and it work real well. hope that helps up. if you have any other questions e mail me at [email protected], thanks , ...tim...
 

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Casey's HVAC said:
your best bet is to pull a 6" flex pipe from out side for your combi air and drop it right beside the furnace.. 12 " off the floor with 6 " hard pipe inside end of the flex pipe. or. at lest thats the way the codes are out here in michigan. and it work real well. hope that helps up. if you have any other questions e mail me at [email protected], thanks , ...tim...

I did this for combustion air into my mechanical room when I built my place. I also added an automatic damper in the vent so it would only open when the boiler was firing and stay closed to keep out the cold air when the boiler shut off.
 
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