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Old 12-05-2011, 07:02 PM   #1
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Outside Combustion Air


I recently converted my garage to a family room. I sectioned one area for a utlility room with my two propane furnances and a new propane tankless water heater (10' x 8' room). The building inspector said I needed to install 2 outside vents (5" x 10") or a 10" x 10" vent into the door. I installed the outside vents, now I have a cold draft and dirt from the outside. My questions are - is it ok to block the outside vents? If I do are there any risk (CO2)? Would a 10" x 10" vent to the attic work? How will I know if I don't have enough combustion air?

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Old 12-05-2011, 07:07 PM   #2
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Outside Combustion Air


if they are newer furnaces they should down if combustion air drops to a certain level... and try to restart a few times then lock out...giving you led codes to help trace out problem...if inspector wanted them he must of had a good reason for such....how long have furnaces been running there...how old are they..

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Last edited by ben's plumbing; 12-05-2011 at 07:55 PM.
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Old 12-05-2011, 07:36 PM   #3
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Outside Combustion Air


I take it these furnaces and water heater rely on inside air for combustion? The general rule of thumb is you need 50 cubic feet of volume per 1000 BTHU of burners although local codes may vary. Otherwise you need those openings in the door or to the outside... or replace everything with high efficiency furnaces and water heater that draw the combustion air directly from the outside via dedicated pipes.
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Old 12-05-2011, 07:59 PM   #4
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Outside Combustion Air


The inspector said I needed 100 sq. in for outside air. I have that much with the space under the door (4" x 30") from the utility room to the family room. What if I leave the vents shut, how will I know if I am not getting enough air? Is there a health hazard? The furnaces have been running for a month now. They are 10 years old.
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Old 12-05-2011, 08:00 PM   #5
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Outside Combustion Air


You need combustion air. So you can't close off those vents.
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Old 12-05-2011, 08:08 PM   #6
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Outside Combustion Air


as well agreeded upon you need combustion air.....as far as where you get it from ..we have used the attic in the past ... and we used crawl spaces..
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Old 12-06-2011, 10:45 AM   #7
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Outside Combustion Air


Thanks for all the input. But how will i know if i don't have enough combustion air? Everything seems to be working fine.
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Old 12-06-2011, 11:10 AM   #8
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Outside Combustion Air


tape a sheet of newspaper inside over the outsie air vent the start th ea furnaces and the HWH see if it drafts the paper off the grill any movement it is drafting.i had a 80yr old SEARS store in downtown Brooklyn N.Y. and they had a stand alone boiler gas fired million BTUs for the house.. in the basement 20' ceilings and the room was 30' square...they had a 3/4 HP fan motor with a squirrel cage up on a wall mounted shelf discharging right from an outside grill blowing out into this room...could see you breath in there on Christmas eve....but specs called for it...but you will chill that room out for sure.see what happens with that paper test.... and whatever is off cold air will be dropping down into the heat exchangers...
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Old 12-06-2011, 12:52 PM   #9
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Outside Combustion Air


I will do that, thank you..
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Old 12-06-2011, 01:06 PM   #10
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Outside Combustion Air


Understand this is mostly just a demonstration for you of what these guys have been telling you.
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Old 12-06-2011, 01:31 PM   #11
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Outside Combustion Air


if you had a Super Bowl party in that room with 20 people and a all day feast,nobody is going to faint from lack of air.the inspector is just concerned(covering his A**)with the furnaces and they(no the people but the furnaces) won't strave for air ever...mount a dryer vent between the 2 furnaces little piec of dryer vent pipe and your safe with minimal chilling effect of the room...if your not ducted from the space into the room on the return and it's louvered doors catching any outside air 20F mix on that return your mixing it with the 65F-70F return air an making the furnace run longer $$$$$ ask the inspector about that one

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Old 12-06-2011, 04:47 PM   #12
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Outside Combustion Air


I think I will close off my outside vents and put a 10" x 10" register above my furnaces into the attic and stop worrying. Thank for the post. Have a great Christmas Holiday.
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Old 12-06-2011, 10:44 PM   #13
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Outside Combustion Air


attic air there you go....

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