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  1. #1
    cts49641's Avatar
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    Default High Efficiency Furnace Combustion intake from inside or outside?

    Hello, I hope someone can lend some advice to a homeowner. I recently had a new high efficiency furnace installed, a Trane XC-95. It was originally installed with the exhaust exiting the side of the house via PVC pipe and the combustion intake drawing air from inside the basement with approx a 1 foot PVC curved elbow that is open on the end. The basement is a finished basement and the house is a 1929 brick tudor home that I would like to retain it's historic value and not poke many holes in it.

    Come inspection time by the local gas utility company, the installation fails due to "short combustion/ventilation to the basement". The inspector suggested it was best to have the combustion intake pull air from the outside which would require another PVC run that could use a concentric kit so that only one hole is in the house. I can see after the first PVC run that there isn't much room for an additional one. If needed it might have to make a turn or two. Does this pose a problem?

    The installer now says to put a vent in the door on the main floor that opens to the basement. Or, remove the door all-together....this is NOT an option. I'm not pleased with either of these options. First I don't want to ruin an original 1929 interior door with a flimsy metal vent. Nor, do I want to remove a door that is used often for privacy and/or sound control.

    I might add that in this small furnace room, there is a louvered door used to access the room. The furnace also shares this room with 2 40-gallon gas water heaters (vented through the chimney) and a side-by-side refrigerator.

    Here is where I am looking for the advice...
    1. Does it matter where the furnace pulls it's combustion air?
    2. Does either inside or outside combustion air have any effect on the furnaces efficiency? Is one better than the other in terms of efficiency.
    3. Is there a back-drafting issue I should be concerned with?
    4. Lastly, and most importantly....Is my installer trying to take a shortcut?

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  2. #2
    Ron Bibler's Avatar
    Ron Bibler Guest

    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    You may want to start with the manufacturer and email these question to them.
    I would go on-line to there web-site and you will find a contact us section.

    Best

    Ron


  3. #3
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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    The installer is required to leave the installation booklet with the unit.

    Read thru the booklet; there are probably several options on supplying combustion air to the unit.

    Also, the total btu's (both furnace and water heater) are need to be added together to determine the amount of air required.

    Darren www.aboutthehouseinspections.com
    'Whizzing & pasting & pooting through the day (Ronnie helping Kenny helping burn his poots away!) (FZ)

  4. #4
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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    Generally you can pull combustion air either from the interior or exterior. I ALWAYS tell people to pull from the exterior. Exterior combustion air draw eliminates the useless wasted time that you are now going through. If the basement is open it isn't a big deal, there's usually plenty of air. In finished basements, furnace enclosures rarely have enough open air space (wall/door vents) to meet BTU requirements.
    Of course your contractor is taking a shortcut. What was stated in the contract? Did he say he would run it outside or not? If he didn't you may need to pay. However, if combustion air in the room is clearly insufficient then you could hold his feet to the fire for performing a non-compliant install.
    From what you describe in your situation, attempting to pull combustion air from the room is a bad idea. You've already got two appliances drawing air and apparently not much ventilation to the room.
    Run the pipe outside and plant a nice topiary in front of it or paint it the same color as the house. Combustion air pipe is passive. I'm not aware of elbow restrictions on the combustion air side.
    Look up the manual for the unit and read it for model specific info.

    www.aic-chicago.com
    773/844-4AIC
    "The Code is not a ceiling to reach but a floor to work up from"

  5. #5
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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    Quote Originally Posted by cts49641 View Post
    Hello, I hope someone can lend some advice to a homeowner. I recently had a new high efficiency furnace installed, a Trane XC-95. It was originally installed with the exhaust exiting the side of the house via PVC pipe and the combustion intake drawing air from inside the basement with approx a 1 foot PVC curved elbow that is open on the end. The basement is a finished basement and the house is a 1929 brick tudor home that I would like to retain it's historic value and not poke many holes in it.

    Come inspection time by the local gas utility company, the installation fails due to "short combustion/ventilation to the basement". The inspector suggested it was best to have the combustion intake pull air from the outside which would require another PVC run that could use a concentric kit so that only one hole is in the house. I can see after the first PVC run that there isn't much room for an additional one. If needed it might have to make a turn or two. Does this pose a problem?

    The installer now says to put a vent in the door on the main floor that opens to the basement. Or, remove the door all-together....this is NOT an option. I'm not pleased with either of these options. First I don't want to ruin an original 1929 interior door with a flimsy metal vent. Nor, do I want to remove a door that is used often for privacy and/or sound control.

    I might add that in this small furnace room, there is a louvered door used to access the room. The furnace also shares this room with 2 40-gallon gas water heaters (vented through the chimney) and a side-by-side refrigerator.

    Here is where I am looking for the advice...
    1. Does it matter where the furnace pulls it's combustion air?
    2. Does either inside or outside combustion air have any effect on the furnaces efficiency? Is one better than the other in terms of efficiency.
    3. Is there a back-drafting issue I should be concerned with?
    4. Lastly, and most importantly....Is my installer trying to take a shortcut?
    It sounds like the room is not supplying the required combustion make-up air. For every BTU you need 50 cu.ft of air space. So if you have 100,000 BTU of burners in that room it needs to be 625sf with an 8' ceiling.

    You might need to go with electric water heaters!

    Scott Patterson, ACI
    Spring Hill, TN
    www.traceinspections.com

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    Post Re: High Efficiency Furnace Combustion intake from inside or outside?

    Here are some short answers to your questions:

    1. No
    2. Yes
    3. Yes
    4. No

    Based on the information provided, the first step should be to consult the installation manual for the appliance. Second, invite the installer out to determine how to effect a correction. It is likely that a fix can be executed without adding a hole to the house envelope, though the current vent hole may need to be enlarged. Third, before you set the manual down after reading it from step #1, pick up the telephone to contact your attorney. You will want to make sure that you don't shoot yourself in the foot as you proceed to find a way through this muck. Fourth, file your lessons learned for future reference. Good luck.


    Randall Aldering GHI BAOM MSM
    Housesmithe Inspection
    www.housesmithe.com

  7. #7
    David Bell's Avatar
    David Bell Guest

    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    The concentric kit will only require that you enlarge the existing vent opening one pipe size,,If you have a 2" exhaust you will need a hole to accept a 3" concentric vent,, If it's a 3" now you will need a 4". Your contractor should have done the calculations to support his decision to pull combustion air from the basement,,ask to see them. Also be sure that the vent terminates above any expected snowfall and not near any means of egress or air intake or exhaust,,such as a dryer vent.


  8. #8
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    Angry Furnace combustion intake in attic.

    Came across this today. Two 6" intake air vents from the second floor furnace cubicle thru the ceiling floor to exit about one foot into the attic. I question the validity of this venting practice for two reasons: 1. Don't you want the air to flow out of the attic thru the roof? 2. Only three vents on roof with a total of 192 sq in total. The furnace cubical isn't air tight but it does not have louvers in the doors. There are also soffet vents.

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    Last edited by Bob Fuhrmann; 02-12-2011 at 05:34 PM. Reason: Additional info

  9. #9
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    Default Re: Furnace combustion intake in attic.

    Quote Originally Posted by Bob Fuhrmann View Post
    Came across this today. Two 6" intake air vents from the second floor furnace cubicle thru the ceiling floor to exit about one foot into the attic. I question the validity of this venting practice for two reasons: 1. Don't you want the air to flow out of the attic thru the roof? 2. Only three vents on roof with a total of 192 sq in total. The furnace cubical isn't air tight but it does not have louvers in the doors. There are also soffet vents.
    That is a good question.
    Hard to say if drawing air down for combustion would even effect the attic air flow of soffit to ridge vent or not, but maybe someone here knows the answer.
    I hate when I have seen it just on the principle it reminds me of putting a hole to the outside of your house ,since the attic should be close to outside air temperature in theory at least.


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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    Quote Originally Posted by Scott Patterson View Post
    ... For every BTU you need 50 cu.ft of air space. So if you have 100,000 BTU of burners in that room it needs to be 625sf with an 8' ceiling.
    That's for every 1000 BTUs.
    I know you know this, Scott, but the OP may not.

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
    www.ArnoldHomeInspections.com

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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    IMO, it's crazy not to use the exterior combustion air vent option. You're paying to heat the air inside the house, and then sending it out the flue when the furnace kicks on. Your most efficient installation is going to be with exterior combustion air in a direct vent configuration. There is also a possibility that you will back draft the water heaters, depending on how big the room is.

    Apart from that, it sounds like the combustion air in the room is the main factor. You need the proper amount for the furnace and both water heaters, one way or another.

    Jim Robinson
    New Mexico, USA

  12. #12
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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    Those two 6" vents have to be DIY. Doesn't make sense that an HVAC guy would do that. Considering that the combustion air pipe for a furnace is 2" or 3" depending on BTU, I don't see an HVAC guy putting in two 6" like that. I would recommend removal and proper installation.
    - Since combustion air intake is passive, in a proper install I doubt the pipe into the attic would effect normal attic vent circulation. With those two pipes it is a possibility.
    - My guess is a lot of cold air will drop down into those and cool the space below beyond just providing combustion air.
    - Great squirrel access
    - My big concern with not running combustion air pipe to the outside is always the problem of providing a path for fire to travel into that space.

    www.aic-chicago.com
    773/844-4AIC
    "The Code is not a ceiling to reach but a floor to work up from"

  13. #13
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    Default Re: Furnace combustion intake in attic.

    Quote Originally Posted by Bob Fuhrmann View Post
    Came across this today. Two 6" intake air vents from the second floor furnace cubicle thru the ceiling floor to exit about one foot into the attic. I question the validity of this venting practice for two reasons: 1. Don't you want the air to flow out of the attic thru the roof? 2. Only three vents on roof with a total of 192 sq in total. The furnace cubical isn't air tight but it does not have louvers in the doors. There are also soffet vents.
    Bob Fuhrmann, you don't give your location or if you are an inspector or home owner but be aware that combustion air installations have a wide degree of variability and is greatly influenced by you home design and furnace configuration. Also regional variations are common, usually just because that is what the builder is familiar with which can be greatly influenced by what "works" in the area. The picture you show IS an acceptable practice according to the building code (2003 IRC G2407.6.1) and is very common in my area. The calculations as far as duct size cannot be done with the information you have provided here, but in general, I would want to see all of the combustion air provided from the attic with weather stripping to seal the furnace closet door so that in effect the closet becomes part of the exterior to avoid air flow around the door and out into the attic. The attic would need to be vented to outdoors at least equal to the furnace venting requirement which is usually not a problem when dealing with a vented attic counting both soffit and roof vents.

    Obviously, a high eff. furnace with outdoor combustion air piped to the unit is the gold standard but the code allows many variations.

    Last edited by Jim Luttrall; 02-13-2011 at 10:48 AM. Reason: added code ref.
    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

  14. #14
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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    Quote Originally Posted by John Arnold View Post
    That's for every 1000 BTUs.
    I know you know this, Scott, but the OP may not.
    Oops, a two year old typo rears it's ugly head! Good catch!

    Scott Patterson, ACI
    Spring Hill, TN
    www.traceinspections.com

  15. #15
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    Default Re: Furnace combustion intake in attic.

    Quote Originally Posted by Jim Luttrall View Post
    Bob Fuhrmann, you don't give your location or if you are an inspector or home owner but be aware that combustion air installations have a wide degree of variability and is greatly influenced by you home design and furnace configuration. Also regional variations are common, usually just because that is what the builder is familiar with which can be greatly influenced by what "works" in the area. The picture you show IS an acceptable practice according to the building code (2003 IRC G2407.6.1) and is very common in my area. The calculations as far as duct size cannot be done with the information you have provided here, but in general, I would want to see all of the combustion air provided from the attic with weather stripping to seal the furnace closet door so that in effect the closet becomes part of the exterior to avoid air flow around the door and out into the attic. The attic would need to be vented to outdoors at least equal to the furnace venting requirement which is usually not a problem when dealing with a vented attic counting both soffit and roof vents.

    Obviously, a high eff. furnace with outdoor combustion air piped to the unit is the gold standard but the code allows many variations.
    Yes Jim, I am a certified inspector in Colorado. Thanks for the building code reference. This is at best an 80% unit. I am going to recommend to the client that they seal the furnace cubical door to essentially do what you say 'make the cubical a part of the exterior'. When I explain to them that you have heated air from the house exiting thru the unsealed door and into the attic, they should be convinced. Thanks again.
    Bob Fuhrmann


  16. #16
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    Default Re: High Efficiency Furnace Combustion intake from inside or outside?

    Bob;

    It's also common here in NJ with townhouses or condos with the mechanical room in the middle of the unit.
    The termination in the room would require one vent located high and one vent located low.

    Darren www.aboutthehouseinspections.com
    'Whizzing & pasting & pooting through the day (Ronnie helping Kenny helping burn his poots away!) (FZ)

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