Power User Conference


Results 1 to 19 of 19
  1. #1
    David Samloff's Avatar
    David Samloff Guest

    Default Combustion Air in Crawl Space

    I inspected a three year old home that has a sealed and conditioned crawl space. In the crawl, is a 90% Lennox gas furnace with no additional source of combustion air. The unit is not only in a "confined space" as defined in M1702.2, I argue that it is a "confined space of unusually tight construction", M1702.3, the size of the crawl is so close to the 50 cu.ft./1k BTU that its hard to tell if it would pass, but I argue that there is no air infiltration as the crawl is pressurized by the unit which will not allow for infiltration. I believe that unit may need to be replaced with a direct vent unit because if you open the crawl to the exterior, you loose the pressurized crawl space. Any ideas?

    Similar Threads:
    RecallChek

  2. #2
    Richard Rushing's Avatar
    Richard Rushing Guest

    Default Re: Combustion Air in Crawl Space

    Welcome to the board. Sounds like you've pretty much got it nailed.

    Question:

    Are you saying the crawlspace is smaller than a 5 x 5 x 2'(deep)?

    That's not a crawl, that's an underground closet without a floor. Me, I've just never seen a crawl that small.

    Rich


  3. #3
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Thanks for the backup Richard. No the crawl space would have to be 3000 cubic feet to properly support combustion for a 60K BTU furnace. The crawl is less than 1500 sq ft @ about 2 feet high on average. The furnace would need 60*50cu ft for proper combustion.


  4. #4
    John Arnold's Avatar
    John Arnold is offline Member
    Join Date
    Mar 2007
    Location
    Philadelphia PA
    Posts
    3,170

    Default Re: Combustion Air in Crawl Space

    I'm confused about the idea of the crawl being pressurized. Wouldn't that be a good thing in terms of combustion air? If the HVAC system is taking air from the living space, and some of that conditioned air is being delivered to the crawl (hence, pressurizing it), then that is at least one source of combustion air, isn't it? I'm not saying it's OK, just trying to understand the problem.

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

  5. #5
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by John Arnold View Post
    I'm confused about the idea of the crawl being pressurized. Wouldn't that be a good thing in terms of combustion air? If the HVAC system is taking air from the living space, and some of that conditioned air is being delivered to the crawl (hence, pressurizing it), then that is at least one source of combustion air, isn't it? I'm not saying it's OK, just trying to understand the problem.
    It is a good thing, as the interior of the foundation walls are insulated and there is a vapor barrier on the ground. The pressure is slightly positive by openings in the supply air ducting. As a source of combustion though, there is not enough going out as is needed to support combustion.


  6. #6
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by Scott Lynch View Post
    Hi Dave,

    Crawl, closet, garage, needs to meet the requirements for combustion air. Sounds like you are on top of it! No pun intended!

    - Did you measure clearances from ground/floor?
    Clearances were fine. The unit sits in a portion of the crawl that is about 42-inches high. The grade slopes under the home.


  7. #7
    Join Date
    Mar 2007
    Location
    Knoxville, TN
    Posts
    2,024

    Default Re: Combustion Air in Crawl Space

    1500 SF crawl, about 2 feet high, but some of it is at least 42" high. I'm going to assume that at the least it's 18" at the lowest. If you claculated it needs at least 3000 cu/ft for combustion, it sounds like there's enough, or close to it.

    While it's good to question the amount of combustion air required, I would be very sure of the actual space available, and confirm there isn't another source of combustion air.

    If it was deficient, could they not run a pvc pipe from the combustion air port to the exterior? I've seen many installations where they have two pipes right until the end, then it joints into a special fitting that allows flow for both combustion and exhaust air.

    JF


  8. #8
    Jerry Peck's Avatar
    Jerry Peck is online now Member
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
    Posts
    22,921

    Default Re: Combustion Air in Crawl Space

    Jack,

    I think this is what David was referring to, and what is the limiting factor:
    - M1702.3 Unusually tight construction.
    Where the space is of adequate volume in accordance with Section M1702.1 or Section M1702.2, but is within a building sealed so tightly that infiltration air is not adequate for combustion, combustion air shall be obtained from outdoors or from spaces freely communicating with the outdoors in accordance with Section M1703.

    Thus, it requires outdoor air, regardless of its size.

    However, being as the space is pressurized by the furnace (as David stated), cutting two openings through to the outdoors would violate the integrity of the space (if I understand David correctly). Thus, installing two openings to the exterior is not an option.



    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  9. #9
    Join Date
    Mar 2007
    Location
    Knoxville, TN
    Posts
    2,024

    Default Re: Combustion Air in Crawl Space

    Jerry,
    I gotcha on the code ref - but the crawlspace may not be unusually tight if there is enough space - right? (X SF x height of crawl = CF of available air)

    If they used a HE unit that gets it's combustion air from outside, and the PVC pipe is sealed tight at the wall penetration, is it still violating the sealed crawlspace?

    JF


  10. #10
    DavidR's Avatar
    DavidR is offline Member
    Join Date
    May 2007
    Location
    Frankfort, KY
    Posts
    326

    Default Re: Combustion Air in Crawl Space

    If there is outside air being introduced into the return side of the duct system with the crawl being pressurized there will be more than enough adequate air to support safe combustion.

    If there is no outside air being introduced you now have a conditioned building envelope that is negative more than likely. This just adds another problem to the mix.

    The only way to verify if that crawl is effecting the combustion process is through combustion testing the furnace.

    Measured Performance more than just a buzzword

  11. #11
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by Jack Feldmann View Post
    If it was deficient, could they not run a pvc pipe from the combustion air port to the exterior? I've seen many installations where they have two pipes right until the end, then it joints into a special fitting that allows flow for both combustion and exhaust air.

    JF
    Jack, this unit has one pipe going to the exterior, the exhaust. The model of this Trane unit does not allow for a direct vent set up.


  12. #12
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by Jerry Peck View Post
    Jack,

    I think this is what David was referring to, and what is the limiting factor:
    - M1702.3 Unusually tight construction.
    Where the space is of adequate volume in accordance with Section M1702.1 or Section M1702.2, but is within a building sealed so tightly that infiltration air is not adequate for combustion, combustion air shall be obtained from outdoors or from spaces freely communicating with the outdoors in accordance with Section M1703.

    Thus, it requires outdoor air, regardless of its size.

    However, being as the space is pressurized by the furnace (as David stated), cutting two openings through to the outdoors would violate the integrity of the space (if I understand David correctly). Thus, installing two openings to the exterior is not an option.

    Jerry, you have the code section right and if I read it as you have, it does not matter what size the space is based upon this section.

    You also hit it correctly that if you open the crawl space to the exterior, you eliminate the pressurized crawl space.

    The only way out of this seems to be to have the furnace replaced with a direct vent type.


  13. #13
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by DavidR View Post
    If there is outside air being introduced into the return side of the duct system with the crawl being pressurized there will be more than enough adequate air to support safe combustion.

    If there is no outside air being introduced you now have a conditioned building envelope that is negative more than likely. This just adds another problem to the mix.

    The only way to verify if that crawl is effecting the combustion process is through combustion testing the furnace.
    David, there is no additional outside air in the system, other than what will be infiltrated into the home. The negative is a potential problem that we proposed to the buyer.

    As far as combustion testing, this is a little beyond our scope and may only become evident after an extended period of heating, several hours possibly.


  14. #14
    David Samloff's Avatar
    David Samloff Guest

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by Jack Feldmann View Post
    Jerry,
    If they used a HE unit that gets it's combustion air from outside, and the PVC pipe is sealed tight at the wall penetration, is it still violating the sealed crawlspace?

    JF
    The existing unit is a mid-efficiency unit that dumps via a PVC pipe on the exterior now, but this particular Trane unit does not accept a combustion air snorkel. A HE unit would work in this case, but the seller is resisting replacement.


  15. #15
    Jerry Peck's Avatar
    Jerry Peck is online now Member
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
    Posts
    22,921

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by Jack Feldmann View Post
    Jerry,
    I gotcha on the code ref - but the crawlspace may not be unusually tight if there is enough space - right? (X SF x height of crawl = CF of available air)
    Jack,

    If it is 'unusually tight construction', there is no 'enough space', the calculation does not matter any more.

    The reason for the calculation is to include more space which could help with the breathing of the appliance within that space, however, with unusually tight construction, it is acknowledged that, by definition of being unusually tight construction, that additional breathing is not going to happen.

    Most modern homes are 'unusually tight construction' nowadays.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  16. #16
    DavidR's Avatar
    DavidR is offline Member
    Join Date
    May 2007
    Location
    Frankfort, KY
    Posts
    326

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by David Samloff View Post
    David, there is no additional outside air in the system, other than what will be infiltrated into the home. The negative is a potential problem that we proposed to the buyer.

    As far as combustion testing, this is a little beyond our scope and may only become evident after an extended period of heating, several hours possibly.
    The cheapest solution for these homeowners David is a fresh air duct to the return plenum with a weighted damper setup on the duct then adjusted to bring the main building envelope to a slight positive or neutral pressure depending on location.

    A one pipe system done this way will actually work more efficient & safer than a two pipe system.

    The combustion testing would more than likely reveal the problem withing 15 minutes if the one taking the readings knows how to interpret them.

    Good luck.

    Measured Performance more than just a buzzword

  17. #17
    Join Date
    Mar 2007
    Location
    Chicago, IL
    Posts
    2,786

    Default Re: Combustion Air in Crawl Space

    David,

    Sounds like a recommendation for combustion testing is in order.

    Is there a written HVAC industry standard HIs can reference to insure that the HVAC tech performing the test is qualified to do so, and that the test is being performed properly, as in:

    "performed by a XXXXX certified technician in accordance with XXXXXX standard XXXXXX")" ?

    ... around here there are HVAC techs who (hard as this may be to believe) seem to understand these issues even less than I do...


  18. #18
    DavidR's Avatar
    DavidR is offline Member
    Join Date
    May 2007
    Location
    Frankfort, KY
    Posts
    326

    Default Re: Combustion Air in Crawl Space

    Quote Originally Posted by Michael Thomas View Post
    David,

    Sounds like a recommendation for combustion testing is in order.

    Is there a written HVAC industry standard HIs can reference to insure that the HVAC tech performing the test is qualified to do so, and that the test is being performed properly, as in:

    "performed by a XXXXX certified technician in accordance with XXXXXX standard XXXXXX")" ?

    ... around here there are HVAC techs who (hard as this may be to believe) seem to understand these issues even less than I do...
    The standards I go by Michael are standard protocol written for anybody certified by the National Comfort Institute in CO/Combustion analysis..

    Jim Davis is the guy you need to talk with there as he had a great deal of input on the national protocol that is in print.
    He is the gentlemen who trained me on the subject, I set out to prove the guy wrong as what he teaches is over the top.
    Over a period of six years that testing has turned me into one of his biggest supporters as well as a close friend, he is dead on in his material.

    Unfortunately combustion related issues in the HVAC field are usually a head in the sand thing, people think if they ignore the issue it will go away.

    If enough of us keep pushing then the severity of the problem will be heard, until then assumptions & rationalization will rule.

    Measured Performance more than just a buzzword

  19. #19
    David Samloff's Avatar
    David Samloff Guest

    Smile Re: Combustion Air in Crawl Space

    There is a good ending to this situation. The company that installed the original system went to the home today. There first phone call was to me wanting to know how I could think that there was a problem with "their" installation. After talking with them for a few minutes and explaining the difference between a "confined space" and "unusually tight construction" the light went off in their head and they realized that they had miscalculated and that this system would not function properly. They provided a recommendation for installing a combustion air pipe connected directly to the furnace. Just as we all seemed to agree upon. The better thing is that since this afternoon when this all happened, I have received phone calls from three of the neighbors to come look at their homes to see if they have the same problem. Seems this HVAC company did a lot of work in this neighborhood and so will I!


Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •