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  1. #1
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    Default sealed crawl space and gas appliances

    House was built in 1989 and someone has sealed the crawl space. 5 year old condensing gas furnace, not direct vent, (~70K btu) and original gas water heater in crawl. Crawl is very large with a walk in area that has an approximate 800 sq ft room with 7' ceiling. Remainder of crawl varies from 3 - 4' and is roughly 2,000 sq ft. That is a lot of volume but there are no methods for combustion air entry. Are there exceptions to the make up air rules based on volume of the "room"?

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  2. #2
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    Jul 2007
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    Chicago
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    Default Re: sealed crawl space and gas appliances

    Here is a reply from Peoples gas calculation guide.

    Calculations
    Confined vs. Unconfined Space
    In new or rehab construction, the most important factor
    governing the installation of gas appliances is whether or not the
    room is classified as a confined or unconfined space. The only
    way to determine whether or not a room is a confined or
    unconfined space is to perform a room volume calculation
    using:
    1) The dimensions of that specific room.
    2) The total Btu per hr input rating of the appliance(s) to be
    installed in the room.
    3) Method #1, Method #2, or Method #3. (See pages 40-42).
    Note: An adequate amount of air is required for safe gas appliance
    operation. The use of exhaust fans, kitchen ventilation systems,
    clothes dryers, bathroom fans, fireplaces, and other fan-assisted
    devices may adversely affect the safe operation of gas-burning
    appliances. Consequently, extra air may be required when
    calculating air requirements for combustion and dilution when
    fan-assisted devices are present.
    Sample Volume Calculation
    Room: 8' H x 33' L x 25' W
    Room Volume = 6,600 ft3
    Appliances: Gas Furnace-Input Rating of 100,000 Btu per hr
    Gas Hot Water Heater-Input Rating of
    32,000 Btu per hr
    Total Btu input rating = 132,000 Btu per hr

    Method #1
    (Total Btu per hr
    input of all appliances) x ( 50 ft3 ) Minimum room
    (1,000 Btu/hr) volume to be
    classified as an
    unconfined
    space.
    (132,000 Btu/hr) x (50 ft3)
    (1,000 Btu/hr)
    A room containing a 100,000 Btu per hr gas furnace and a 32,000
    Btu per hr gas hot water heater shall have a minimum room
    volume of 6,600 ft3 in order to be classified as an unconfined
    space.
    A room containing a 100,000 Btu per hr gas furnace and a 32,000
    Btu per hr gas hot water heater that has a volume less than 6,600
    ft3 is classified as a confined space.

    Method #2
    (Room volume in ft3) x (20 Btu) = Maximum total Btu per hr
    input of all appliances that this
    room volume will support.
    (6,600 ft3) x (20 Btu) = 132,000 Btu per hr
    A 6,600 ft3 room (8' H x 33' L x 25' W) is large enough and has
    sufficient air volume to support gas appliances with a maximum
    total Btu per hr input rating of 132,000 Btu per hr.
    A 6,600 ft3 room (8' H x 33' L x 25' W) containing gas appliances
    with a maximum total Btu per hr input rating of 132,000 Btu per
    hr is classified as an unconfined space.
    A 6,600 ft3 room (8' H x 33' L x 25' W) containing gas appliances
    with a total Btu per hr input rating greater than 132,000 Btu per
    hr is classified as a confined space.

    Method #3
    (Room volume in ft3) x 1,000 = Maximum total Btu per hr input
    (50 ft3/Btu) of all appliances that this room
    volume will support.
    (6,600 ft3) x 1,000 = 132,000 Btu
    (50 ft3/Btu)
    A 6,600 ft3 room (8' H x 33' L x 25' W) is large enough and has
    sufficient air volume to support gas appliances with a maximum
    total Btu per hr input rating of 132,000.
    A 6,600 ft3 room (8' H x 33' L x 25' W) containing gas appliances
    with a maximum total Btu per hr input rating of 132,000 is
    classified as an unconfined space.
    A 6,600 ft3 room (8' H x 33' L x 25' W) containing gas appliances
    with a total Btu per hr input rating greater than 132,000 is
    classified as a confined space.
    Most appliance rooms in residential construction are confined
    spaces. There are special air requirements for the appliances
    contained in these rooms. (For more details, see Air Requirements
    on page 13.)

    Hope that makes sense.


  3. #3
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    Default Re: sealed crawl space and gas appliances

    It doesn't really make sense. If it is a totally sealed crawl space, the oxygen is going to get used up no matter what the volume. I guess that the worst that could happen is the oxygen would be gone and the burners would no longer ignite.


  4. #4
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    Default Re: sealed crawl space and gas appliances

    Quote Originally Posted by Benjamin Thompson View Post
    It doesn't really make sense. If it is a totally sealed crawl space, the oxygen is going to get used up no matter what the volume. I guess that the worst that could happen is the oxygen would be gone and the burners would no longer ignite.
    I doubt it is hermetically sealed.
    Some air is always bound to get in.


  5. #5
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    Default Re: sealed crawl space and gas appliances

    In most cases. I have seen a couple that had concrete floors and urethane foam sprayed on the walls and ceiling. I think I would have concerns about gas appliances in those no matter the size.


  6. #6
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    Default Re: sealed crawl space and gas appliances

    Quote Originally Posted by Randall Clark View Post
    Are there exceptions to the make up air rules based on volume of the "room"?
    Some places allow use of combustion air systems for make up in areas without enough combustion air. I would think the current system is quite inefficient by pulling in room air from the ducts and leaks from the heated space to be used for combustion.


  7. #7
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    Default Re: sealed crawl space and gas appliances

    one thing to try is obseving the flames when door to entrance is open vs closed. having door closed for a while then opening it and observing the flames might tell you if the area is getting air from there or somewhere else. if there is a difference then suggest that they get some outside supply air coming in but maybe an air exchanger might be needed to warm up the outside air before it is used by the furnace and water heater. otherwise refer to a qualified furnace person. knowing the sizes of the room does not make the whole picture. know where the air supply for the combustion is coming from is what really matters.


  8. #8
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    Default Re: sealed crawl space and gas appliances

    Either way make sure to have a Carbon Monoxide detector under 5 years old down there.


  9. #9
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    Feb 2009
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    Indiana
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    Default Re: sealed crawl space and gas appliances

    Bob,

    Using the methods you have listed for determining if a room is large enough to support combustion is only allowed if it is KNOWN that the ACH (air changes per hour) are greater than .4. If the area is sealed, it is likely the air changes are less than .4, and another calculation must be used, which often results in a MUCH larger area being required.

    For fan assisted appliances:
    Required Volume >= 15cu. ft/ ACH * BTU of appliance/1000

    For non-fan assisted:
    Required volume >= 21cu. ft/ ACH * BTU of appliance/1000

    For the subject appliances (assuming the minimum ACH of .4, and since the original poster didn't disclose the water heater size, assume a standard 40,000 BTUh)

    Furnace: 15/ .4 * 70,000 BTUh / 1000 = 2625 cubic feet

    Gas water heater: 21/ .4 * 40,000BTUh/ 1000 = 2100 cubic feet

    Totaled together = 4725 cu. ft.

    If the air changes per hour are only at .3, it goes up to 6300 cu. ft., and at .2 ACH, the requirement goes to 9450 cu. ft.

    Biq question is, just how "sealed" is the room?
    Also, does the appliance "room" exchange air freely with the rest of the crawl space? If it does, the rest of the crawls area can be added to the appliance room volume.


  10. #10
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    Default Re: sealed crawl space and gas appliances

    Quote Originally Posted by Jerry Shipman View Post
    Bob,

    Using the methods you have listed for determining if a room is large enough to support combustion is only allowed if it is KNOWN that the ACH (air changes per hour) are greater than .4. If the area is sealed, it is likely the air changes are less than .4, and another calculation must be used, which often results in a MUCH larger area being required.

    For fan assisted appliances:
    Required Volume >= 15cu. ft/ ACH * BTU of appliance/1000

    For non-fan assisted:
    Required volume >= 21cu. ft/ ACH * BTU of appliance/1000

    For the subject appliances (assuming the minimum ACH of .4, and since the original poster didn't disclose the water heater size, assume a standard 40,000 BTUh)

    Furnace: 15/ .4 * 70,000 BTUh / 1000 = 2625 cubic feet

    Gas water heater: 21/ .4 * 40,000BTUh/ 1000 = 2100 cubic feet

    Totaled together = 4725 cu. ft.

    If the air changes per hour are only at .3, it goes up to 6300 cu. ft., and at .2 ACH, the requirement goes to 9450 cu. ft.

    Biq question is, just how "sealed" is the room?
    Also, does the appliance "room" exchange air freely with the rest of the crawl space? If it does, the rest of the crawls area can be added to the appliance room volume.
    I agree but am going with Peoples Gas because they know a hell of a lot more than I do.
    Still C/O unit is well advised and I would sure hate to be crawling around down there if not sure.

    One only needs to remember ventless fireplaces are used in large volume rooms yet it is always advised to have a fresh air source.


  11. #11
    Ted Menelly's Avatar
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    Default Re: sealed crawl space and gas appliances

    No ventilation to the exterior from the crawl space is just as bad as no combustion air source. There needs to be both in this area same as a closet. You may be able to draw enough combustion air but I doubt it is adequate. Flue leaks, gas leaks in confined spaces. Yes put a carbon monoxide detector in but add some ventilation and combustion air source.

    You as an inspector has no positive way of determining if in fact there is enough of either combustion air or ventilation. You would be guessing at best. I doubt it but someone may have done a bang up job sealing this space off.


  12. #12
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    Default Re: sealed crawl space and gas appliances

    Quote Originally Posted by Ted Menelly View Post
    No ventilation to the exterior from the crawl space is just as bad as no combustion air source. There needs to be both in this area same as a closet. You may be able to draw enough combustion air but I doubt it is adequate. Flue leaks, gas leaks in confined spaces. Yes put a carbon monoxide detector in but add some ventilation and combustion air source.
    In the Southeastern and Northeastern US ventilation in the crawl space is inadvisable due to humidity. Given that there are fuel burning appliances, a combustion air source is likely to be required for the appliances, suggesting need for an HRV or other device which can admit air but also control for humidity.


  13. #13
    Ted Menelly's Avatar
    Ted Menelly Guest

    Default Re: sealed crawl space and gas appliances

    Quote Originally Posted by Corn Walker View Post
    In the Southeastern and Northeastern US ventilation in the crawl space is inadvisable due to humidity. Given that there are fuel burning appliances, a combustion air source is likely to be required for the appliances, suggesting need for an HRV or other device which can admit air but also control for humidity.
    Actually the ventilation in a crawl whether it be Southeast or Northeast would be detemined by the home itself. Trapped moisture in a crawl if it is wet is far worse than humid air travelling through the crawl space. Retrofitting old homes as in sealing off a crawl space is not cheap and if not done well can have an adverse affect. As far as a furnace and water heater in a crawl space with no outside air coming in for combustion and no ventilation for the in-case situation can be extremely hazardous.

    We also cannot determine if there is enough air leaking into a crawl space for that equipment. (obviously there must be because the units have been running for 5 years) We as home inspectors are also not the ones to determine whether there is adequate ventilation in those crawl spaces for the what ifs so all we can do is advise further evaluation to our explanation as to why we are writing it all up. 2000 square feet with a stand up area and the rest is crawl that is 3 to 4 ft high? Are we really going to do all those calculations? We see 2 gas fired units and see no incoming combustion or outgoing ventilation? "It has been sealed, as in fairly recently" Has been sealed could mean extremely well or just a crappy job and air leaking all over the place. We just do not know.

    All we can do for the future home owner, our client, is look out for the health and well being in this particular situation.

    We see a newly sealed crawl space, no ventilation and no combustion air source. We must write up everything above. Write up everything above as in that is what I do to let them know why I am writing it up including the part that the system appears to have been functioning for the past years and is now since it has been sealed. Give the client as much info a needed and necessary. Oh yeah, Realtors love that, right?


  14. #14
    Join Date
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    Default Re: sealed crawl space and gas appliances

    The problem in the Northeast and Southeast is that it is very rare for the air in a crawl space to be more humid than the air outside of it. The idea of ventilating crawl spaces as a matter of course is long outdated; building scientists have been saying for decades that the decision needs to be region-specific.

    That said, the appliances need combustion air. The best way to provide that air without introducing unwanted humidity is through a device that can control for both. An HRV is one such option.

    But getting back on point, I'm curious about this condensing gas furnace that doesn't have a source of combustion air. What category is it? Does it have a combustion air inlet that might be supplied with piping from the exterior? What material is the flue made of and where and how does it discharge? How is the condensate discharged?


  15. #15
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    Default Re: sealed crawl space and gas appliances

    Beside the furnace there is an old 'original' gas water heater. I would simply call for the addition of a combustion air vent, in this case, maybe a 5" round duct from the exterior to the furnace area.

    Then if somebody wants to get technical, or simply ignore the recommendation or install an HRV system, the inspector did his job.
    The bit of humidity is irrela-vent (pun), because the air get burned in the appliance. Correct sizing is important, but you need to know the BTU's for that.

    John Kogel, RHI, BC HI Lic #47455
    www.allsafehome.ca

  16. #16
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    Default Re: sealed crawl space and gas appliances

    someone has sealed the crawl space
    How was it sealed?
    Was there open ceiling joists or finished ceiling?
    Where the rim joists cavities/sill plate insulated?
    Any duct work running from furnace into the area?


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