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  1. #1
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    Default Combustion air for gas appliances in the home

    I see a lot of older homes with the WH and Furnace installed in interior closets with vents on the doors.

    I know if a furnace or water heater are installed on the interior of the home the appliance should be installed in a sealed room and the combustion air needs to come from the exterior.

    Why is this required? CO posioning ?
    Or is there a problem with using the interior air for combustion air ?

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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Dan Harris View Post
    I see a lot of older homes with the WH and Furnace installed in interior closets with vents on the doors.

    I know if a furnace or water heater are installed on the interior of the home the appliance should be installed in a sealed room and the combustion air needs to come from the exterior.

    Why is this required? CO posioning ?
    Or is there a problem with using the interior air for combustion air ?
    Not required.

    ' correct a wise man and you gain a friend... correct a fool and he'll bloody your nose'.

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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Dan Harris View Post
    I know if a furnace or water heater are installed on the interior of the home the appliance should be installed in a sealed room ...
    Only applies to bedrooms (sleeping rooms).

    Why is this required? CO posioning ?
    Or is there a problem with using the interior air for combustion air ?
    So you don't wake up dead ...

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Jerry Peck View Post

    So you don't wake up dead ...
    .
    Last time that happened to me , what a headache, disoriented, cotton mouth, blurred vision, unsettled stomach, laying on the floor with a Strange Woman snoring in bed.

    Wait ? Someone told me later Don't Wake up Hungover !
    .

    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Rick Cantrell View Post
    Not required.


    Well I'll be
    . On all of the new construction here the water heaters and or furnaces on the interior of the home are installed in a room, usually in the hall with the doors sealed. The combustion air comes from vents to the attic.

    Phoenix AZ Resale Home, Mobile Home, New Home Warranty Inspections. ASHI Certified Inspector #206929 Arizona Certified Inspector # 38440
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    Default Re: Combustion air for gas appliances in the home

    Taking combustion air from outdoors (preferably with a direct vent unit) is the best practice with regard to energy efficiency, back drafting, etc. but the code allows various methods to supply combustion air.

    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Dan Harris View Post
    The combustion air comes from vents to the attic.
    "to the attic" or 'through the attic to the outdoors'?

    To the attic simply means that there will be migration to/from the other living areas as nothing is sealed tight.

    Through the attic to the outdoors means that it comes from/goes to the outdoors, is mixed with outdoor air, then migrates back into the living space through the exterior walls as other outdoor air does.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion air for gas appliances in the home

    Dan,

    What code does Arizona use? There may be some requirement in it for what you are describing, but I would be curious as to its wording about 'to the attic' or 'through the attic'.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Jerry Peck View Post
    Dan,

    What code does Arizona use? There may be some requirement in it for what you are describing, but I would be curious as to its wording about 'to the attic' or 'through the attic'.
    IRC: I never looked up the code, just going by what I see on new homes and on homes as old as 15 years old
    The metal vents with a screen are axp 4"X12" just go into the attic, apx 4-6" above the insulation. The attics are vented

    Phoenix AZ Resale Home, Mobile Home, New Home Warranty Inspections. ASHI Certified Inspector #206929 Arizona Certified Inspector # 38440
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Dan Harris View Post
    IRC: I never looked up the code, just going by what I see on new homes and on homes as old as 15 years old
    The metal vents with a screen are axp 4"X12" just go into the attic, apx 4-6" above the insulation. The attics are vented
    Being as the 2006 IRC is still the most widely adopted version: (I know, I only stated "bedrooms" and it includes "closets" which is where you said they were, but I took - and still take - that to mean closets for the appliances, not storage closets.)(The requirements below are *if* the appliances are located where 'prohibited ... except' ... )(I think I included all the referenced sections so you can go from one section to another.)
    - G2406.2 (303.3) Prohibited locations. Appliances shall not be located in sleeping rooms, bathrooms, toilet rooms, storage closets or surgical rooms, or in a space that opens only into such rooms or spaces, except where the installation complies with one of the following:
    - - 1. The appliance is a direct-vent appliance installed in accordance with the conditions of the listing and the manufacturer’s instructions.
    - - 2. Vented room heaters, wall furnaces, vented decorative appliances, vented gas fireplaces, vented gas fireplace heaters and decorative appliances for installation in vented solid fuel-burning fireplaces are installed in rooms that meet the required volume criteria of Section G2407.5.
    - - 3. A single wall-mounted unvented room heater is installed in a bathroom and such unvented room heater is equipped as specified in Section G2445.6 and has an input rating not greater than 6,000 Btu/h (1.76 kW). The bathroom shall meet the required volume criteria of Section G2407.5.
    - - 4. A single wall-mounted unvented room heater is installed in a bedroom and such unvented room heater is equipped as specified in Section G2445.6 and has an input rating not greater than 10,000 Btu/h (2.93 kW). The bedroom shall meet the required volume criteria of Section G2407.5.
    - - 5. The appliance is installed in a room or space that opens only into a bedroom or bathroom, and such room or space is used for no other purpose and is provided with a solid weather-stripped door equipped with an approved self-closing device. All combustion air shall be taken directly from the outdoors in accordance with Section G2407.6.

    - G2407.6 (304.6) Outdoor combustion air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with Section G2407.6.1 or G2407.6.2. The minimum dimension of air openings shall be not less than 3 inches (76 mm).
    - - G2407.6.1 ( 304.6.1) Two-permanent-openings method. Two permanent openings, one commencing within 12 inches (305 mm) of the top and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors.
    - - - Where directly communicating with the outdoors, or where communicating with the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4,000 Btu/h (550 mm2/kW) of total input rating of all appliances in the enclosure [see Figures G2407.6.1(1) and G2407.6.1(2)].
    - - - Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of not less than 1 square inch per 2,000 Btu/h (1,100 mm2/kW) of total input rating of all appliances in the enclosure [see Figure G2407.6.1(3)].
    - - G2407.6.2 (304.6.2) One-permanent-opening method. One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or through a vertical or horizontal duct to the outdoors, or spaces that freely communicate with the outdoors (see Figure G2407.6.2) and shall have a minimum free area of 1 square inch per 3,000 Btu/h (734 mm2/kW) of the total input rating of all appliances located in the enclosure and not less than the sum of the areas of all vent connectors in the space.

    "The metal vents with a screen are axp 4"X12""

    That equals 48 square inches, what size closets and what Btu input ratings?

    The requirements below are for installations which have vents to the indoors in addition to the outdoors:
    - G2407.7 (304.7) Combination indoor and outdoor combustion air. The use of a combination of indoor and outdoor combustion air shall be in accordance with Sections G2407.7.1 through G2407.7.3.
    - - G2407.7.1 (304.7.1) Indoor openings. Where used, openings connecting the interior spaces shall comply with Section G2407.5.3.
    - - G2407.7.2 (304.7.2) Outdoor opening location. Outdoor opening(s) shall be located in accordance with Section G2407.6.
    - - G2407.7.3 (304.7.3) Outdoor opening(s) size. The outdoor opening(s) size shall be calculated in accordance with the following:
    - - - 1. The ratio of interior spaces shall be the available volume of all communicating spaces divided by the required volume.
    - - - 2. The outdoor size reduction factor shall be one minus the ratio of interior spaces.
    - - - 3. The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section G2407.6, multiplied by the reduction factor. The minimum dimension of air openings shall be not less than 3 inches (76 mm).

    - G2407.5.3 (304.5.3) Indoor opening size and location. Openings used to connect indoor spaces shall be sized and located in accordance with Sections G2407.5.3.1 and G2407.5.3.2 (see Figure G2407.5.3).
    - - G2407.5.3.1 (304.5.3.1) Combining spaces on the same story. Each opening shall have a minimum free area of 1 square inch per 1,000 Btu/h (2,200mm2/kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.06 m2). One opening shall commence within 12 inches (305 mm) of the top and one opening shall commence within 12 inches (305 mm) of the bottom of the enclosure. The minimum dimension of air openings shall be not less than 3 inches (76 mm).
    - - G2407.5.3.2 (304.5.3.2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more openings in doors or floors having a total minimum free area of 2 square inches per 1,000 Btu/h (4402mm2/kW) of total input rating of all appliances.

    How large is the vent to the indoor space?
    - "but not less than 100 square inches" (see above)

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion air for gas appliances in the home

    “Tight” buildings (with weather stripping
    and caulk to reduce infi ltration),
    may require special provisions for
    introduction of outside air to ensure
    satisfactory combustion and venting,
    even though the furnace is located in
    an unconfi ned space.

    An unconfi ned space is an area including all
    rooms not separated by doors with a volume
    greater than 50 cubic feet per 1,000 Btuh of the
    combined input rates of all appliances which
    draw combustion air from that space. For example,
    a space including a water heater rated
    at 45,000 Btuh and a furnace rated at 75,000
    Btuh requires a volume of 6,000 cubic feet [50 x
    (45 + 75) = 6,000] to be considered unconfi ned.
    If the space has an 8 foot ceiling, the fl oor area
    of the space must be 750 square feet (6,000
    / 8 = 750). In general, a furnace installed in an
    unconfi ned space will not require outside air for
    combustion.

    WARNING:
    Furnaces installed with combustion
    air drawn from a heated space which
    includes exhaust fans, fi replaces, or
    other devices that may produce a negative
    pressure should be considered
    confi ned space installations.


  12. #12
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Dan Harris View Post
    I see a lot of older homes with the WH and Furnace installed in interior closets with vents on the doors.
    One thing to consider: the intake through those doors is specified in terms of net free area, and the NFA of (for example) a wooden louvered grill is typically calculated as 20-25% of the grill area.

    Michael Thomas
    Paragon Property Services Inc., Chicago IL
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    Default Re: Combustion air for gas appliances in the home

    Dan

    Where the air for combustion comes from is not a problem. It is what happens to the combustion gas that matters. What that air vent to the attic is doing is making sure there is not a negative air pressure in the room/closet where natually vented gas appliances are operating.

    In todays tighter homes when kitchen fans, bath fans, a fireplace etc are pulling air out of the house it is possible to pull air back in through a flue.

    To me it doesnt make sense to build a tighter home only to put an uncontrolled air leak in. If a home has the turbine roof vents or a power vent they would suck a lot of air out of the house. Even normal roof vents could lead to negative pressure and the loss of conditioned air from the home.


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    Default Re: Combustion air for gas appliances in the home

    I see this all the time here in GA. Invariably, if a home has a gas forced air furnace in a basement, the water heater is next to it, and someone finishes out the basement, these appliances get shoved in a confined space with no air for combustion, dilution and or makeup. It is one of the more frequent issues I see. Friday's inspection had this make up: the 66,000 BTU furnace was installed in a small closet off a basement bedroom and the 80 gallon water heater was installed in a closet in a teenager's basement bedroom. I call this out as hazardous every time and the sellers are generally clueless. This is the outcome when no building permits are pulled for basement finish outs. It's a wonder more people don't die from backdrafting CO into bedrooms.


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    Cool Re: Combustion air for gas appliances in the home

    MUA vents pulling from the attic don't work. The warm air in the mechanical closet will tend to draft up those vents like chimneys further depressurizing the room or fighting for any MUA that does infiltrate from elsewhere. Air is dumb and goes only where the pressure gradient forces it to go. Air cannot read code books.

    Keep the fire in the fireplace.

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    Default Re: Combustion air for gas appliances in the home

    [quote=Robert Hronek;195810]Dan

    Where the air for combustion comes from is not a problem. It is what happens to the combustion gas that matters. What that air vent to the attic is doing is making sure there is not a negative air pressure in the room/closet where natually vented gas appliances are operating. I don't understand this last sentence.

    ....

    To me it doesnt make sense to build a tighter home only to put an uncontrolled air leak in. If a home has the turbine roof vents or a power vent they would suck a lot of air out of the house. Even normal roof vents could lead to negative pressure and the loss of conditioned air from the home. There was a good article posted about this recently - I think the thread was called "attic ventilation"[/quote]

    Where the combustion air comes from can be a concern. If it is not rich enough in oxygen (for example, if it's in a sealed closet with no air inlet), the furnace won't burn as efficiently.

    I think the answer to Dan's question depends on what type of furnace it is. Wouldn't a condensing furnace be fine in a sealed room?

    (Just an aside: Doing my insurance inspections, I'm supposed to report as a concern any time a furnace or boiler is less than 36" from combustibles. I don't quite understand it.)

    Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.
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    Cool Re: Combustion air for gas appliances in the home

    Passive makeup air supplied don't work. An ASHRAE study proved this.

    When you put a duct into the ceiling 'drawing' from the attic, it will exhaust into the attic or exfiltrate--not infiltrate. The top of the house is under positive pressure so attic air will tend Not to infiltrate but exfil. Then, you warm that room up and the warm air will create stack effect in the duct and draft into the attic. The only way to pull attic air down into the CAZ is with a fan.

    yes, attic vent fans can depressurize the attic thus tending to exacerbate the drafting out of the attic and preventing infiltration. If you want to equalize the CAZ pressure, install a powered MUA system.

    Keep the fire in the fireplace.

  18. #18
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by Bob Harper View Post
    Passive makeup air supplied don't work. An ASHRAE study proved this.

    When you put a duct into the ceiling 'drawing' from the attic, it will exhaust into the attic or exfiltrate--not infiltrate. The top of the house is under positive pressure so attic air will tend Not to infiltrate but exfil. Then, you warm that room up and the warm air will create stack effect in the duct and draft into the attic. The only way to pull attic air down into the CAZ is with a fan.

    yes, attic vent fans can depressurize the attic thus tending to exacerbate the drafting out of the attic and preventing infiltration. If you want to equalize the CAZ pressure, install a powered MUA system.
    Just to make sure I'm getting this right, you're talking about exfiltration and infiltration into the CAZ, not the attic, right? And is CAZ combustion activity zone?

    The passive MUA study you talked about - where was the air from? How supplied? I have a duct going through a basement window to supply air (I don't think it's necessary, but it's [local?] code), and it seems to work insofar as it sucks in cold air in the winter...I don't see why it wouldn't work elsewhere.

    Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.
    - James Burgh, 1754.

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    Cool Re: Combustion air for gas appliances in the home

    The CAZ is the Combustion Appliance Zone. I'll have to look for a link but it's on the web (ASHRAE study). Anytime you rely on a hole inthe wall/ ceiling, it is a passive MUA system. Air does not know which way to go--the pressures dictate. You may normally have infiltration until the wind shifts putting your MUA intake in a negative pressure zone that sucks air out of the CAZ. There's no guarantee which way the air will flow unless powered.

    Since the attic is a separate zone, it, too is subject to positve and negative pressures so accordingly it, too can undergo infiltration and exfiltration depending upon the pressure gradients.

    A 4" duct under a 5 Pascal negative pressure gradient with a short, smooth duct can deliver about 12-14 cfm on a good day. There are studies by the Canada Mortgage and Home Corp that documented actual delivery of MUA through passive intakes at various pressures and it ain't nowhere near what is required by the appliance even when it is working in the appliance's favor. I've seen MUA kits cause backdrafting.

    Keep the fire in the fireplace.

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    Default Re: Combustion air for gas appliances in the home

    I looked for the ASHRAE study, but one has to be a subscriber to read a lot of the research. Did find another interesting article about ventilation. Bob, you've been talking a lot about pressure, but what about temperature? On a cold winter day in MN, when the furnace would be on most, the air loss out the top of the house would create a significant depressurization in the basement. My basement is drafty enough that I'm not worried about not having enough air for combustion so I tie a plastic bag loosely over the indoor end of the MUA vent, and it is always puffed out (drawing air into the building).

    It seems like there are so many variables governing how air moves through a house and its ventilation systems it's hard to make a lot of generalizations.

    Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.
    - James Burgh, 1754.

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    Smile Re: Combustion air for gas appliances in the home

    Basic building science holds that all things considered equal, the lower part of a building closed in (doors and windows closed) typically be under negative pressure or depressurized with respect to both outdoors and to the upper part of the home. The upper part is usually under a positive pressure With Respect To (WRT) downstairs and outside. The magical cross-over point is referred to as the Neutral Pressure Plane or Point (NPP)or Neutral Pressure Zone (NPZ). This cross-over undulates and fluctuates with pressure gradients to the home and outdoors. Can't teach the whole class here but you can have local effects in one zone of a structure WRT another. For instance, while the whole basement may be negative WRT outdoors, the mechanical closet may be positive WRT the adjoining room where you're hoping your passive MUA grille will pull air from. Well, if the gradient is positive in the CAZ then air in the CAZ will flow through that dumb grille into the adjoining room. Wind can put an outdoor air intake in a negative zone that can pull MUA outdoors.

    The effect you described in winter is Stack Effect. Warm air rising within the structure. If you have upper level losses, you will also suffer exfiltration. The more Stack Effect and exfiltration the worse the basement goes negative in response. If you want to balance the house, don't cut holes in the ceiling--seal them, then provide relief down low. Tight up top/ loose down low.

    Now, you add forced air to the house. Let's say you have a confined space mechanical room (CAZ) that needs MUA. The adjoining room (we'll call it the supply room) is finished with forced air ventilation and you have the code prescribed grilles in the intervening wall. Will the MUA migrate from the finished space into the CAZ? It depends upon the pressure gradient across that wall. Let's say the return ducts and air filter slot are tight and the ducts in the adjoining supply room are balanced. Very little air will move into the CAZ unless something else generates a negative gradient WRT the supply room. Now, when you find the water heater still backdrafting, you decrease the return air from the supply room or add another supply register and poof! the water heater vents because the supply room becomes positive WRt the CAZ so MUA can flow to the WH.

    Now, flip this around: no sealed return or filter slot and imbalanced ducts. The leaky ducts generate a huge depressurization in the CAZ WRT the supply room trying to pull air in. However, the supply ducts are undersized or closed off so very little air is being pumped into this room. That room begins to pull from outdoors and upstairs while the WH still backdrafts. So, a supply grille is cut into the supply plenum within the CAZ and adjusted for the minimum flow to balance the CAZ and the WH vents. You seal the ducts and filter slot so now there's more conditioned air for the rest of the home. You seal the attic stairs and install ICAT luminaires and you can just about close off that register on the supply.

    Yes, it gets confusing at first. My best friend is still my TiCl4 chemical smoke puffer. It demonstrates which way the air is flowing. All I have to do it teach the air which way I want it to go.

    Last edited by Bob Harper; 04-18-2012 at 04:48 PM.
    Keep the fire in the fireplace.

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    Smile ASHRAE article referenced

    Article enclosed. Since this article there have been many other various articles, primarily based upon anecdotal evidence that corroborates these findings. My personal findings certainly do to a greater extent as this study was limited in scope and appliances.

    Enjoy!

    Keep the fire in the fireplace.

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    Default Re: Combustion air for gas appliances in the home

    Wow! Thanks for the excellent post. I think I'm going to have to draw some figures of different scenarios to get it fully into my brain. There are so many variables, and some change from season to season or even minute to minute.

    Thanks for the article, too. I read some of it, will have to read more and give it some thought.

    So am I right that none of this really applies to condensing furnaces, since they use outdoor air for combustion and spit it outside again?

    You've been very informative, as usual! Great posts. I like that dry sense of humor, too. I did wonder if you were joking about the cripple wall...I mean, you had to be, eh?

    I haven't had to learn so many acronyms since grad school!

    Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.
    - James Burgh, 1754.

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    Smile Re: Combustion air for gas appliances in the home

    A CAT IV condensing furnace is relatively immune to these pressure gradients (there is always a limit). For gas direct vent fireplaces, they are tested against a 25 Pascal positive vent pressure for leaks. I've seen a 27 Pa depressurization cause one to ghost out (oxygen starvation) in a pub where the exhaust fans were imploding the building. That one took $10K in MUA to correct.

    Now, if a condensing furnace is only one piped, you have a problem because it is drawing air from the CAZ and it is depressurizing the CAZ. Still appproved in many places but a horrible practice.

    Thx!

    Keep the fire in the fireplace.

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    Default Re: Combustion air for gas appliances in the home

    10 k sure is a lot of money to open a window.


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    Default Re: Combustion air for gas appliances in the home

    That window would had to have a net free opening of about 50 square feet minimum. Instead of cold air in winter, this system has duct heaters and misc. safety controls. Hard for the boogie man to break in through a MUA system, too.

    Open windows don't always work.

    Keep the fire in the fireplace.

  27. #27
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    Default Re: Combustion air for gas appliances in the home

    Hi,

    Brand new here. I am a carpentry contractor for the last 29 years. I have never thought much about combustion and make up air before because I use licensed HVAC guys or plumbers when having any heating work done. I only started to become interested in it while researching a vent-less gas heater that I wanted to use for my basement as supplemental heat.

    I soon became entangled in non stop research that shows me how improperly supplied my boiler and water heater are alone, forget about the additional heater. I have seen tons of finished basements that were done well after the original construction and I am sure that many of these are improperly done with considerations to air/make up air supply.

    Anyway I want to know and understand this subject not to become an HVAC or heating guy, but so that I may be able to see/foresee problems in existing or future basement renovations. I understand as a GENERAL rule the 50 cubic feet per 1000 btu input guideline which gives the basis for combustible air requirements. But there is a portion of the correction options listed here and on other sites that I am still having trouble understanding. Here is my personal situation right now and my question will follow.

    -I am finishing my basement for recreational use.
    -I have an 80% efficient fairly new Utica gas boiler (116,000 btu input)
    -I have a standard flue vented gas water heater (24,000 btu)
    -in the "big room" where the mechanical room is I have 4000 cubic feet of available air
    -I can not get more air from adjoining rooms in basement
    -The basement is fairly "tight" with new drywal walls, new double glass windows and new door

    Using the 50 CF per 1000 btu rule you will see that I have enough air for 80,000 btu's when my furnace and WH can use up to 140,000 leaving me with a 60,000 BTU deficit. I also wanted to use a 30,000 btu vent less heater to run only when someone is down there (one or twice a week for a couple of hours).

    I was goint to introduce the lacking air needed for the mechanical room (lets forget the additional heater I was going to get for a minute) and do a combo of all ready available air (using a full louvered door on room and a duct to take in outside air. After reading multiple posts from many sites I see that I will probably have a huge draft issue with this method making my basement even colder.

    After more research I decided that I would do a solid door on the mechanical room, feed the WH and boiler with outside air only and then I would have plenty of combustible inside air if I wanted to do a supplemental heater.

    Now after this long wind up here is my question/issue. I see there are 3 methods for outside air. 1-single hole on outside wall (cant do that), 2-vertical duct to outside air (cant do that), 3-two horizontal ducts to outside air (I am interested in this)

    Vertical duct allows a 4000 btu per square inch ratio while horizontal is cut down to 2000 btu per square inch. I get it as horizontal does not draw/vent as easy. the thing is with the first two methods of fresh air vent it appears that once you see what your square inch needs are it is accomplished with one duct that size. With the horizontal method I am getting the impression that I need TWO ducts EACH one being done in the size needed by the square inch rule.

    I was hoping that since there were two ducts (one high one low) that I could take my square inch needs divide that number by two and run the two ducts by that calculation. I.E. My air needs for the WH and boiler require a 10" round duct (aprox 76 square inches) why do I have to run two 10" ducts for the horizontal method instead of two smaller ducts that equal the same 76 square inches? Why the high low thing too? I dont quite get what the high ones purpose is. I see it listed as "outlet air duct" in drawings but I want to understand what is happening exactly in a horizontal two duct system.

    Thanks for your patience. This forum has opened my eyes to potential health and equipment hazards. I will never look at a finished basement (existing or new) the same way again.

    -John

    PS- I dont want to seem out of line but I get the feeling that some home and town inspectors in my area may not even think about this issue as much as they should. I am sure I have seen homes with finished basements that probably don't meet the code.


  28. #28
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    Default Re: Combustion air for gas appliances in the home

    Welcome John,

    Why not enclose the Gas fired appliances in a sealed Supply Closet with the proper sized outside air intakes?

    *Electric Heat does not require "Makeup Air."
    **the high,low is for the combustion process Fresh into the bottom and combustion gas by products drawn out the high.

    Last edited by Billy Stephens; 02-16-2014 at 12:42 PM.
    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  29. #29
    John Demmer's Avatar
    John Demmer Guest

    Default Re: Combustion air for gas appliances in the home

    Thanks for the welcome,

    to respond to your post:

    1-I am planning an enclosed room with outside air. That is what the post is about

    2-there is no electric heat proposed. The supplemental unit I am talking about is a 25,000 btu vent less gas unit

    3-if my water heater and boiler are connected to a chimney then what gases are we talking about expelling through the higher horizontal vent? Are the two horizontal vents proposed by code for intake air or is the high one some kind of exhaust and if so what is it exhausting?

    I may have another solution to this. It's a product by Field Controls and it is a combustion air system. I would use their cas-4 for my needs. It is basically a fan powered air intake that is linked to my boiler. When the thermostat calls for heat the cas-4 comes on and brings in plenty of fresh air. There is a safety set up where the boiler can not come on unless the cas-4 is operating. The best part is that I only need a single 4" pipe based on my needs and location.


    i still have some questions for the company regarding whether or not there is cold air spillage into the room when the unit is off but it does have a weighted terminal hood that I am sure will help keep drafts out. Either way one 4" duct will bring in a lot less cold air than two 10" ducts.

    has anyone here used a system like this? What are your general feelings or comments on this?

    John


  30. #30
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by John Demmer View Post
    Thanks for the welcome,

    to respond to your post:

    1-I am planning an enclosed room with outside air. That is what the post is about

    2-there is no electric heat proposed. The supplemental unit I am talking about is a 25,000 btu vent less gas unit

    3-if my water heater and boiler are connected to a chimney then what gases are we talking about expelling through the higher horizontal vent? Are the two horizontal vents proposed by code for intake air or is the high one some kind of exhaust and if so what is it exhausting?

    I may have another solution to this. It's a product by Field Controls and it is a combustion air system. I would use their cas-4 for my needs. It is basically a fan powered air intake that is linked to my boiler. When the thermostat calls for heat the cas-4 comes on and brings in plenty of fresh air. There is a safety set up where the boiler can not come on unless the cas-4 is operating. The best part is that I only need a single 4" pipe based on my needs and location.


    i still have some questions for the company regarding whether or not there is cold air spillage into the room when the unit is off but it does have a weighted terminal hood that I am sure will help keep drafts out. Either way one 4" duct will bring in a lot less cold air than two 10" ducts.

    has anyone here used a system like this? What are your general feelings or comments on this?

    John
    1. I am purposing to enclose the boiler & WH separate to the living space.

    2. I am purposing electric heat as an alternative to gas heat.

    3. Dependent upon the configuration, liner material and past use of the "chimney" it may not be allowed to vent the boiler & WH. Does the boiler & WH manufacturer allow a power fan for these installed appliances? Check with your AHJ before proceeding on this project.

    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  31. #31
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    John Demmer Guest

    Default Re: Combustion air for gas appliances in the home

    Hi again,

    1-yes the two units will be in there own room completely separated from adjoining living space

    2-electric heat is out but I am not even focused on that I just want to get enough air to my WH and boiler in their new room

    3-the WH and boiler currently vent into the chimney which they have been for years so that has not changed

    4-why would it matter how a unit gets its air as long as there is enough air?

    5-I need to know what AHJ means. I am new to this forum

    Thanks for trying to help me understand all of this

    john


  32. #32
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by John Demmer View Post
    Hi again,

    1-yes the two units will be in there own room completely separated from adjoining living space

    2-electric heat is out but I am not even focused on that I just want to get enough air to my WH and boiler in their new room

    3-the WH and boiler currently vent into the chimney which they have been for years so that has not changed

    4-why would it matter how a unit gets its air as long as there is enough air?

    5-I need to know what AHJ means. I am new to this forum

    Thanks for trying to help me understand all of this

    john
    1. Great if it is Air Tight Sealed from the living space.

    2. ok

    3. Just because the Gas Appliances have used this chimney for years doesn't mean it is right.

    4. The installed gas appliances need to be listed and rated for the installation method.

    5. Authority having Jurisdiction ( Building Code Enforcement )

    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  33. #33
    John Demmer's Avatar
    John Demmer Guest

    Default Re: Combustion air for gas appliances in the home

    Ok thanks,

    In regards to number 4 I will see if I can pull up specs on my boiler and WH to see if mention is made as to whether or not a power air supply can be used. I will also call the company that makes the unit to see if they have concerns. There literature makes it seem as if there product is designed to work with any boiler since it's just supplying air but we will see.

    I am hoping that others have opinions on this too


  34. #34
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    Default Re: Combustion air for gas appliances in the home

    The following is from the 2012 IRC regarding mechanical combustion air:
    - G2407.9 (304.9) Mechanical combustion air supply.
    - - Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from the outdoors at a rate not less than 0.35 cubic feet per minute per 1,000 Btu/h (0.034 m3/min per kW) of total input rating of all appliances located within the space.
    - - G2407.9.1 (304.9.1) Makeup air.
    - - - Where exhaust fans are installed, makeup air shall be provided to replace the exhausted air.
    - - G2407.9.2 (304.9.2) Appliance interlock.
    - - - Each of the appliances served shall be interlocked with the mechanical air supply system to prevent main burner operation when the mechanical air supply system is not in operation.
    - - G2407.9.3 (304.9.3) Combined combustion air and ventilation air system.
    - - - Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air.

    You could seal off the area/room the equipment is in, do away with all other sources of combustion air, seal the room, the door, etc., and then provide 0.35 cu ft/min of mechanical combustion air with an interlock which prevents the main burner from operating (as you described the interlock on the system you installed).

    What is the total Btu/hr input (in thousands of Btu/hr) of all appliance installed and planned on being installed in that sealed off area? Multiply that total by 0.35 to find out the cu ft/min required for the mechanical combustion air.

    water heater = 24,000 Btu/hr max input rating
    furnace = 116,000 Btu/hr max input rating
    Total = 140,000 Btu/hr max input rating
    140 x 0.35 = 49 cu ft/min minimum mechanical combustion air into the room

    The ducts for the combustion air for the mechanical combustion air shall be:
    - G2407.11 (304.11) Combustion air ducts. - - Combustion air ducts shall comply with all of the following:
    - - - 1. Ducts shall be constructed of galvanized steel complying with Chapter 16 or of a material having equivalent corrosion resistance, strength and rigidity.
    - - - - Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one required fireblock is removed.
    - - - 2. Ducts shall terminate in an unobstructed space allowing free movement of combustion air to the appliances.
    - - - 3. Ducts shall serve a single enclosure.
    - - - 4. Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air.
    - - - 5. Ducts shall not be screened where terminating in an attic space.
    - - - 6. Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air.
    - - - 7. The remaining space surrounding a chimney liner, gas vent, special gas vent or plastic piping installed within a masonry, metal or factory-built chimney shall not be used to supply combustion air.
    - - - - Exception: Direct-vent gas-fired appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s instructions.
    - - - 8. Combustion air intake openings located on the exterior of a building shall have the lowest side of such openings located not less than 12 inches (305 mm) vertically from the adjoining finished ground level.

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

  35. #35
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    Default Re: Combustion air for gas appliances in the home

    Thanks Jerry,

    That provides a lot of useful information. My needs are for 150,000 potential btu's in the sealed room so multiplying 150x.35 means I need 52.5 cubic feet per minute to satisfy code. I will review the CAS-4 Unit I am interested in to see if they give that information. I only recall seeing total btu limitations based on run length.

    It seems that I can make this unit work but will research it some more and I all ready have an email out to the fan manufacturer who i hope to speak with tomorrow. I will let all here know what the outcome is.

    Please feel free to continue commenting if more information is available.

    Thanks,

    John


  36. #36
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    Default Re: Combustion air for gas appliances in the home

    Quote Originally Posted by John Demmer View Post
    I only recall seeing total btu limitations based on run length.
    John,

    The manufacturer probably listed the maximum Btu/hr limitation based on the same formula and factored in pressure loss per foot of duct length as the length of the run does effect the CFM capabilities of a given motor size/rpm/fan blade configuration, as does the size of the duct, whether the duct run slopes uphill or downhill, elbows in the duct run, and any louvers on the inlet to at the intake end of the duct.

    Go by their calculations and go with a larger fan capacity as needed so that you are not pushing the maximum limitations of the fan as the cfm will drop as the motor deteriorates, the fan blade becomes dirty, the intake louvers become dirty/clogged, etc.

    Remember, the cfm in the code is the "minimum", not a "maximum", not even "good design practice" ... just the minimum - I explain code this way: code is the most unsafe one is legally allowed to do the work. From your posts I believe your goal is not to do it the most unsafe you are legally allowed to.

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

  37. #37
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    Default Re: Combustion air for gas appliances in the home

    Got it!

    My configuration is WELLLLLL under the companies max limitations according to their charts. I did just write them again to see what the cfm ratings are for my specific situation just to see what they come up with anyway.

    Yes anyone who designs something to code minimums is definitely asking for trouble.

    -John


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