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    Combustion air (25 Posts)

  • Mpj Mpj @ 9:27 PM
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    Combustion air

    Went to look at a job today. The building inspector wanted the boiler room to have a fireproof door, sheet rocked and the entire room sealed. I was giving a price to install a "air in a can" for combustion air when the inspector showed up.
    He told me that I needed to install fresh air from outside for the boiler to burn, I told him I was installing the"air in a can". He then told me that I also needed the vents from outside both high and low (proper size of course). I then told him that the "air in the can" replaces the duct work. He then told me that it will not provide exhaust for the boiler room. I did not understand the that statement about the exhaust.
    He told me thats why they want a high and low duct- one for intake for the boiler (combustion) and the second for boiler exhaust (room heat?) in which I disagreed.
    Can anybody explain why you needed one high and one low duct for combustion air if done that way? I understand that you need x amount of square inches for btu input of the equipment. I just never wondered why two vents.
  • don don @ 8:32 AM
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    Great question

    And one you could ponder for days or just simply do as he want and get on with life.I'm with you matter not at what part of that wall you are pulling from as long as the appliance get enough o2 for combustion what it matter.
    I guess you could always think in terms of safety if someone happen to block off the one a foot off the floor, you have the one a foot from the ceiling to take its place.
    If you were to look in the code book they  have a few section that has outdoor air (condition one) and another (condition two).Which they speak of not only combustion air but dilution air as well.
    A word of advise just do as the inspector want no matter how bad you want to have a debate with him on the issue.
    This post was edited by an admin on March 9, 2012 8:36 AM.
  • hotpipe hotpipe @ 4:26 PM
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    Don is right on

    The inspector may be looking for the correct amount of air changes in the boiler room, but regardless, the quickest and most painless road is to follow the authority having jurisdiction's guidelines.
    Don't blame me, I voted for the old war hero and the business expert!!!!
  • Mpj Mpj @ 10:51 PM
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    Combustion air

    I agree to do what the inspector says to do. I was just trying to find out why two vents high and low? Is it because of possible block like Don mentioned or another reason?
  • Paul48 Paul48 @ 11:12 PM
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    To

    prevent the heated air from creating a positive pressure in the room,there-by restricting the flow of combustion air through a single vent. With one high and one low, you get a convected supply of fresh air.
  • Jean-David Beyer Jean-David Beyer @ 8:18 AM
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    Air vents, high and low.

    I get to deal with a small room with two 125,000 BTU/hour gas burning forced hot air furnaces in it. The building is almost 200 years old and originally had no heat. It then acquired three pot bellied stoves that burned coal. Then an oil burner under the floor in a crawl space. Then they dug a hole in the ground and put an oil burner in there. Most of that hole was below ground, and there was a single vent about 1 foot by 1 1/2 foot rectangle. It was about 6 feet above the ground level to get it above the usual height of the snow. They then put two 100,000 BTU/hour gas furnaces in there. We never got harassed by inspectors.  The air intake on those furnaces entered through a three-inch diameter hole that was obviously a fitting to pick up outside air if we wanted to. Similarly, the exhaust pipe was three inch. So I would guess a 10 square inch vent would be almost enough. (I do not know the code.)

    When the heat exchangers started leaking, we installed the new furnaces. Then the inspector failed us because we did not have a high and low vent. He calculated the venting we needed, but he could not do math. I mean the existing vent seemed to let the natural gas burn blue, and he said we needed about 100 square feet of vents. That was nuts. We now have two vents: the existing one, and a new one at just about ground level, so if the snow gets deep, it is covered.. Even so, the low one is about 8 feet above the floor: hardly within 12 inches of the floor. and the other one is about 3 feet down from the ceiling, hardly within 12 inches of the ceiling. Luckily, we did not get the same inspector, and the new one passed our installation. Bah! Humbug! Can you imagine a 100 square foot vent? We would have had to take a whole wall off.
  • don don @ 6:24 AM
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    Thanks Paul

    That a great explanation.Now i wonder has anyone of us ever ran into a positive pressure in a mechanical room with two opening or even one for that matter? I would assume if one has a positive pressure in a mechanical room.then that would be even better for maintaining a proper draft thu the appliance.
  • Mpj Mpj @ 6:21 PM
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    Combustion air

    Thanks Paul,
    That is what I was looking for. I looked in the code book today and I did notice the duct work had written on it intake and exhaust.
    Thanks again.
    Do I still need an exhaust vent with the air in a can?
  • Paul48 Paul48 @ 6:35 PM
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    Right or Wrong

    You need just what the inspector requires, and maybe a coffee and a donut.
  • furnacefigher15 furnacefigher15 @ 8:14 PM
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    Show him

    The international mechanical code.

    Mechanical fans can be used to supplement combustion air, or used to supply all of it.
    The stipulation is that all appliances that are to receive combustion air from the mechanical fan must be electrically interlocked to prevent operation of the appliance in the event the fan fails.

    But fresh air need to be sized to replace all air - dryers, furnaces, boilers, etc...

    mechanical fans need to be sized to 1 cfm per 2400 btu.

    Most inspectors don't actually know the codes, so when challenged with documentation proving your rightness they will cave.

    I keep current code books on the truck when performing installs, and I usually call the inspectors in advance to get there blessing before I even turn a wrench.

    Some are more bull headed then others, but if you let them run you over, you can end up loosing your shirt on a job.
  • VictoriaEnergy VictoriaEnergy @ 1:24 AM
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    why you sometimes need high & low air supply vents

    I think the rule goes something like this (plz don't do without checking 1st)

    For passive vents you need 7.5 sq inches per 100 000 BTUs if there are no draft hoods (or barometric dampers) in the room.  Air supply vent usually terminates at floor level.

    You double that to 15 sq inches per 100k BTUs if ANY draft hoods are present. Plus add second vent the same size located as near the ceiling as practical and must be higher than the highest draft hood.

    All vents must be above highest anticipated snow level.  If the air supply vents are ducted, they also have to be separated by at least 36" on the exterior

    The intent of the requirement is to recognize that many installations will spill through the draft hood for a brief period of time on cold start up while the vent is warming up and developing adequate draft.  This creates a need to have a path for the spillage to exit the mechanical room and avoid recirculating back to the burner and making it barf out very high CO.
    Home Owners Please Note:


    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
  • Tim McElwain Tim McElwain @ 6:13 PM
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    There are many air requirements

    especially in "Confined Spaces" You may need "Air for Combustion" which can be supplied by using two openings directly connected to outdoors (NFPA 54 9.3.3).

    Or two openings communicating with a room in contact with out doors.(NFPA 54 9.3.4)

    Or air from indoors using standard method room volume being 50 cubic feet per 1,000 BTU/Hr.(NFPA 9.3.2)

    Known Air Infiltration Rate Method (NFPA 54 9.3.2.2)

    There is also a provision in the code since 1999 for one opening under some specific sizing applications. NFPA 54 9.3.3.2)

    Then you may also size openings using NFPA 54 9.3.2.3 Combining spaces on the same story or combining spaces in different stories.

    You can use the "Fan-in-a Can" in lieu of some of these procedures, that is up to the local AHJ many times.

    Another rule that is often overlooked by everyone including inspectors. It is the rule of ROOMS LARGE IN COMPARISON WITH SIZE OF APPLIANCES which is concerned with combustible materials and keeping rooms with gas equipment cool. The rule is "Volume equal to 12 times the total volume of a furnace or AC, 16 times the total of a boiler". I have a feeling that may have been what the inspector was looking for. He wants the room to be able to exhaust the residual heat developed by the equipment. The fan in the can only satisfies the "air for combustion rule".

    Then again if one or more of the appliances is "Fan Assisted" the rules change as there is no draft hood on that equipment so dilution air is not required.

    I can tell by the postings here that much training is needed on air for combustion, dilution air, ventilation air etc.

    Then there is "Make up air" air required to replace air removed by mechanical exhausting.

    The rest of the problem on air in the combustion zone concerns dryers (NFPA 54 10.4.3.1)

    MY Fundamentals of Gas Volume II addresses all of this along with Venting.
  • VictoriaEnergy VictoriaEnergy @ 1:24 AM
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    x

    Home Owners Please Note:


    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
    This post was edited by an admin on March 11, 2012 1:26 AM.
  • Mpj Mpj @ 11:22 PM
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    Combustion air

    Thanks for the reply Tim,
    I'm not sure what you meant by "The rule is "Volume equal to 12 times the total volume of a furnace or AC, 16 times the total of a boiler". I have never heard or read about this.

    This particular boiler room is about 9' X 5" and has an old American Standard oil fired boiler and the room is sealed tight. I do not think I need make up air; nothing else is in the boiler room.
  • Mpj Mpj @ 11:23 PM
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    Combustion air

    Thanks for the reply Tim,
    I'm not sure what you meant by "The rule is "Volume equal to 12 times the total volume of a furnace or AC, 16 times the total of a boiler". I have never heard or read about this.

    This particular boiler room is about 9' X 5" and has an old American Standard oil fired boiler and the room is sealed tight. I do not think I need make up air; nothing else is in the boiler room.
  • icesailor icesailor @ 7:22 AM
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    Sealed Tight:

    What do you mean, you don;t need "make-up air"?
    When people hyperventilate when they are excited, they get too much oxygen in their blood stream. So, you breathe out of a bag and suck in the CO2 from your breath.
    I have tried for years to find a third world instrument that I could use to show a drop in pressure in a boiler room for lack of make-up air. I thought of an aircraft altimeter but those are usually ruined in plane crashes.
    But maybe this will work. Take your digital combustion analyzer and with the door open and the burner settled down and reading a steady state, close the door. If after 15 minutes, the numbers don't change, you might have an argument with the AHJ's. But you won't win it. You need make-up air. Period. One to get the hot air out, and two, to get fresh cool air in.
  • Tim McElwain Tim McElwain @ 10:46 AM
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    I hate to be such a buster on

    this business of air. MAKE UP AIR is by definition air required to make up air removed by MECHANICAL EXHAUSTING it is not COMBUSTION AIR Combustion Air is air required for burning. So you do not need Make Up Air. The reason that is important is that it changes the size of openings required when calculating for Make up Air typically one single opening sized by determining the CFM of the mechanical exhausting.

    Combustion air is determined as to the BTU content of the equipment.

    As to room to large rule it is from NFPA 54 Section 10.3.2

    From Definitions section of the code:

    3.3.90 Room Large in Comparison with Size of Appliance

     
    Rooms having a volume equal to at least 12 times the total volume of a furnace or air-conditioning appliance and at least 16 times the total volume of the boiler.

     
     
    10.3.2 Clearance. 10.3.2.1 Listed central heating furnaces and low-pressure boilers installed in a room large in comparison with the size of the appliance shall be installed with clearances in accordance with the manufacturer's instructions. (See 3.3. 90, Room Large in Comparison with Size of Equipment)

     
     
    10.3.2.2 Central heating furnaces and low-pressure boilers in­stalled in rooms that are NOT large (such as alcoves and closets) in comparison with the size of the appliance shall be listed for such installations. Listed clearances shall not be reduced by the protection methods described in Table 10.2.3(b) and illustrated in Figure 10.3.2.2(a) through Figure 10.3.2.2(c), regardless of whether the enclosure is of combustible or noncombustible material.

     
     
    10.3.2.3 Unlisted central heating furnaces and low-pressure boilers installed in rooms that are large in comparison with the size of the appliance shall be installed with clearances not less than those specified in Table 10.2.3(a). 10.3.2.4 Central heating furnaces and low-pressure boilers (listed and unlisted) installed in rooms that are large in comparison with the size of the appliance shall be permitted to be installed with reduced clearances to combustible material pro­vided the combustible material or appliance is protected as described in Table 10.2.3(b) (see 10.3.2).



    What this is saying is that if the room is not larger than the appliance by the numbers given then special clearance rules apply of air must be supplied for keeping the room cool. This is air over and above air for combustion or make up air.



    This does not apply to equipment designed for installation in closets or alcoves.
     
  • Jim Davis Jim Davis @ 11:02 AM
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    Combustion Air

    It amazes me that to this day the Code still hasn't figured out how much combustion air equipment actually requires.  It also is interesting that ASHRAE did a study of passive combustion air (high & low openings) and found that under many actual field conditions they failed to provide adequate combustion air to mechanical rooms.  Their study went on the say only mechanical combustion air works.  The Code does allow for mechanical combustion air but still no clue on how much you actually need other than a guess.
    100% of all gas appliances were certified to operate in a positive pressure mechanical room or at least minimum negative.  In other words, all equipment has been tested in mechanical rooms with mechanical combustion air only!
    High and lows are only somewhat functional if they enter and exit at a high and low location.  Piping from a high location to the floor does not create a low any more than piping from the floor to the ceiling creates a high. 
    I love when some state that the purpose of the high is a secondary flue in case the appliance flue gets blocked.(this was in the ASHRAE Report)  Shouldn't equipment contain safeties that shuts it off when the flue gets plugged.  Well according to GAMA No! but then what does she know?  Might as well ask GAMPAH too.
    If a room is running out of air the draft in the flue will start reducing.  This can be so minor that it may be hard to see unless we have a digital gauge that can read as low as -.0001' w.c.
    Combustion air affects venting.  If venting is interferred with flue gasses will start to spill, even if there is measured draft.  Flue gasses are mostly CO2.  CO2 is heavier than air.  The best way to measure quickly if there is a combustion air or venting problem is to locate a CO2 detector on the floor of the room.  You have to be careful because even our breathing and exhaling of CO2 will cause the floor level to rise, so you might have to leave the room and give it a few minutes to develop some readings.
  • Mark Eatherton Mark Eatherton @ 8:27 PM
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    GAMA said that GAMPAH doesn't know what he's talking about...

    You crack me up Jim... Good one.

    Interesting input to this question. I was told a long time ago, by someone I consider an accurate heating historian, that the requirement for high and low air came from the good ol' days when solid fuel fired appliances, which were poorly insulated, typically had a high mechanical room temperature. In addition to providing the air required for combustion and draft maintenance, the thought was that by having a high vent, the hot air could be let out of the room and cool air let in near the floor. Makes some sense, except that what REALLY causes mechanical room discomfort is the Mean Radiant Temperature, which is not significantly affected by convective currents. He went on to say that when natural gas was used to displace solid fuels, that if there were a natural gas leak, with natural gas being lighter than air, it would have a means of escape before accumulating to any dangerous degree. And honestly, gas leaks were quite common in the older boiler tombs that used the tapered body, spring loaded A cock B cock gas valve systems...

    I guess if you had propane, you were flat S.O.L.....(sorta outta luck).

    Personally, I like to use a completely sealed combustion appliance, which requires no additional free air inlets into the room. I've seen WAY too many frozen/broken water lines from the required free air.

    I was once told by an obstinate inspector that arguing with an inspector is like wrestling with a pig in mud. The harder you wrestle, the more the pig likes it...

    HTH

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
    This post was edited by an admin on March 20, 2012 8:28 PM.
  • Jean-David Beyer Jean-David Beyer @ 9:20 PM
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    I have one of those.

    "Personally, I like to use a completely sealed combustion appliance,
    which requires no additional free air inlets into the room. I've seen
    WAY too many frozen/broken water lines from the required free air."

    The former homeowner made a little room in the garage where the boiler is from drywall. Some of it was cut away to allow for maintenance. There were two vents, one about half way up the wall, and another about 3/4 of the way up the wall. This may have been OK, but not code for the old GE oil burner that got all its air from the garage.

    When I replaced it by a W-M Ultra 3, the inspector warned me not to seal off that little room. Strange. Even if I did, one of those two vents was there. So I did not wall it off. I would not have done that anyway. So around here, the AHJ does require those vents. He did not measure their size though, and did not worry that one was not high enough, and the other was not low enough. Around here, you would not want them low enough anyway because they would be blocked by the snow in the wintertime.

    Later I was having a problem with that boiler (due to sloppy installation) and it got so the manufacturer's rep came out and first thing he said was to close off that vent (which I did). I chose not to confuse the inspector by calling for a new inspection. This had nothing to do with the problem, but the boiler has run just fine for a year now with no ill effects, and even passed testing with a digital combustion analyzer after I blocked off one of the vents.
  • Paul Paul @ 12:15 PM
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    book

    tim, how do i get your book on venting/comb. air    thank you
  • Tim McElwain Tim McElwain @ 11:15 PM
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    Paul

    just e-mail me at gastc@cox.net and I will send you my catalog. You can order it from there.
  • Mpj Mpj @ 9:03 PM
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    Just an update

    Plumbing inspector came by last week to do a final inspection. He looked in the boiler room, saw the air in can and said it was fine. Everything passed.
    I'm posted in my first post that an inspector came to look at the boiler room, he was the building inspector not the plumbing inspector. Sorry about that.
  • Mpj Mpj @ 9:03 PM
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    Just an update

    Plumbing inspector came by last week to do a final inspection. He looked in the boiler room, saw the air in can and said it was fine. Everything passed.
    I'm posted in my first post that an inspector came to look at the boiler room, he was the building inspector not the plumbing inspector. Sorry about that.
  • Mac_R Mac_R @ 12:12 PM
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    Formula

    The formula I was always told to use for any fuel is 50Cuf per 1,000Btu input.  So with an oil fired equipment burning at one gallon per hour you need 7,000 cubic feet of air per hour.  Using 140,000Btus/ Gallon of oil.  Take your burner output divide by 1,000 then multiply by 50.  Presto there is your combustion air needs.  Another thing you need to take into account is the change in the amount of oxygen during summer and winter.  You need more air in the summer than in the winter. Because there is less oxygen in a hot cubic foot of air than a cold cubic foot of air.
    appliances require a lot more air than most people think.  I cant tell you how many times I had units that would continually soot up on me.  I install outside air and presto.  No more soot. 
    The other nice thing about running outside air is you will also reduce the amount of infiltration into the home.  The air for the system to burn has to come from somewhere.  So it comes in around the windows, under doors, any place it can.  Now we are sealing up our homes to reduce or eliminate these "drafts" and installing ventilation fans over cooking equipment and in the bathrooms.  We no longer have the air in a house for proper combustion.  Plain and simple.  Do your self and your customers a favor and install outside air. 
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