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Combusion Air Vents for Boilers, Dryers & Water Heaters?

NTL1991 Member Posts: 103
Hello there.

I've done the BTU input measurements of the fuel burning equipment in my basement. My 2 gas dryers, 3 boilers, and 3 water heaters total 532,000 BTU/hr. My basement, where these appliances are located, is 5,288 cubic feet in area. According to my calculations, my basement is only supplying enough air for 79,000 BTU/hr of combustion. I am at 9.9 cu. ft. per 1,000 BTU, and I need to be at 50 cu. ft. Therefore, I need 110 square inches of vent area to the outdoors for intake air, with each square inch supporting 4,000 BTU of combustion.

Now, my basement has four 29x12 basement windows, each with four 7-3/4"x9-1/2" panes of glass. With a 6" round vent hood, each gives me ~28 square inches of intake air. Or, I could remove two panes of glass and screen and hood the openings instead, giving me a total of ~147 square inches of intake air, enough for my appliances.

BUT, my problem is with the air... If I used 4 6" vents, none of them come with flaps (like in a dryer hood, but opening in the opposite direction) inside to stop cold air from coming in when the appliances are not in use. If I used a dryer vent sort of flapper, just turned in the opposite direction, would the negative pressure in the basement from the running appliances be enough to open the flapper? If so, where could I get just a 6" flapper?

Any ideas for satisfying the combustion air while also not letting every cold breeze into my basement when they're not in use?

Here's the boiler/water heater area. On the opposite wall are the dryers, with an identical window above them.:

<img src="http://i12.photobucket.com/albums/a212/NTL1991/Basement/UtilityArea.jpg" width="800" height="600" alt="" />
Nick, Cranston, RI


  • MikeyB
    MikeyB Member Posts: 696
    Combustion Air

    I know Timmie McElwain will be able to answer you questions, and you might be able to use a product made by Tjernlund called the In-Forcer, www.tjernlund.com
  • MikeG
    MikeG Member Posts: 169
    Wind tunnel

    If everything is running, how cold will it be in the basement with all that air coming in?   Wii that create another issue related to heat loss infiltration?
  • NTL1991
    NTL1991 Member Posts: 103

    I previously had a single 6" round air intake vent in the glass portion of the window to the right of the boilers. When I moved into the house, I removed it, not knowing any better. Whoever installed it didn't do the calculations, as obviously, this single 6" round vent was clearly not enough. Now that I know the problems with lack of combustion air for the burning of fuels, I'm trying to rectify the issue.

    I never had any draft issues in the basement of cold floors or whatnot. (I'm in the 1st floor apartment, and I have hardwood floors). In the winter time, the basement stays a constant 65 degrees or so, and the working boilers and hot water heaters, along with all of the piping runs in the basement keep it at a nice temperature. I'm sure two 9x7 vents wouldn't cause an issue in that regard.

    Also, with an apartment that has a heat loss of 35,000 BTU/hr, and a boiler that's putting out 100,000 BTU/hr, there's not much of anything that could cause my apartment to become uncomfortable even on the coldest of nights.

    For complexity, I'd like to find a simple setup with no inducers or whatnot, as it just seems unnecessary...
    Nick, Cranston, RI
  • Ironman
    Ironman Member Posts: 7,350
    I've Seen Those Boilers Before

    Can I offer a more practical solution. The code requirement for combustion air assumes a tightly sealed house or mechanical room. Yours appears to be loose to average. Your boilers are over-sized.

    My solution would be to have a savvy tech come in who has a combustion analyzer and knows how to use it. Have him down fire the oil fired boilers as much as the manufacturer will allow. Then turn on all the equipment at the same time as well as bath and kitchen exhaust fans. Then check for proper draft in each appliance. If they all have proper draft, then you should be OK. If not, then start letting outside air come in incrementally until the draft is OK with all appliances and exhaust fans on.

    I would also install a CO detector
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    A 5 gallon solution...

    Terminate the lower incoming combustion air pipe inside of a 5 gallon bucket. This will create a cold air "trap" and will keep the cold air held back at bay. You can do the same thing with the upper combustion air by using a series of elbows to crete the equivalent of a plumbing P trap.

    As for combustion air requirements, depending upon where you are and the code being enforced, the introduction of outside air is usually between 1 square in per 4,000 or 5,000 btuH, 1/2 from within 1' of the ceiling, and the other half introduced within 1' of the floor. My assumption is a direct connection to the outside air through the outside wall. If the vent has to travel horizontally, the number per goes down, and if the air is being introduced vertically, it goes up.

    DO NOT attempt to use any kind of flapper, unless approved by code. I believe there are provisions for using electrically controlled dampers, but they MUST be interlocked with safety end switches.

    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.
  • NTL1991
    NTL1991 Member Posts: 103

    Does the infiltration category of tight, normal and loose only apply to the mechanical room? The basement is normal to loose, as you stated, due to air leaks around the wooden windows. The bottoms of the wall cavities are fire-stopped, and anything that passes through the foundation is insulated around. The apartments on the other hand, are tight.

    I do have a CO detector installed on the beam just above the lally column in the picture, along with a smoke detector. It has never gone off before.

    I've heard of induced systems which are linked to the boiler controls which are active only when a burner is active, which would provide a damper to keep cold air out.

    Down firing would also make sense as the input BTU rating would be lower, which would lower the overall intake air requirements.
    Nick, Cranston, RI
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Personal preference...

    I'd pipe ALL combustion/ventilation air in from the outside, and NOT assume partial supply through the infiltration. I do this for numerous reasons.

    1. If you are dependent upon infiltration air from the combustion process, you take a BIG chance of freezing and breaking pipes that are near the vicinity if the incoming air (sill edge with copper heating pipes).

    2. If someone decides to "tighten up" the building envelope, then all of the free air you'd anticipated will be gone with the wind....

    The makers of CO detectors warn against placing them directly in the combustion zone for a reason. With as many BTU's as you are dumping into a large stack, under certain conditions (cold weather, cold start boilers) you may struggle to establish draft, and some spillage can be expected (actually allowed under the provisions of the code). This will cause nuisance alarms. The fact that yours has never gone off (that you are aware of...) indicates to me that you have plenty of draft to maintain the proper flow of flue gasses in the right (Up, up and AWAY) direction.

    Our code authorities USE to allow for the free air in a basement to compensate for C/A-V/A, but realized that in most cases, the basement would be finished eventually, and the proper C/A-V/A is NOT piped correctly to the new confined mechanical room space, so they now require 100% outside air be piped to the appliance to insure proper C/A-V/A.

    Better safe than sorry. Waking up dead is NOT a good way to start the day ;-( If you haven't done so, I'd have a trained, qualified service technician with a combustion analyzer check the operating conditions of ALL combustion appliances in the building (include ovens, ranges, stoves unless electric). Even that is not a guarantee against CO poisoning, but if you don't test, you won't know.

    FACT: In a recent commercial boiler room study that I completed for the State of COlorado, nearly 50% of the boilers tested had CO in excess of allowable standards. No spillage severe enough to cause any health concerns, but it just takes a wind out of the wrong direction to cause severe spillage, and death and destruction of peoples lives. What is sad, is that there is a State boiler inspector in ALL of these boiler rooms once a year. I am in the process of trying to get the inspectors set up with CO detectors, and then I will volunteer my time to go and train them as to the proper use of the CO sensing device. I really don't want them to perform a full flue gas analysis, just a simple check for the production and presence of CO. It will require the State to modify their protocal to allow for follow up if high CO is found, but at least we'd know if there were any combustion issues in the first place.

    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.
  • icesailor
    icesailor Member Posts: 7,265
    CE detecectors:


    What kind of CO detectors are they going to use?

    I have a UEI # CO71A that I carry everywhere. It is idiot proof and seems to be very accurate. It wasn't expensive for what it does.

    I'm really surprised at the places I get alarm readings.

    The one I have I consider like a seat belt. It's only good if you turn it on. I've had times when I thought that something might be there and there wasn't. And times when I knew that things were OK and they weren't. Not dangerous mind you, but knowing that I checked.
  • NTL1991
    NTL1991 Member Posts: 103

    Thanks Mark, great info.

    My chimney is about 35 feet tall from the ground, and I believe my flues (one for my fireplace, and the other for the boilers/water heaters) are 8"x12."

    My brother came over today, and while I was showing him some improvements I've done to the basement, I talked about cleaning and doing combustion analysis for each boiler, pointing out the fact that when the two new boilers were installed by my plumber who removed the original steam system, combustion analysis were never done on them, as I have no holes in the breeching. Also, the last combustion analysis on my Weil McLain was in 01 when it was producing steam.

    I also talked about downfiring my WM, and he said that depending on how far I go, I might need to change the retention head out with one designed for a smaller nozzle.

    I looked at the Weil McLain manual for my SGO-4, and they made a SGO-2 which used a .70 nozzle rather than .85 which I currently have in my boiler (Original was 1.20GPH). BUT, the AFG that my boiler uses currently has an F-4 head on it, whereas the SGO-2 uses an L1 head. Can these heads be changed out? I see a part listed as "Air Tube Combination - AFG50MBAS" which is the L1 head. My F-4 head is listed as "AF44WHPW". The pump pressures are the same 100PSI; the only difference being the .70-75B nozzle rather than my current .85-80B.

    The same goes for the LAARS Newport which is an NP-85, with a .85-70A nozzle. They also made the NP-75 with a .75-70A, but I'd need to get "A LOW FIRING RATE BAFFLE" as the installation manual states. The part number for such a baffle (for the F3 head) is not listed in the manual...

    So, my brother said he'd downfire the boilers, do the combustion analysis, smoke tests, and draft measurements for all three boilers, which will give me a current baseline for efficiency and also intake air. It's not a good idea to make changes to the amount of intake air in my basement if the boilers will soon be downfired and burning up less oxygen.
    Nick, Cranston, RI
  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126
    How 'bout make a room?

    I'm looking at the windows on both sides and thinking exterior chimney.  It'll ordinarily have lots of draft with 35', but on a cold night when the boilers are all off due to setback stats and infrequent cycling of the water heaters, there's an opportunity for a chimney that's exposed to the weather to cool too much and start to reverse flow(become a source of make up air to the building; aka "downdraft").  As you improve the efficiency of the boilers, the chimney will run at lower temps increasing the risk.

    It looks like you have lots of space to wall it off & build and a room around the 6 units.  Seal it off fully up in the joist spaces.  Use one of the window wells for venting.  Plan a provision for the future service and removal of all of the appliances, especially the tall Hwh that will be in the corner.

    A good option for air supply would be to size passive venting for the combined air requirements of the HWHs, and a motorized damper with end switch arrangement interlocked to the boilers.(similar to the vent damper rig on the gas boiler)

    Sounds like you've done some work on the house yourself, so you could handle the construction at minimal cost, and yer bro the pro; could handle the air supply and control rig.
    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
  • NTL1991
    NTL1991 Member Posts: 103
    Good Idea

    Hmm. I never thought about enclosing the boilers and hot water heaters in a utility room. That would allow me to vent through a couple of panes in the window while also keeping the rest of the basement warm.

    I did, in fact, build a room in the basement already, but it's on the opposite corner of the house. It's a workshop and laundry room, where my gas dryer and washing machine are located. The other two dryers and washers are in the open area of the basement. You're right, if I do build a utility room, I'd have to put in a 36" door and give enough clearance around the appliances for service and replacement...

    Nick, Cranston, RI
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