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Can I exhaust 2 heaters through the same concentric vent?

baltik
baltik Member Posts: 16
edited August 2 in Radiant Heating
I have a wall mount DHW system and am adding a wall mount boiler, Given tight spacing and decorative stucco finish on my home, I am reticent to make an additional hole for venting purposes:
My DHW is currently vented through one of these: https://m.media-amazon.com/images/I/81Qg-gnnuzL._SX522_.jpg

can I vent both of my heater through it? (intake is easy through a crawl space vent below the heater)

Comments

  • Ironman
    Ironman Member Posts: 6,893
    No, not unless the appliance manufacturer approves it.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EdTheHeaterManHomerJSmith
  • baltik
    baltik Member Posts: 16
    Fair enough - I guess I dont quite understand the why. If both units have a full 2" unobstructed exhaust run, what's the problem?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    Agree with Ironman. Here is some Info for you:
    https://www.rses.org/assets/serviceapplicationmanual/630-152.pdf
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • HVACNUT
    HVACNUT Member Posts: 4,624
    Get the I&O manual for each appliance and see what they say. 
    And what you're showing isn't a concentric vent. It's a separate exhaust through the face while combustion air is drawn from the perimeter. 
    I'm betting neither manufacturer shows a venting diagram for what you're proposing. And your new boiler vent needs to be a certain distance away from the existing, plus all the other obstacles. 


  • Jamie Hall
    Jamie Hall Member Posts: 19,150
    or to look at it from a somewhat more simple minded direction -- what happens if both appliances are running at once? Unless there is a positive lockout system, they will from time to time. Then the vent will be half as big as it needs to be in terms of area, with four times the pressure loss and neither one will work properly. Use a bigger vent? Ah but now -- now when only one is running, the vent is twice what it needs to be, and that won't work either.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EdTheHeaterMan
  • JUGHNE
    JUGHNE Member Posts: 9,822
    These exhaust fans have just enough power to get the gases outside, and sometime not even that much.
    That is why the distance, fittings and length are critical to operation.

    If there were 2 exhaust streams that met in one fitting, there could be back flow thru the system that was not running.

    Just put your hand over the exhaust you have now and the system should shut down fairly quickly.
    EdTheHeaterMan
  • Ironman
    Ironman Member Posts: 6,893
    The only approved common venting of category IV appliances that I know of is when both are the same model and size and they have a common communication network where they work in tandem.

    Everyone that I’ve seen has a back draft damper and keeps the combustion fan running on both (or all) when any one is firing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    GGross
  • baltik
    baltik Member Posts: 16
    I think there is some confusion here... Each exhaust has it's own isolated run of 2" piping (my run is short sub 6 feet) the only possible restriction point is at the concentric cover. The exhaust streams will not mix in the above scenario since they are each exiting through isolated openings. Presumably the cover has sufficient flow as it can provide ample intake and exhaust flow for a single unit, not sure why it would be too restrictive for an exhaust/exhaust configuration.
  • heathead
    heathead Member Posts: 191
    That cover is only one out and an intake in. You need to look at both the manuals as HVACNUT has said. They will spec out how close to other intakes and exhausts you can run. If you have them too close they can cross contaminate the intake or worst poision you if wind pressure on one side off the house, is applied to one working appliance and then pushed in the non working one with wind pressure and that exhaust the gas inside the house.
    EdTheHeaterManIronman
  • PC7060
    PC7060 Member Posts: 739
    Two major points 
    1. All exhaust configurations must match OEM approved configurations. Running two boilers into a single vent is not a approved configuration.
    2. That vent outlet is not designed to support the flow produced by two boilers simultaneously. There is nothing to prevent exhaust from one system backflowing into the second boilers exhaust line.  
    EdTheHeaterManIronman
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    edited August 3
    baltik said:

    I think there is some confusion here... Each exhaust has it's own isolated run of 2" piping (my run is short sub 6 feet) the only possible restriction point is at the concentric cover. The exhaust streams will not mix in the above scenario since they are each exiting through isolated openings. Presumably the cover has sufficient flow as it can provide ample intake and exhaust flow for a single unit, not sure why it would be too restrictive for an exhaust/exhaust configuration.

    Have you ever heard the saying "A chain is as strong as its weakest link". Well, an exhaust system is as clear as its smallest opening. If two exhausts are exiting at the same opening. and the opening you are showing is enough for one appliance, then when both appliances operate simultaneously (that means at the same time... and that will happen) the opening is not large enough for separate vent pipes that converge there at the opening.

    Unless you are trying to say that there are 2 pipes existing at that termination. Can both pipes be exhaust? The answer is still NO! You see the reason the exhaust gas vent instructions are so detailed and restrictive is because there is a deadly gas called Carbon Monoxide (CO) that must not be allowed to find its way into your bedroom while you are sleeping, or your den while you are watching the Super Bowl, or into the return duct of your air conditioner while you happen to be living in your home on any given Tuesday.

    Some one has already tried what you are asking about and has fell victim to this poisonous gas, an insurance company has paid a claim, an inspector has filed a report and a member of a board of underwriters has recommended to the uniform building code board members they rewrite the code to specifically tell everyone that they can not safely do what you want to do no matter how nice it looks.

    But it's your home, we will see you in the funny papers. ...or maybe as a news story with the local fire chief telling how CO poisoning is serious business.

    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    edited August 3
    To further explain the 2 existing pipes can not both be exhaust because that cover would be a restriction on the pipe that exits under the solid portion of the cover.

    Why not drill a 2" hole in the cover?

    That might work but, where is the intake air going to come from? You mentioned the crawlspace, and possibly extending that (those) intake pipe(s) via the crawlspace.

    It will work until there is a significant wind condition. the reason both the intake and exhaust are so close to each other is so they are in the same pressure zone. If the wind is from the east and the intake is on some other side of the house, the wind pressure could overcome the combustion fan pressure causing the vent to be ineffective. This will cause the burner to stop in the best case but could force exhaust gas into the home in some cases.

    The Philadelphia Housing Authority found out the hard way when they installed Lennox Pulse furnaces in thousands of units in order to eliminate the buildings central boilers heating system. Someone made an executive decision to put all the exhaust pipes on one side of the high-rise buildings and all the intakes on the opposite side of the buildings. Everything worked fine until there was a high wind condition. The pressure of the wind overcame the pulse furnace ability to force the exhaust gas out the vent pipe. The details of this may be off a little but the concept is there.

    If Philadelphia only had a Dan Holohan to get their steam systems to work properly!


    The mistakes were made already and the intake and exhaust pipes need to be near each other, but not so close that the exhaust gas will find its way to the intake. Those measurements are pretty specific.

    That is not to say that I have not gotten pretty inventive with some of my designs. But they followed the minimum and maximum dimensions and clearances. Safety First!
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • baltik
    baltik Member Posts: 16
    Thanks Ed, your point is well taken. The exhaust is far from any windows/doors so I am not concerned about Monoxide. The high wind condition imbalance is an interesting point, though I've never seen a manufacturer requirement that intake and exhaust be on the same exposure. Hypothetically speaking if both were far and shielded from wind, would that change your perspective? image for refrence
  • HomerJSmith
    HomerJSmith Member Posts: 1,624
    edited August 3
    While the manual shows concentric venting, I have never ever been a fan of concentric venting. If done properly it does work. If exhaust fumes works its way into the air intake you can have big problems. If installed on the windward side as opposed to the leeward side of the building exhaust can move into the air intake leading to gas reversion which will over time destroy the appliance.

    I recommend separate appliance venting, exhaust and air intake, at the appropriate distances, above the snow line.

    Unapproved venting will probably void the warranty.
    EdTheHeaterMan
  • watermover
    watermover Member Posts: 1
    edited August 3
    Consult your install manual for engineered operational installation procedures. Most systems that operate common vent have communicating induced draft motor fans that turns on both units fan motors during either units operation cycle. The venting is always upsized to accommodate this action
    EdTheHeaterMan
  • GGross
    GGross Member Posts: 188
    The vent manufacturer might not approve it.
    The equipment manufacturers definitely don't approve
    Code probably does not approve it (not allowed here anyway if the mfr does not approve)

    Please don't try to common vent your 2 different appliances. The best case scenario you could even hope for with using unapproved venting is that it may negatively impact your warranty, thats the BEST case. Just don't do it
    EdTheHeaterMan
  • HVACNUT
    HVACNUT Member Posts: 4,624
    baltik said: 
    No. Just no. 
    EdTheHeaterMan
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    THIS IS A SUGGESTED DIAGRAM. THIS IS NOT AN APPROVAL OR ENDORSEMENT

    This may be very close to the manufacturers design. You would need to submit this to the manufacture of each appliance, with pipe size, length, number of elbows, and distance apart. The air intake from the crawlspace must be piped to the same side as the exhaust, and be at least 12" above the expected snow line. (if the snow ever gets above the air intake due to unusual snow amount, you will need to clear the snow so the appliance can have combustion air)

    We had a condo owner that wanted to have radiant floor heat in the entire 4 level unit. There were no provisions for any boiler venting in any of the units and there were no outdoor appliances allowed to hang on the building. With little or no options we reached out to Weil McLain for a determination on whether we could conduct the vent pipe down thru the floor to the crawl space then to the exterior of the building. Within a week we had a drawing approving a drop vent with with a provision for a condensate drain tee in the crawl space.

    It looks something like this, but it was all we needed to get the inspector to approve the boiler installation under the steps on the first floor of the condo.
    (I did not tell Weil McLain that we needed this for only one of the 350 condos in the complex.)



    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
    GGrossPC7060baltik
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    edited August 3
    baltik said:

    Thanks Ed, your point is well taken. The exhaust is far from any windows/doors so I am not concerned about Monoxide. The high wind condition imbalance is an interesting point, though I've never seen a manufacturer requirement that intake and exhaust be on the same exposure.

    I Disagree with you on that point, I believe you have seen this illustration or something similar in about every manufacturer I/O manual or venting supplement for Category IV Direct vent.

    Some Appliance are approved for "Power Vent" where the combustion air can come from within the structure, as long as it is not considered a "confined space". If you don't know what that means then you need to study more about venting before you decide to reinvent the wheel on gas appliance venting.

    Are you a DIY installing this? I would strongly recommend getting the plumbing and fire inspector to sign off, both appliance manufacturers to sign off and even consider getting a Professional installer to make sure this is save? Some (not all) will come inspect your work and use the needed combustion testing equipment to verify your completed project. This depends a lot on your DIY ability to do it right.


    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
    baltikGGrossHomerJSmith
  • HVACNUT
    HVACNUT Member Posts: 4,624
    @EdTheHeaterMan, I may be mistaken but I believe the OP is looking to exhaust 2 fan assisted appliances into one common termination.
    Your diagram is showing two exhaust terminations. The OP is showing one. The word "concentric" shouldn't be used here.
    EdTheHeaterMan
  • baltik
    baltik Member Posts: 16
    Fair point on the concentric. supplyhouse lists the horizontal termination as concentric, but I don't believe it technically is
    https://www.supplyhouse.com/DiversiTech-HVENT-2-2-Horizontal-Termination-Kit
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    HVACNUT said:

    @EdTheHeaterMan, I may be mistaken but I believe the OP is looking to exhaust 2 fan assisted appliances into one common termination.
    Your diagram is showing two exhaust terminations. The OP is showing one. The word "concentric" shouldn't be used here.

    I agree that concentric is the wrong term
    I also know the there are two 2" PVC pipes associated with that cover. If that cover were removed, and both pipes were able to be used for exhaust (as I have illustrated) then the stucco wall finish need not be disturbed where it is anywhere visible. (I think this is the goal here)

    In order to accomplish this, the less restrictive pipe, (air intake does not need to worry about condensate removal) could be run into the crawlspace and out the foundation then piped up above the snow line. Without exact dimensions of the building, crawlspace, distance from exterior wall, and other factors, I can not make the determination as to whether this will work. That is why I recommend getting manuf. to sign off. But, you should agree the plan has merit, don't ya think?

    I might drill a 1/2" hole in the air intake pipes in the crawlspace, just in case water did find its way in there.

    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
    GGross
  • JUGHNE
    JUGHNE Member Posts: 9,822
    Many condensing appliances allow single pipe exhaust with the combustion air taken from the basement.
    (Provided there is adequate cubic area freely available....for both units.)

    If inside combustion air is used it will be drawn into the basement thru air leaks you have.
    It may dry out the air in the house.

    If so, then you could use the 2 holes in the outside wall you have now for exhausting the two burners.

    But, they are probably too close together so you would have to elbow one of them, possibly up some distance, and the other down towards the ground.

    It will clutter up the side of your house worse than the 2nd terminal like you have now.

    I have a stucco house and have cut into several others for the purpose of venting, exhaust fan vents and electrical etc.
    the work can be done neatly and the vent hoods have plenty of flange to cover ragged edges.

    Study both I&O manuals for each unit.

    IIWM, I would just add a second terminal like you have, if approved.

    Space them far apart to avoid recirculation of gases and freeze up of inlet air openings.....happens all the time.
  • Tim McElwain
    Tim McElwain Member Posts: 4,517
    Simple answer both manufacturers have to approve what you are attempting to do. In addition the Authority Having Jurisdiction (Local Building Inspector) must also approve the installation.
    EdTheHeaterMan
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,336
    edited August 4
    Agree with both @JUGHNE (except for the elbow away from each other part) & @Tim McElwain However, here is a detail that needs dimensions added along with the other illustrations with dimension and detail added so you can ask for approval from each manufacturer to make it easier to get Authority Having Jurisdiction approval.
    A real craftsman with a drill press and the proper hole saw could make this look nice!
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • Ironman
    Ironman Member Posts: 6,893
    @baltik
    I think you’re making this more painful than it needs to be. If both appliances are indirect vented (combustion air taken from inside), then both exhaust pipes can exit side by side as long as the minimum separation between them is maintained. Check both manufacturers I/O manuals and use the larger number. It’s probably 12-24”.

    Skip the idea of trying to use the pancake termination with some kind of modification. Just put the pipes through the wall maintaining the proper clearances.

    If you want some kind of fancy trim behind them, that’s fine.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    hot_rodkcopp
  • PC7060
    PC7060 Member Posts: 739
    edited August 4
    I pre drill the guide hole and used a 4” diamonds core bit to cut the opening through 2” of masonry stucco for a 3”vent on a listed historic house. Took some time but got a nice clean hole. 


  • Larry Weingarten
    Larry Weingarten Member Posts: 2,588
    Hi, I'm going to agree with @PC7060 , A good drill and operator can deal with stucco and not make a mess of things. That clearly would be a simpler and probably safer approach.

    Yours, Larry