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  1. #1
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    Default water heater and furnace venting

    I recently replaced my old furnace and water heater and have a question about how they should be vented. The furnace is an 80% 75K btu unit. The water heater is a standard 40 gallon unit. Currently, these two appliances are venting into a common flue. There is a 4" vent pipe from the furnace and a 3" vent pipe from the water heater that connect to the common 5" flue (type b vent), which extends about 15' vertically to the roof. The furnace connects to the common flue below the water heater. Should these two appliances be vented together into a common flue or vented separately? So far the contractors that have provided bids say that a common flue is ok and venting separately isn't necessary. However, some of what I've read on the internet says that these two appliances should be vented separately. Can someone help clarify this for me? I am asking this because I am particularly sensitive to indoor air pollutants and am concerned that common venting the furnace and water heater could allow leakage of the flue gases into the living space.

  2. #2
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    Default Re: water heater and furnace venting

    There are rules in the form of vent tables published in the Fuel Gas Codes that govern this.

    The short answer is: Yes, it is permissible and safe to vent the two appliances together into a common vent.

    The conditions peculiar to your application are that the height and size of the common vent allow for a total BTU input of 159,000 BTU's. The sizes of the vent connectors, that is, the individual vents that connect each appliance to the common vent, are also within the limits of what the vent tables allow.

    This is assuming that the common vent rises thru the roof without much offsetting and without excessive horizontal sections. If the common vent is fairly straight, you have no worries.

    You should also do a simple test to ensure that the operation of the furnace blower does not cause de-pressurization of the furnace room, which, in turn, would cause the WH to backdraft:

    With the furnace blower running, all doors and windows closed, and the kitchen and bath exhaust fans running, go inside the furnace room and close the door. Feel along the bottom of the door for air rushing into the furnace room. If it's a lot, that means the furnace room is being de-pressurized, and there is a risk of backdrafting the WH.

    If that happens, post back for suggestions on how to deal with it.

  3. #3
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    Default Re: water heater and furnace venting

    Look at your installation manual all the do's and donts are in there for proper venting
    Gizmo

    Cut it 3 times & it's still to short.
    Inventor of the Miter Master Plus.

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  4. #4
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    Default Re: water heater and furnace venting

    Quote Originally Posted by rdesigns View Post
    You should also do a simple test to ensure that the operation of the furnace blower does not cause de-pressurization of the furnace room, which, in turn, would cause the WH to backdraft:

    With the furnace blower running, all doors and windows closed, and the kitchen and bath exhaust fans running, go inside the furnace room and close the door. Feel along the bottom of the door for air rushing into the furnace room. If it's a lot, that means the furnace room is being de-pressurized, and there is a risk of backdrafting the WH.

    If that happens, post back for suggestions on how to deal with it.
    Sorry but this makes little sense to me. The furnace blower circulates air through the duct work and returns. It does not pick up air from the furnace room.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  5. #5
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    Default Re: water heater and furnace venting

    Quote Originally Posted by rdesigns View Post
    You should also do a simple test to ensure that the operation of the furnace blower does not cause de-pressurization of the furnace room, which, in turn, would cause the WH to backdraft:

    With the furnace blower running, all doors and windows closed, and the kitchen and bath exhaust fans running, go inside the furnace room and close the door. Feel along the bottom of the door for air rushing into the furnace room. If it's a lot, that means the furnace room is being de-pressurized, and there is a risk of backdrafting the WH.

    If that happens, post back for suggestions on how to deal with it.
    some your post has merit this last section doesn't.

    The normal combustion cycle of both the furnace and the water heater would draw air toward the appliances so you would feel the " air rushing into the furnace room. " this would be normal for the water heater and an 80% furnace.

    If the exhaust bath fans were creating a negative pressure this would draw air away ---- air rushing out of the furnace room ---- causing back drafting.

    If the blower is causing issues of back drafting there are other issues.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #6
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    Default Re: water heater and furnace venting

    Quote Originally Posted by JLMCDANIEL View Post
    Sorry but this makes little sense to me. The furnace blower circulates air through the duct work and returns. It does not pick up air from the furnace room.
    Jack
    Contractors miss this all the time. It is a real danger, and I routinely check every new house that I inspect. I find at least half a dozen every year that fail this test, and they cause the WH to backdraft. In these cases, the moment you shut off the furnace blower fan, the backdrafting stops.

    As you say, one might never suspect the furnace and duct system of causing the water heater to backdraft, but here's how it happens:

    The return air of most duct systems is more leaky than the supply, especially if building cavities like joist and stud spaces have been used for returns.

    The return ducts operates at a negative pressure compared to air in the house, but in cases where the ducts are enclosed in chases and furr-downs, such chases become negative as well, due to the leakage. The place where the chases can "pull" air is where they open up in the furnace room. Beause of this, the furnace room itself goes negative, and if it's in the range of 5 Pascals (0.02" water column) or more, it is enough to backdraft the water heater.

    If the simple test I suggested reveals such a condidtion, I use a draft gauge and manometer to actually measure the amount of de-pressurization.

    There is no question that the furnace and duct system is the culprit, because shutting off the blower instantly stops the condition.

    BTW, this is not due to a lack of combustion air being ducted into the furnace room. All the houses I test have at least a 6" combustion air duct. I had one where there were two 8" CA ducts that would still backdraft the WH.

    I should also mention that this problem is almost always in houses where the the furnace room is in the basement. But it can happen in houses with the ducts in the attic, if the supplies there are leaky. In that case, the attic goes positive due to supply leaks and the house goes negative.
    Last edited by rdesigns; 02-04-2010 at 09:55 AM. Reason: omitted word

  7. #7
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    Default Re: water heater and furnace venting

    Quote Originally Posted by Gizmo View Post
    Look at your installation manual all the do's and donts are in there for proper venting
    Installation manuals for Category I furnaces (80% efficiency, non-condensing, non-positive vent pressure) usually do not include the vent tables, but instead refer you to the code or to GAMA's vent table, which are identical.

    For Category IV furnaces, (condensing, positive vent pressure), the installation manuals always include the venting instructions, and the Fuel Gas code requires you to go to the manufacturer's instructions for proper venting of Category IV appliance.

  8. #8
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    Default Re: water heater and furnace venting

    rdesigns,
    Interesting, thanks for the detailed explanation
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  9. #9
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    Default Re: water heater and furnace venting

    Quote Originally Posted by JLMCDANIEL View Post
    rdesigns,
    Interesting, thanks for the detailed explanation
    Jack
    Past editions of the fuel gas codes used to have an appendix explaining how to do the test I mentioned to the OP, but I haven't seen it for the last several code cycles.

  10. #10
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    Default Re: water heater and furnace venting

    Quote Originally Posted by rdesigns View Post
    Contractors miss this all the time. It is a real danger, and I routinely check every new house that I inspect. I find at least half a dozen every year that fail this test, and they cause the WH to backdraft. In these cases, the moment you shut off the furnace blower fan, the backdrafting stops.

    As you say, one might never suspect the furnace and duct system of causing the water heater to backdraft, but here's how it happens:

    The return air of most duct systems is more leaky than the supply, especially if building cavities like joist and stud spaces have been used for returns.

    The return ducts operates at a negative pressure compared to air in the house, but in cases where the ducts are enclosed in chases and furr-downs, such chases become negative as well, due to the leakage. The place where the chases can "pull" air is where they open up in the furnace room. Beause of this, the furnace room itself goes negative, and if it's in the range of 5 Pascals (0.02" water column) or more, it is enough to backdraft the water heater.

    If the simple test I suggested reveals such a condidtion, I use a draft gauge and manometer to actually measure the amount of de-pressurization.

    There is no question that the furnace and duct system is the culprit, because shutting off the blower instantly stops the condition.

    BTW, this is not due to a lack of combustion air being ducted into the furnace room. All the houses I test have at least a 6" combustion air duct. I had one where there were two 8" CA ducts that would still backdraft the WH.

    I should also mention that this problem is almost always in houses where the the furnace room is in the basement. But it can happen in houses with the ducts in the attic, if the supplies there are leaky. In that case, the attic goes positive due to supply leaks and the house goes negative.
    5 pascals isn't always an indication there is indeed a serious back drafting issue since this is also dependent on the outside temperature --- 30 degrees F and below can be in that range for natural vented ( atmospherically ) appliances. Besides you can also use the old smoke test without the need for manometers to indicate draft flows.


    Pressure within the home will fluctuate constantly from atmospheric changes along with air exchange from the home. Thus affecting the natural vented appliance exhaust which back draft regularily anyway ---- it's the duration that's important. This has been always the case ever since central types of heating have been used --- whether it has been fire places or mechanical combustion equipment.
    When the combustion equipment fires usually there is enough air flow to counteract the back draft .
    There is documented information measuring the amount of CO from a back drafted water heater being less than a gas range being used in a home.

    In the case of natural vented appliances they rely on air within the home to provide the needed air for combustion to occur.
    Back drafting is more of an issue when the home is depressurized from inside air in the home exiting and not being replaced at the same volume. The typical culprits are bath exhaust fans , over sized kitchen exhaust fans ( > 250 cfm ) , clothes dryer ----. these remove air that is being used for the combustion appliances.
    Hence the requirement by all codes to provide a fresh air intake which is to also provide the " make up air " needed to offset the depressurization from the exhaust fans.

    Since a forced air heat system recirculates the air within the home they don't tyically starve the combustion equipment and cause back drafting.

    Though one thing that has been implied is having the combustion equipment enclosed in a room ( usually too small ) with a door.
    Consider closing off the combustion equipment with a door and not allowing the free flow of air the appliances need. Generally this type of setup starves the equipment then add any depressurization from exhaust fans and you end up with a receipe for disaster.

    To be honest a blower door test is the most accurate method to calculate air exchange values and properly evaluate if and how much depressurization is occuring and how much " make up air " is required.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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