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Thread: Gas water heater down draft

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    rollerdawg is offline New Member
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    Gas water heater down draft

    I have an A.O. Smith FSG 75 gal gas hot water heater. On very cold nights, a reverse downdraft is created in the fluepipe and it takes a very long time for the draft to change direction when the hot water heater fires up. During this time, the spent gas fumes are being exhauted into my basement.

    Making matters worse, my fireplace flue shares the same chimney with the gas hot water heater. Yep - you guessed it, when I start a fire in the fireplace, the strong downdraft in the water heater flue sucks smoke down into my basement. The only way to avoid a smoky basement is to first turn the water heater way up to warm the flue pipe, then I'm OK to start a fire. Later in the evening however when the water heater cools back down and the draft again reverses, I again get some smoke in the basement, along with (I'm sure) spent pilot gas from the water heater.

    Is there any way to add a "anti draft shield" into the 4' section of metal flue pipe that extends between the draft diverter and the basement wall? I'm hoping this would prevent the backflow of cold air into the basement when the water heater was not on. Also, can a draft inducer be added to a conventional hot water heater flue to help exhaust the hot gas fumes?.

    Any advise would be appreciated. BTW, I pointed this out to the contractor when we bought the house - of course nothing was done about it.

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    HayZee518 is offline Deity
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    Simple - put a chimney cap on the chimney. Also check to see the stack is the highest object above the roof. Outside temp has a lot to do with it too. 55 or below should provide adequet "pull" for the chimney.

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    rollerdawg is offline New Member
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    HayZee, does a chimney cap have a flapper of some sort that would prevent backdraft (i.e. one way direction of airflow only?) I used to have a chimney cover that covered both fireplace and hot water tank flue pipes but that actually made matters worse. Thanks for the response!

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    No flapper just covers the top and allows the gases to escape. The top of the hot water tank should have a bonnet like an inverted funnel that the flue pipe goes through. This is to induce a draft into the three inch fluepipe going to the chimney.

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    rollerdawg is offline New Member
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    HayZee,

    My hot water tank does have the diverter funnel but it does not create a strong enough updraft unless the hot water tank has been on for a few minutes. As I mentioned, on very cold nights, air flows backwards into the basement. I must have severe negative pressure in the house and that's the reason I'm looking for a way to stop the backdraft.

    Jim

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    HayZee518's Avatar
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    I have seen an electrically driven flapper in the flue pipe right where it enters the chimney. When the stack temperature reaches a set point the flapper opens and the super hot gases go up the chimney producing a positive draft. Check home depot.

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    rollerdawg is offline New Member
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    Thanks for the tip - I'll check into it.

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    LazyPup is offline Deity
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    I suspect there are a number of things which are contributing to this problem.

    1. The Codes expressly prohibit connecting a water heater flue to a masonary flue that serves a fireplace.

    "No Connection to a fireplace flue"
    REF: International Residential Code IRC-2427.5.6.1
    Uniform Plumbing Code UPC-802.4(M)

    Connecting a water heater to a masonary flue can be a very challenging problem. Whenever we connect two appliances to a common flue, the flue must be sized to the required cross sectional area for the largest appliance plus 50% of the required cross sectional area for the smaller appliance, BUT the cross sectional area may not exceed the Maximum cross sectional area for either appliance.

    Under the International Residential Code the MAXIMUM cross sectional area is determined by multiplying the appliance flue cross sectional area x 7.

    Cross sectional area for a round duct is determined by multiplying the diameter x Pi where Pi = 3.1416.

    Assuming the water heater has a 3" flue pipe the cross sectional area would then be:

    3" x 3.1416 = 9.4248 sq.in.

    Multiplying the cross sectional area of the flue pipe by the code standard of 7 =

    9.4248 x 7 = 65.97 sq.in.

    The cross sectional area of a square or rectangular duct or flue is determined by multiplying length x width therefore the maximum cross sectional area of a square duct could be determined by computing the square root of the maximum cross sectional area.

    the square root of 65.97sq.in = 8.122"

    Therefore the maximum internal opening of the masonary flue cannot exceed 8 1/8" x 8 1/8".

    Under the Uniform Plumbing Code the code standard multiplier is 4.

    Again using the cross sectional area of a 3" flue (9.4248 sq.in) we derive:
    9.4248sq.in x 4 = 37.692sq.in.

    The maximum size of a square flue would be the square root of 37.692sq.in = 6.139 x 6.139 or 6-1/8" x 6-1/8"

    When a flue is larger than the maximum allowable cross sectional area the flue gasses cool too much to maintain an adequate updraft.

    I would suspect that another contributing factor to your problem is home of fairly new construction that is essential air tight construction with no provision for ducted combustion air intake to the HVAC system. This would result in the water heater flue gasses being drawn out of the water heater draft hood and into the basement or utility room air where it eventually is drawn to the HVAC system.

    The proper solution to this problem would be to install a dedicated flue for the water heater, and install a combustion air intake duct for the furnace. The combustion air intake could be as simple as just running a 3 or 4" diameter PVC pipe from outside and terminating it near the furnace combustion air intake.




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    rollerdawg is offline New Member
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    Gas water heater problem

    Well it's fireplace season again and my problem is still there. To clarify my situation, I have two fluepipes side by side in the same chimney. One flue is for the water heater, the other for the fireplace. The flue for the water heater consistently goes into downdraft mode whenever it is very cold outside. It acts like a vacuum cleaner and sucks smoke down into the basement from the fireplace flue. To stop this problem, I have to crank up the water heater to create an updraft.

    I do have a newer home that is fairly airtight. Your suggestion to provide outside air for the HVAC system might work.

    Any other ideas? Is there such thing as an electric heater wrap for the section of flue pipe that connects from the draft inducer to the basement wall? My thought is if I keep the pipe hot enough, the downdraft won't start in the first place.

    Thanks!

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    Hube is offline Journeyman
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    Rollerdawg; your house is obviously under a "negative" pressure because of its tightness.
    To over come this negativity, the home must have some 'make-up air' or a HRV (heat recovery ventilator) This will create a more neutral or slightly positive pressure within the home and it should put an end to this so-called downdraft problem your having. Make sure if your engaging an HVAC pro for the installation of an HRV unit, that it is properly setup with a 'magnehelic instrument' to get the proper in/out portions of air flow.

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