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Thread: Combustion order from water heater fan

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    DIY Junior Member paulcat's Avatar
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    Default Combustion order from water heater fan

    This seems to happen only during the winter time. The vent on top of the WH has a somewhat open back side and I can feel a draft coming out of this opening instead of up through the vent pipe and my basement fills with a strong combustion odor. Now if I open my glass sliding door in the basement, the air then goes out the vent and the smell disappears. Any advice? Could it have something to do with my furnace air flow since this only happens in the winter? My furnace is only a few feet from the WH. My basement is a very open environment, also. Thanks for any help.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Combustion devices actually can suck a lot of conditioned air out of a house. In the process, they suck unconditioned air in the house. The newer devices use closed combustion, where the burner gets all of its combustion air from outside, and then dumps the exhaust there as well. So, basically, combustion devices rely on sucking air in from outside to burn properly. It's better to do this in a controlled manner rather than have it do it through all of the cracks in the house. If the house is tight, it becomes worse, the device won't work right. By that, one possibility is incomplete venting - dumping it in the house rather than outside. Or, if there's not enough air, make some CO rather than CO2, which could kill you. If you don't have one, suggest you get yourself a CO detector and use it. You may need to provide an outside air vent to the area where the burners are.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by paulcat View Post
    This seems to happen only during the winter time.
    I can feel a draft coming out of this opening instead of up through the vent pipe and my basement fills with a strong combustion odor. Now if I open my glass sliding door in the basement, the air then goes out the vent and the smell disappears.
    Probably 'cause cold air sinks. You may need a draft inducer. Also, with 3 CO detectors you'll have aircraft reliability, 99.9999%, the so-called "six-nines".

    Some of your brain cells have already died if you smell that odor or if your eyes smart. With nausea and headaches, flu-like symptoms, you are close to dying. One of my customers had that, and survived.

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    In the trades Dana's Avatar
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    Flues open to the basement would normally put the basement under negative pressure from the "stack effect". Opening the slider allows outdoor air in to relieve that pressure differential, and the air/combustion exhaust to flow up the flues as-designed. But if there are other induced- exhaust points (kitchen/bath fans, or clothes dryers), it can create sufficiently negative pressure when the slider is closed that air comes down the flues, a condition called "backdrafting".

    Backdrafting combustion appliances are a serious health hazard- take this deadly-seriously!

    If there is a powered draft-inducer on the furnace it may be THE problem (and not the usual backdrafting) since the forced draft of the furnace would pressurize the flue forcing some of it's exhaust into the basement. If the furnace is atmospherically drafted, adding power draft inducer just on just the hot water heater could create reversed draft problem at the furnace whenever the hot water heater is running.

    Is the furnace a forced-draft version?

    Almost all 83%+ furnaces & boilers are induced draft, and should never share a flue with an atmospheric-drafted hot water heater. Do the furnace & hot water heater share a flue, or are they on separate flues?

    If they're both atmospheric-drafted and share a flue, sometimes adding 4-5' to the top of the chimney is enough. Alternatively, a turbine-vent or venturi-vent chimney cap can often do the trick.

    Sometimes the flue has too large a bore for the BTU rating(s) of the burner(s) attached, and would draft better with a narrower-bore liner installed.

    Properly diagnosing the cause may lead to different "right" solutions depending on the particulars.

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