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  1. #1

    Intermittent gas smell in AC vents tied to hot water usage

    Seeking advice on a dangerous problem with an elusive cause.

    After running hot water in the master shower for a few minutes, a strong rotten egg odor is emitted from the AC vent. The smell is present in all vents serviced by this AC unit. The smell dissipates and does not recur even with continued hot water usage. This will happen again on another day, but is not always repeatable.

    The gas line and systems have been checked for leaks recently and come out clean. But the problem wasn't occurring at the time of the gas line test.

    The Furnace/AC system is a dual system. Both units are old but have been under home warranty and have had many parts replaced. The water heaters are new and have been checked for the source of the smell.

    The water heaters are installed in parallel rather than series. Based on speculation by the latest plumber to look into this problem could the cause be something related to small off-venting of gas pressure caused by the spike/dip in gas pressure based on both water heaters firing? This would fit the symptoms as the source of the gas leak would be the furnace/AC unit but caused by the gas usage of the dual water heaters.

    What would cause a small discharge of gas in the AC unit tied to hot water usage?

  2. #2
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,929
    I never heard of gas that smells like rotten eggs. Maybe it's flu gas being pulling into the air handler's return. You should have someone test to see if your water heaters are venting properly, and/or whether your air handler is creating a negative pressure that causes your water heaters to spill flu gas. Could be something died in your ductwork.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,895
    Water smelling like rotten eggs isn't that uncommon, particularly with well water and certain anode rods in the water heater.

  4. #4
    Join Date
    Feb 2011
    Location
    Peoria, IL
    Posts
    2,149
    Quote Originally Posted by BaldLoonie View Post
    Water smelling like rotten eggs isn't that uncommon, particularly with well water and certain anode rods in the water heater.
    Being a plumber as well, i agree, and its not just limited to well water, if your public system is on a water tower, i have had same problems, you can turn the temp up on the water heaters, or better yet change out the magnesium anode rod for an aluminum. (Its bacteria attacking and feeding off of the magnesium that creates the smell. Turning the temp up some will kill them, or replacing the rod will get rid of there food. (that rod is in there to protect the tank from hungry water, so it still needs one, or it voids the warranty if they find out its out.)

  5. #5
    Join Date
    May 2012
    Location
    Upper Michigan
    Posts
    3,588
    Quote Originally Posted by BaldLoonie View Post
    Water smelling like rotten eggs isn't that uncommon, particularly with well water and certain anode rods in the water heater.
    Yes I have smelled some really bad well water, they may also not be drafting initially, then once the flu warms they start to draft.

  6. #6
    I don't believe the problem is the water. The water smells fine. The smell comes from the AC duct. Only in the summer, no problem with any smell in the winter.

    I thought it was natural gas, but having just visited with the gas company, the smell is more sulfur like.

    I don't think it is anything dead in the vents because the problem has persisted for over 6 years. The water heaters are new. Plumber has checked the anodes. Both plumber and gas company verified venting of the water heaters.

    The smell only occurs in the afternoon on warm days when the hot water in the master bath is run.

  7. #7
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    How did they verify the venting? A combustion analyzer taking multiple readings and a draft gauge is the correct answer. Blowing out a match and sticking it under the draft diverter is the wrong answer.

  8. #8
    Join Date
    Feb 2011
    Location
    Peoria, IL
    Posts
    2,149
    you use hot water in winter, and on cold days, (Both the water heaters are used then as well as the furnace blowing air) Try not running hot water at all in this bathroom, and make sure you do not smell this smell. Or better yet if you smell this every day, now that its warm out, shut the water heaters of and see if the smell go's away, as a test.

  9. #9
    Join Date
    Feb 2011
    Location
    Peoria, IL
    Posts
    2,149
    Also are the water heaters, and furnace, in a enclosed space? As there may not be enough combustion air for all 3

  10. #10
    Quote Originally Posted by martyinlincoln View Post
    How did they verify the venting? A combustion analyzer taking multiple readings and a draft gauge is the correct answer. Blowing out a match and sticking it under the draft diverter is the wrong answer.
    Match method.

  11. #11
    Quote Originally Posted by garychance View Post
    Also are the water heaters, and furnace, in a enclosed space? As there may not be enough combustion air for all 3
    They are together in an unfinished space that represents about 1/4 of the basement space. There is plenty of room for storage so I expect enough air is present for combustion.

  12. #12
    Join Date
    May 2006
    Location
    Raleigh N.C.
    Posts
    276
    Possible causes; 1- gas leak after the gas valve,. 2-dirty sock(or in this case dirty fart syndrome). Or most likely a dry condensate trap possibly connected to the sewer system
    Damn phosgene

  13. #13
    A question relating to this problem is that while this may be a difficult thing to determine should I buy this house without knowing the source of the problem? How expensive could the fix be? How unsafe of a hazard is the root cause and sulfur smell? Is it solvable? How much worse might it get? What trade specialty can diagnose the problem and find the cause?

    Do I walk away or own the problem and cost to resolve or consequence of not being able to resolve over the next 20 years my family will live in the house?

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