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Thread: Odor of gas

  1. #1
    DIY Senior Member ToolsRMe's Avatar
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    Default Odor of gas

    I have a whole-house fan.

    Occasionally, when I turn it on, the pilot light goes out on one of my two water heaters.

    Then I smell gas.

    The fan is in a crawl space attic. The heaters are in my basement.

    Question: Should I smell gas if the pilot goes out? Or is it normal to smell gas when a pilot light goes out?

    I also have a Nighthawk gas detector two feet from the heater and it is not blaring a warning. Could it be defective? (It is plugged in about a foot above the basement floor. I know natural gas rises because it is lighter than air. Is there a way to test it safely?

  2. #2
    Plumber Cass's Avatar
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    The whole house fan is causing negative pressure that is sufficient enough to draw air back wards through the water heater vent.

    If the heater is on at the time the flame is going out but it is taking a while for the gas valve, by way of the thermocouple / flame sensor, to know the flame has gone out and raw gas is going out into the room until the valve shuts. That is the odor of gas.

    If just the pilot is on you will get the raw gas from the pilot being out but it is minimal and again only till the gas valve shuts.

    Depending on air flow / direction it may be by passing the detector all together because the air flow is so strong.

  3. #3
    DIY Senior Member ToolsRMe's Avatar
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    Quote Originally Posted by Cass
    If the heater is on at the time the flame is going out but it is taking a while for the gas valve, by way of the thermocouple / flame sensor, to know the flame has gone out and raw gas is going out into the room until the valve shuts. That is the odor of gas.
    Many thanks for your answer.

    For a properly operating thermocouple, how long should it take for the thermocouple to detect that the flame has gone out?

    Does the thermocouple only detect the heat of a pilot light or does it also pick up the heat of the burners? Thus it would take longer for a thermocouple to cool off if it was detecting the burners, too. Right?

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Default gas

    if the burner flame is large enough to keep the thermocouple activated, it would probably also relight the pilot, but the pilot flame and thermocouple are usually not where they are affected by the main burner, because that could also cause a problem if the t'couple gets too hot. The shutdown is fairly rapid, a few seconds at most usually.

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    Jack of all trades frenchie's Avatar
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    You should be opening windows & doors when you run the fan.

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    DIY Senior Member ToolsRMe's Avatar
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    Quote Originally Posted by frenchie
    You should be opening windows & doors when you run the fan.
    I know this is really stupid but I want to make sure:

    Does it have to be
    --- windows AND doors
    or can it be
    --- windows AND/OR doors
    ?
    Last edited by ToolsRMe; 06-24-2007 at 01:26 PM.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    It depends on the air volume moved by the fan. What you may want to consider is an automatic vent. Note, you don't get much airflow unless you open something in each room, which usually means at least windows. If you don't, you end up sucking air through the walls, outlet boxes, switchplates, cracks, etc. It puts a bigger load on the fan and decreases the efficiency. In a worst case, it could suck the exhaust fumes through the house verses letting them flow up their respective flues. The only fixtures that that won't happen with are closed combustion systems where they route air directly to/from the outside of the house.
    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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    Plumber jimbo's Avatar
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    This is a bigger problem than the pilot and the gas smell. Given the negative pressure, if the water heater, a stove, or a gas furnace was running at the time, you would back draft that appliance and draw Carbon Monoxide into the home. So, in theory, either the house will explode from the gas, or you will be dead in the morning from the CO.

    Whole house fans draw enormous CFM's. You need to have a lot of open doors and/or windows to prevent serious negative pressure build up/

  9. #9
    DIY Senior Member ToolsRMe's Avatar
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    Quote Originally Posted by jimbo
    you would back draft that appliance and draw Carbon Monoxide into the home. So, in theory, either the house will explode from the gas, or you will be dead in the morning from the CO.

    Whole house fans draw enormous CFM's. You need to have a lot of open doors and/or windows to prevent serious negative pressure build up/
    This is most interesting and informative! Thanks!

    Is there any way to measure whether we have enough open windows/doors? Barometer?

  10. #10
    Plumber jimbo's Avatar
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    You mentioned a Nighthawk. Is is a gas detector? You should also always have a CO detector in the house somewhere.

  11. #11
    DIY Senior Member ToolsRMe's Avatar
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    Quote Originally Posted by jimbo
    You mentioned a Nighthawk. Is is a gas detector? You should also always have a CO detector in the house somewhere.
    It is a combination CO and gas detector.

    I probably need to move it higher on the wall.

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