Odor of gas

Discussion in 'HVAC Heating & Cooling' started by ToolsRMe, Jun 24, 2007.

  1. ToolsRMe

    ToolsRMe New Member

    Messages:
    145
    Location:
    CO
    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. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    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. ToolsRMe

    ToolsRMe New Member

    Messages:
    145
    Location:
    CO
    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. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,508
    Location:
    Cave Creek, Arizona
    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.
  5. frenchie

    frenchie Jack of all trades

    You should be opening windows & doors when you run the fan.
  6. ToolsRMe

    ToolsRMe New Member

    Messages:
    145
    Location:
    CO
    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: Jun 24, 2007
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    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.
  8. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    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. ToolsRMe

    ToolsRMe New Member

    Messages:
    145
    Location:
    CO
    This is most interesting and informative! Thanks!

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

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    You mentioned a Nighthawk. Is is a gas detector? You should also always have a CO detector in the house somewhere.
  11. ToolsRMe

    ToolsRMe New Member

    Messages:
    145
    Location:
    CO
    It is a combination CO and gas detector.

    I probably need to move it higher on the wall.
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