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Furnace smell, suspect HX

EBEBRATT-Ed Member Posts: 14,543
32 year old Comfortmaker condensing furnace.

Customer complained of a smell. Of course with the age of this thing the first thing you think of is the HX.

They have carbon monoxide detectors installed on every floor. One of my tricks is to put a plastic baggie over the air intake pipe and one over the flue pipe taped on and with the burners off of course start and stop the blower motor and look for any movement in the plastic bags....they didn't move.

Combustion tested and was ok

Got any other tricks??


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    What sort of smell was it?—NBC
  • pecmsg
    pecmsg Member Posts: 4,389
    Forget the UL listed CO detectors there worthless. Wont alarm until 70 PPM after 1 hour

    You should have a Personnel Low Level CO detector that will alert around 9PPM.

    Pull the heat exchanger and inspect!
  • icy78
    icy78 Member Posts: 404
    Condensing furnace would have a negative pressure htex. Not likely the smell is from there while the unit is firing.. (Supported by your CA test.)
    Leave the analyzer in ,and running, until the unit shuts down. (So nothings energized.) Did the C0 rise after the gas valve deenergizes? If so, it's not sealing properly and could make bad odors, from incomplete combustion caused by hot heatex and raw gas, that could be noticed if the house is drawing the flue gas in thru a soffit or similar.
  • DZoro
    DZoro Member Posts: 1,048
    On the left hand side of the furnace, there is a plastic transition box on the secondary H/E there. That is where you will find cracks and holes. You can look down from the top to see, or cut a hole in the left side and patch if needed. Pretty sure you will find it bad tho.
    HVACNUT Member Posts: 5,478
    You can drill a hole in the supply plenum above the furnace or evaporator and put the analyzer in to check for CO.
    Is it sealed combustion?
    Any way to take an over fire draft?
    With oil fired, an over fire draft with the regulator locked closed, you'll know when the blower motor comes on and the gauge goes towards positive.
    EBEBRATT-Ed Member Posts: 14,543
    We put clear plastic bags over the air intake and flue pipe outdoors and taped them on. With the burners off (of course)
    We started and stopped the indoor blower motor several times and watched the plastic bags...not even the slightest indication of a problem.

    I didn't smell anything but what the homeowner described did not sound combustion related
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    Dried up floor drain, or sink trap?--NBC
    rick in Alaska
  • DZoro
    DZoro Member Posts: 1,048
    What is the model#
    JUGHNE Member Posts: 10,909
    HEX condensate drain losing it’s prime somehow?
    Or dead mouse.
    EBEBRATT-Ed Member Posts: 14,543
    I don't think that there is a problem with the HX.

    What I was looking for is how you check a furnace HX short of pulling it out of the furnace. Seems to me that the plastic bag trick is pretty good just wondering what others do.
  • Grallert
    Grallert Member Posts: 599
    edited January 2020
    I'm not sure how effective this is but it's worked for me on some pretty bad heat exchangers. I've used my analyser. I put the probe in the supply plenum and look for any change in the o2 reading or in the co reading.
    Miss Hall's School service mechanic, greenhouse manager,teacher and dog walker
  • rick in Alaska
    rick in Alaska Member Posts: 1,441
    With your air and exhaust capped like you did, put a manometer in the exhaust line before your "cap", and turn the fan on. There should not be a reading.
  • SuperTech
    SuperTech Member Posts: 2,024
    I would pull the blower assembly and inspect the secondary heat exchanger. I use a bore scope inspection camera and drill holes to inspect the primary heat exchanger. But I rely on my combustion analyzer primarily. High CO output or changes in O2/CO2 when the blower is energized are usually good indicators.
    If the heat exchangers are good I would checked the condensate drains and trap next.
    JUGHNE Member Posts: 10,909
    Ed, what kind of water seal trap is on the HEX drain?
    If it stays dry, then exhaust gases would escape.

    If it were a simple loop tube, someone may have “fixed” that pigtail by straightening it out.

    Some of those had a float to block the exhaust if the drain failed and the pressure switch would shut the burner down.
    Not that you have a shut down problem there could be something amiss inside the drain tee.
  • Lance
    Lance Member Posts: 257
    True story: Cust called, we, Melroy, replaced a new furnace with another one. Cust called complained of fumes. Next we installed a sealed combustion furnace. Customer complained again. This time we researched why. Found out this was furnace number 4. Others had tried and failed. After a little research, the new furnace was perfect but the problem was the old wood fired flue into the bedroom in the rowhouse was being used for an air duct. It was soot lined. Suspected he had developed an allergic reaction. Found he lived in front of a scrap yard. I recommended he run a new air duct to the Bedroom, or move to a radiator heated home in the country. He called another contractor. Years later he wrote me and tells the Paul Harvey line: After the 5th furnace failed, he told the guy to install the duct I recommended. His problems went away. He then decide I was right and moved to a radiator heated home and is very happy again. Not all symptoms are caused by the same thing. Although a 32 yr old furnace should be looked at carefully.
    EBEBRATT-Ed Member Posts: 14,543
    @rick in Alaska

    That's a good idea with the manometer.


    That's a good reminder
    I didn't see it myself, my brother was there I forgot to ask him about the condensate drain.