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Gas fireplace back drafting

GringoGringo Member Posts: 6
I have a gas only fire place that is vented thru the wall in my master bedroom. The vent exits the wall about 24in above the top of the fireplace and the vent hood faces north. The problem I’m having is that I have a open field and a pond on the north side of my house and the prevailing north wind in the winter hits the north side of my house and causes my fireplace to backdraft. Does anyone know of a wall mounted vent hood that will prevent back drafting for this situation?


  • lchmblchmb Member Posts: 2,996
    Check with the unit manufacturer. They may have a high wind cap or some suggestions to assist you...
  • ZmanZman Member Posts: 6,288
    Where does the combustion air come from? Is it a direct vent?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GringoGringo Member Posts: 6
    The manufacturer told me to contact the installer and they weren't much help. The installer is telling me that the unit isn't backdrafting and they say the smell is simply part of the break in process. After two winters, and several attempts to use the fireplace, it gases us out if there is any wind from the north. I have another unit in our great room that did have some odor on start-up but it now has no odor (it was vented thru the roof).

    Combustion air comes from a connection on the back of the fire box - it pulls air from the chase behind the fireplace.
    Another item, at times we have a significant amount of cold air blowing in around the sides of the glass on the front of the fireplace. I have had to shove insulation in the gap around the glass at times to stop the cold air from blowing in.

    I did find a high wind termination kit that basically looks like a round piece of stainless pipe that is open on top and bottom. Not sure if it will be compatible with the existing flue pipe, but i may be able to get creative and get it to fit.

    Thanks for your input.
  • ZmanZman Member Posts: 6,288
    What is the make and model? Do you have the I&O manual?
    It sounds like you have an installation issue.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GringoGringo Member Posts: 6
    Lennox MLDVT-35PM
    No IOM
  • ZmanZman Member Posts: 6,288
    That is a direct vent model. It should be pulling it's combustion air from a concentric style termination. A picture of the termination on the outside would help verify. It should not be pulling air from the chase.

    Tell us more about the "backdrafting". Does it set off CO detectors? Is it obviously combustion byproducts?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GringoGringo Member Posts: 6
    After more investigation, the unit does in fact pull combustion air from a concentric termination hood not from the chase as I thought.
    The backdrafting doesn’t set off a co2 detector but frankly I’m not sure if my detector is any good, it’s never gone off. I need to replace it.
    I notice combustion odor and get a headache when I run it for more than 30 minutes. It only seems to occur with north wind. I have run it before with little or no odor, but odor is present more often than not.
    I’m new to heating help and not sure how to add pics. The wall termination hood is a standard Lennox vent for the model. (I did find a manual online)
    The vent hood has a large piece at the top that angles away from the house to deflect heat I’m sure. Unfortunately it seems to make an excellent air scoop for catching the north wind and channeling it into the combustion air space around the flue.
    I have an idea about shortening the height of the heat deflector to try and reduce the air scoop effect. It should still deflect heat away from the side of the house.
  • ZmanZman Member Posts: 6,288
    There is an "attach file" tab along top of the leave comment window. It looks like a piece of paper with the corner folded over.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SteamheadSteamhead Member Posts: 14,173
    @Gringo , DO NOT operate that unit until this is resolved.

    Since you "notice combustion odor and get a headache when (you) run it for more than 30 minutes" when the wind blows from the north, that's a good indication of poor combustion, which produces carbon monoxide. Yes, I know you have a CO detector, but most such detectors do not alarm right away.

    Is it possible to vent this thing up the chimney?
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • GringoGringo Member Posts: 6
    I can’t take the vent up thru the roof but I could take it up another 2-3 feet and vent it out an adjacent wall that doesn’t face north. I’d have to open up the side of my house or go in thru the drywall over the fireplace to reroute the the vent. It would be tight working in the chase, but it could be done.
    I was hoping to find a high wind termination hood that would solve the problem but Lennox uses 4.5in flue size which appears to be non standard so I haven’t been able to find any high wind wall caps.
    My plan at this juncture is to cut down the metal on the top of the hood to reduce the air scoop effect. It’s a pretty simple modification that could solve the problem. (See attached detail of the wall termination cap). I’m going to take off about 1.5in and bend the metal down a bit. It should still vent combustion gases effectively. I’ll take pic tomorrow after I do the modification.
  • lchmblchmb Member Posts: 2,996
    Does the unit show any signs of poor combustion? Is the glass sooted over? Does the flame pull away from the burner? Is the unit for the proper gas? Has anyone checked the vent pipe to confirm it is connected? Check around for a Lennox dealer and have someone come in.. you need someone who understands the unit and the product your burning.. Where are you located?
  • GringoGringo Member Posts: 6
    No signs of poor combustion. Glass isn’t sooted at all and the flame seems good, it burns the same as my other fireplace.
    I had the installer out after we had used it about 6 times during the first season of use and they said nothing was wrong that the smell was normal break in. That was two years ago. I’ve used it sparingly since because of the fumes.
    Im not sure if they checked the vent connection when they came out. I have access to a pipe camera so I can check the vent to make sure it’s connected.
    I live near KC Mo.
  • Bob HarperBob Harper Member Posts: 856
    Odors are very difficult to diagnose. I'd recommend you have Bart Ogden from your area look at it. He's a hearth product dealer and chimney sweep. FYI, it's a "CO" alarm-not CO2, which is what you're exhaling. There may be something in the wall off-gassing from the heat and the wind is just forcing it into the building those days. Need a pro certified in carbon monoxide to test with an analyzer to be sure no CO

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