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Lennox G8-137T-1A

Russ_14Russ_14 Member Posts: 7
I am looking to replace the heat exchange on the Lennox G8-137T-1A. Does anyone know where I may find one? Barring not finding one, I am trying to figure out how I am going to replace it as nobody seems to be much help. I have the G8, which for years the single attached garage converted into a rec room was always cold. It only has one 9" duct to it. I sped up the fan 20% by way of changing the pulleys & gave it more intake airflow. Now it is the warmest room in the house. As much as I like the G8 I know its a dinosaur, but I would rather fix it than buy a new one as I know it works.
I've had 3 furnace companies come in for estimates, (about 5 years ago). They all assured me that what they had quoted would move the air adequately. When asked if they would put that in writing, not one of them would. In short, I'm worried that a new DC direct drive fan won't move the air enough to heat the rec room. I should mention that I have an electronic filter system & the furnace runs 24-7-365 so I have no need for variable speed bla bla bla. Does anyone have any recommendations as to what I can do to figure out what is required to ensure heat to that room? The main floor is 1600 sq/ft with 1200 ft basement. The 9" duct for the rec room is 5 ft long off of a main duct 30 ft from the furnace. Any and all help would be appreciated.


  • JUGHNEJUGHNE Member Posts: 6,985
    How old is that furnace? With the 137 in the model number does that indicate the input of 137,000 BTUH. That info would be on the nameplate inside the burner compartment maybe.

    If it is 137,000 input, that is a lot for a 1600sq/ft house. Maybe twice what is needed.
  • Russ_14Russ_14 Member Posts: 7
    It looks like it was manufactured Dec 1966. Yes 137,000 BTU input. 109,000 BTU Bonnet capacity whatever that means.
  • Harvey RamerHarvey Ramer Member Posts: 2,214
    I'm guessing the current furnace is a belt drive blower which is why you got away with speeding it up by 20%. Most, if not all new residential units will have a direct drive blower/motor. They depend on the air they are moving for cooling. It's a calculation of how much work they are doing versus how much air they are moving, determining if you will have problems or not. What you need to do is check the ESP (external static pressure) of the furnace. Most new units are rated at .5 ESP although can be safely, but not efficiently, run up to .7 ESP.

    Personally I'd ditch the monster and get something new. Keep looking until you find someone that knows how to handle airflow. And if they need to change some ducting around to make it work, let them have at it.
    Ramer Mechanical
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  • JUGHNEJUGHNE Member Posts: 6,985
    137 in and 109 out means that Lennox assures you that 20 % for sure of your gas money goes up the chimney. You could probably fry eggs on the single wall flue coming out of the furnace.
    In reality probably 30% of your gas dollars are leaving thru that pipe. Some will say even 40%. Over the heating season.

    Speeding up the air flow might have contributed to the "early" demise of the heat exchanger. Too much air flow will make the exchanger too cool and cause condensation......think of a car muffler that never gets hot....they rust out because of condensation inside. Heat exchangers of that day could take a lot of abuse so maybe this is a moot point.
    Finding a replacement would be amazing.

    As much old school that I am, one who repairs some really old stuff, If the heat exchanger is out I recommend a new furnace.
    There are other items that may fail in the near future, not terribly expensive but will bite you with 200 to 400 dollar repairs in the future. IMO
  • Russ_14Russ_14 Member Posts: 7
    Thanks guys, it says .75 ESP. Probably right regarding the condensation. I just don't feel confident that a new furnace would push that amount of air. I've seen the DC motor/fan setups and it looks like they hooked up a Kia to do a tractor trailer's job. One guy I talked to, (who had a heat exchange for sale for a smaller unit), said the setup I have is moving appx. 1100 -1200 FPM. Like I said Nobody, would put it in writing that it'll push properly. So what do I do? Give someone a bunch of money who can't even say for sure if it will work? Are new furnaces not measured in CFM? It seems fairly straightforward to replace. Match the BTU, match the CFM, but no-one has given me CFM when quoting. They talk about tons which I though was an air conditioner term. When trying to make the sale, they say things like, "I'm sure it'll be fine". Or "I don'tthink it'll be an issue. They won't put it in writing, so now I'm gunshy & very suspicous.
  • Russ_14Russ_14 Member Posts: 7
    Jughne said its too big of a furnace, but I live in Edmonton, which is a 16 hr drive north of Salt Lake City. It gets cold here. I had a brand new Carrier 80-90%, (I forget which) efficiency 90,000 BTU, that my friend gave me out of a house that was just remodeled for a flip. The new owner hired my friend to demolish the house & build a new one. I had a furnace guy come to look & he said that it was too small. I ended up giving it to another friend.
  • SWEISWEI Member Posts: 7,356
    Russ said:

    I had a furnace guy come to look & he said that it was too small. I ended up giving it to another friend.

    I'm sorry, but that is nowhere near enough justification to declare an appliance 'too small'. How many square feet is the building/unit?
  • Russ_14Russ_14 Member Posts: 7
    The main floor is just over 1600 on the main level & 1200 in the basement. Its a detached house.
  • SWEISWEI Member Posts: 7,356
    edited August 2016
    Assuming the basement is heated:

    96% efficiency furnace = 30.9 BTUs/ft²
    92% efficiency furnace = 29.6 BTUs/ft²
    82% efficiency furnace = 26.4 BTUs/ft²

    Barring a truly horrid envelope or ductwork that vaguely resembles Swiss cheese, that 90k unit (in the hands of a competent installer) should have worked just fine.
  • Russ_14Russ_14 Member Posts: 7
    Ok, that may be correct but the real issue is making sure that a DC fan has enough CFM& pressure to push the air into the rec room.
  • SWEISWEI Member Posts: 7,356
    Fixing bad ductwork by installing a bigger fan motor rarely turns out well. To address a shortage of airflow in one area, I would first try to fix the duct work or (in the event that proves impossible) consider adding a small duct booster.
  • Russ_14Russ_14 Member Posts: 7
    I think the ductwork is ok. I don't see a better way to do it, other than cutting through concrete to install a return in that room. I originally installed a duct booster. It was so loud that I only had it on there for about 5 minutes before I ripped it off. That's when I came up with the idea to speed up the fan. Do new units not move 1100 CFM? If they do, there is another thing to consider which is the pressure of the CFM. I remember that I had a 36" portable type fan. It said it moved x amount of CFM. It wasn't doing the job I needed as I was running a fiberglassing plant making bathtubs. I ordered a proper fan for a paint booth $2000. The salesman told me it moved x amount of CFM. I said "the portable one moves more than that". He explained that the CFM was rated at static pressure & when given a load to pull the portable one would be nearly 0 CFM. The paint booth fan would continue to pull the rated CFM even in a vacuum situation. I think it was rated in Bar. So my point is would a new furnace push the air strong enough through the ductwork taking into account the friction loss of the ductwork to get it far enough out to heat that room?
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