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didn't there used to be a category for scorched air (re armstrong lenox A96DFMV)

archibald tuttle
archibald tuttle Member Posts: 1,043
edited December 2019 in THE MAIN WALL
are we trying to reconcile with forced air folks . . . or is there a category i'm missing where i should have put this?

anyway, trying to help remote diagnosis of lennox A96DFMV and suggest repair parts to keep around but those cheesy buggers don't have an actual parts diagram or parts number listing in their manual which covers 70 MBH to 110 MBH models i think.

present problem is the exhaust air pressure sensor isn't making per error code. no apparent blockage and this is long installed and running so i don't believe there is a vent sizing issue. but a little blowing in the tube and pulling contacts on and off and it was off the races although not 100% sure what is now right that was wrong before. I don't have pictures yet and the sister-in-law didn't indicate that she saw more than one such sensor, but the manual shows two pressure sensors for high and low heat. I don't know if i am mistaking plenum/circulation sensors but they are safeties in the gas valve circuit from what i can see, so maybe there are two exhaust sensors set for different levels for a two stage or variable operation or maybe i'm just missing the actual exhaust air pressure sensor on the diagram. it is a little unclear but they seem to go through K5 which isn't designated anywhere and i don't recognize the electric simply which is kind of like a barbell with the center of the bar missing.

anyone who might have had a look at one of these able to help me better grasp theory of ops. thanks, i'm trying to get some pictures from the origin as well and pictures of venting setup and make sure intake is checked and no frozen condensate in exhaust, etc. simple stuff. and get the full model as the numbers they sent last night stopped short of the fully designating the unit.




  • ratio
    ratio Member Posts: 3,544
    This is similar the the problem I mentioned in another thread, the operating pressure was just shy of keeping the switch closed after ignition on a cold start. After a handful of attempts, things would be warm enough to stay lit (but it took manual intervention to get there). New inducer, pitched the flue better, checked the secondary HX, everything looked good. Combustion numbers looked good, no waffling around when the blower came on. After some time on the phone with Trane, he condemned the HX, because the pressures weren't right. IIRC it was 10-12 years old.

    We passed it off to the original installer since they had the warranty, so I don't know what happened after that.

  • ratio
    ratio Member Posts: 3,544
    I checked the trip point of the pressure switch, it was just about nuts-on. I ended up with a photohelic tee'd into the lines to watch it.
  • archibald tuttle
    archibald tuttle Member Posts: 1,043
    got it. so should have pressure notably above sensor set point ( didn't see the setpoint or even the sensor itself called out skimming the manual - and still seem to be running into indication there are two sensors, one for high and one for low fire, which if the sensors are closely calibrated makes sense). inference from the service call you were on is if you are just barely making the sensor that pressure is being lost into the conditioned air circuit? wouldn't surprise me if this were possibility here as i think the unit is similar age. so i'm gonna suggest they get the exhaust pressures measured, although would be nice if target info were in manual.

    back in the day when i had oil scorched air in my place and the heat exchanger went, i just wired the system fan to come on with call for heat instead of waiting for hot plenum and that kept the pressure in the plenum high enough that we didn't get much exhaust leakage into the house air. those were the days when i didn't have two nickles to rub together to put in anything else. managed a couple years that way. and then put in a boiler and warm floor and left the scorched air behind. Do you recall how long the guarantee on the heat exchanger was?

    thanks for reply,

  • ratio
    ratio Member Posts: 3,544
    IIRC it was a 10-15 year warranty on the HX, but like I said another outfit serviced the warranty so I'm sketchy on details. If it does have a cracked HX, I'd condemn it & get an immediate replacement, even if it were mine. Maybe especially…

    Depending on the flue & intake, you could try to close it off & inject something like a little peppermint oil (old plumbers trick) or pressurize it (gently) to look for a crack. I always thought a combustion analizer would be the one sure way to diagnose a cracked HX, but now I guess not.

  • John Mills_5
    John Mills_5 Member Posts: 950
    Bad pressure switches, especially those made in Costa Rica, were a known problem with that furnace. Can't recall the latest & greatest part, I'm thinking 77W44 but check with Allied distributor if you order one.