Lennox G71 question...
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  1. #1
    Join Date
    Aug 2009
    Posts
    154

    Lennox G71 question...

    Contractor installed a new Lennox G71 furnace, XC21 AC unit, and corrected some of the undersized ductwork that was original to the home. The AC ability was dramatically improved, however we suddenly have noticed 1 small problem with the heating.

    There are 2 second floor bedrooms which are the furthest rooms from the furnace. Despite being so far, they actually used to heat much better. The ac is fine with them (blower speed is definitely pressurizing all the ducts appropriately).

    What I assume the problem is... is that the furnace works in 1% increments from 40% up to 100% and when the furnace is only working at say 40-60% the blower speed may not be enough to pressurize all the ducts. I don't think the furnace ever really kicks on at full speed - it seems to only run at low speed to maintain the temperature. I assume if it kicked on at 100% that it would heat fine, but then I lose the efficiency of the G71... Anyone else have this problem?

  2. #2
    Join Date
    Jul 2009
    Location
    Ontario, Canada
    Posts
    913
    Had a call similar to yours this weekend, only the G71 was only running in low fire. Have the contractor ensure the settings on the Tstat are set for 2 stg operation and the dip switches on the circuit board are set for variable modulation. Air flow settings are obviously different for heat and cool. The contractor can adjust air flow, if needed, for low fire, med. Fire, and high fire.

    Keep in mind this furnace will operate completely different from what you usedto have in your home

  3. #3
    Join Date
    Sep 2006
    Location
    Montana
    Posts
    983
    Quote Originally Posted by tomwk View Post
    Contractor installed a new Lennox G71 furnace, XC21 AC unit, and corrected some of the undersized ductwork that was original to the home. The AC ability was dramatically improved, however we suddenly have noticed 1 small problem with the heating.

    There are 2 second floor bedrooms which are the furthest rooms from the furnace. Despite being so far, they actually used to heat much better. The ac is fine with them (blower speed is definitely pressurizing all the ducts appropriately).

    What I assume the problem is... is that the furnace works in 1% increments from 40% up to 100% and when the furnace is only working at say 40-60% the blower speed may not be enough to pressurize all the ducts. I don't think the furnace ever really kicks on at full speed - it seems to only run at low speed to maintain the temperature. I assume if it kicked on at 100% that it would heat fine, but then I lose the efficiency of the G71... Anyone else have this problem?
    While the furnace is running, have you looked at the circuit board on the furnace to see what stage of heat you're running in? Did you have a 2-stage thermostat installed with the furnace? When the furnace is running, you should see a letter H followed by numbers, which indicates what stage of heat it's in. Those are followed by an A with numbers which indicates what the cfm of the blower is. Your cooling speed is higher than any of your heating speeds, so that's probably why you notice a difference. It's also possible that the furnace is not running up into the higer stages. I would call the installer back out to verify the dip switches on the furnace would set correctly and the thermostat was set up correctly.

  4. #4
    Join Date
    Aug 2009
    Posts
    154
    The contractor unfortunately skipped out on a lot of people leaving us high and dry. Fortunately I was confident that the actual install guys did a fairly good job.

    I went with a HW VisionPro IAQ thermostat with 2 remote sensors. I verified in the advanced menu screen that it is set for 2 stage heat. I now have to verify the dip switch settings on the furnace and I will look at what indicators or stages it is showing.

  5. #5
    kenney t Guest
    you can't run 2 sensors, read instructions it says 1,4,9,12 it must be done in square numbers

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,616
    2 remote indoor sensors?
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    Join Date
    Aug 2009
    Posts
    154
    My mistake... 1 indoor sensor. The contractor installed 2 but we had to disconnect 1 so it would work.

  8. #8
    Join Date
    Aug 2009
    Posts
    154
    The situation has improved...

    Verified that my HW VisionPro IAQ was set to 2 stage heat and that my Lennox G71 was set to modulating. I was able to increase my low speed heat by 15% and decrease my high heat speed by 7%.

    It has been 2 days and the house already feels better. I had no idea that the furnace was this adjustable - what a great unit. Too bad my installer could not help.

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