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  1. #1
    Join Date
    Aug 2007
    Posts
    273

    manual J question/infinity gas furnace

    The manual J of my home comes out essentially as a toss up between the 100,000 btu and 80,000 btu carrier infinity gas furnace-- house is 3 levels, 2700 square feet, located in washington dc built in 1930's. Other than storm windows and insulation in the attic, not very tight. Since the Infinity is a two stage unit, with the 100,000 btu model cycling down to 60,000 btus in low speed, does this make the decison less critical since it will be operating at 60,000 btus in milder weather? Am I off base thinking this way? Thanks

  2. #2
    Join Date
    Apr 2005
    Location
    new jersey
    Posts
    752
    Your furnace max BTU heat output should match your manuel J load. With the mild temps the low stage would run most often saving you fuel And then only on extreme temps both stages would operate.

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    It's my understanding that low stage ,will not save fuel,just longer run times to help obtain more comfort.

    So,in this case the 80,000 btu furnace would give you more comfort.Can you add insulation,etc.,to it the corect size?

    What if you did the Man. J with a slightly cooler indoor design temp.,how much cooler would make it the correct size,and can you live wth that in extreme weather??

    Additionally ,the smaller furnace moves less air,which might be a big plus with older,possibly undersized ducts.

  4. #4
    Join Date
    Aug 2007
    Posts
    273
    Would the two speed function mitgate the issue more than a one speed furnace?

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by ron3637 View Post
    Would the two speed function mitgate the issue more than a one speed furnace?
    Yes!

  6. #6
    Join Date
    Mar 2003
    Location
    PA/DE area
    Posts
    1,535
    Make sure if you go with smaller heater ,you have the cfm to drive a/c system
    It's NOT the BRAND,it's the company that installs it!!!!!

  7. #7
    Join Date
    Aug 2007
    Posts
    273
    So going with the 100,000 btu unit may benifit the ac functionality?

  8. #8
    Join Date
    Oct 2007
    Location
    STL Metro East
    Posts
    20
    The 100,000 btu and 80,000 btu carrier infinity gas furnaces are both available with 2,000 CFM blowers. The 80,000 btu model can be ordered as a 1,400 or 2,000 CFM model. According to Carrier, for a 4 ton or larger AC, you would need the 2,000 CFM model.

  9. #9
    Join Date
    Aug 2007
    Posts
    273
    So it makes no differnce if you go with the larger blower. But from what I take away here the 100.000 btu unit, since it has two speeds, would not likely be too oversized in any event. make any sense?

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    The smaller furnace would be beter,if it would "cover" the heating load.

  11. #11
    Join Date
    Nov 2007
    Location
    Chicago
    Posts
    76
    If you are borderline, go with the bigger furnace. With two stages, the gas consumption on both will be negligible. The price difference between the two air handlers should also be negligible. Better to be safe and have a little bit too much heat than not enough.

  12. #12
    Join Date
    Aug 2007
    Posts
    25

    Infinity Gas Furnace Selection

    Quote Originally Posted by hvacconsultant View Post
    If you are borderline, go with the bigger furnace. With two stages, the gas consumption on both will be negligible. The price difference between the two air handlers should also be negligible. Better to be safe and have a little bit too much heat than not enough.
    Thanks for your emphatic recommendation to GO WITH THE BIGGER FURNACE. There seems to be some confusion since other times you hear that in HVAC work, "bigger is not better". I listened to my dealer's advice and went with a smaller furnace last November and now he is delaying replacement with a 5 ton system (25HNA9 plus 58MVB100-20).

    Both my 4 ton hp and furnace have operated more than 45% in high stage. This made me suspect that they were undersized since the hp is supposed to run 80% of the time in low stage. Just when the high stage of the hp (48,000 BTU) has been reached, the 49,000 BTU low stage of the furnace can barely keep up and shifts into high stage (70,000 BTU) also. If I'm undersized, my furnace's 49,000 BTU low stage is virtually useless.

    I'm gambling that I would be using more of the 5 ton furnace's low stage (61,000 BTU) than it's 93,000 BTU high stage. Would be nice if Carrier had a medium stage of say 75,000 BTU for their 5 ton gas furnaces. Any thoughts.

  13. #13
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    I can't see doing 3 floors with one system. I'd be looking at multiple systems for that many levels.

    Heck, I'm zoning 3200 sf on 2 levels.
    Perhaps you should have read the instructions before calling.

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