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  1. #1
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125

    Leave Trane xv95 in first stage, problems?

    Hi,
    When I got my new air conditioner, I was told the manual J heat need was 91k btu. My oversized heater puts out 114/78 btu. I've dropped some money on upgrading my returns and supplies in an attempt to get the 2000cfm that I apparently need for my the second stage furnace. The ducts are certainly better, but by no means do they meet this boards level of perfection.

    I am thinking about trapping the furnace in first stage (rather than 1 stage 1 zone, >1 zones--second stage). I highly doubt i need the second stage.

    Is this going to damage the furnace? Does it somehow need to be exercised in the higher stage (on the off chance we get a tremendous cold spell and I need the extra heat)?

    Thanks
    Steve

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    2000CFM is nice to have, but 1750CFM is enough to keep you off the high limit switch (60F temp rise).... 1600CFM for 65F, and some go as igh as 75F, so only 1400CFM for thsoe models. What the max heat rise on your furnace (listed on namplate inside the unit).

    High standards are nice, but sometimes your stuck with what you have. My downstairs furnace is oversized, ducts are too small and unevenly distributed, I've balanced it the best I can, upgraded the air filter added 1 supply, but I still run it close to the max temp rise of 75F. Oh, and my oversized AC (3.5 tons, need 2 tons), runs at only 350CFM/ton.

  3. #3
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Motoguy128-thanks. It says air temp rise range 40-70.

    It ran for 2 years kicking into second stage. I guess it would have shut down if the limit was reached, so it isn't a safety issue, but just efficiency. Is there a somewhat easy way for me to know if the unit has enough return air on second stage?
    Thanks
    Steve

  4. #4
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    A related question, is it sort of impossible to crack the heat exchanger on the xv95 because there is the discharge air sensor or some other sensor? The unit that the xv95 replaced was a 15 year old trane xe90 120 btu single stage heater. That heat exchanger cracked. Are those older units lacking some sort of signal for too high a temp?

    Either way, i've increased the return a bit, and at least the stages help the zoning a bit (but don't by any means match perfectly).

    Thanks

    Steve

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,880
    Newer heat exchangers are thinner then the older ones. So they crack easier.
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    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by central nj View Post
    A related question, is it sort of impossible to crack the heat exchanger on the xv95 because there is the discharge air sensor or some other sensor? The unit that the xv95 replaced was a 15 year old trane xe90 120 btu single stage heater. That heat exchanger cracked. Are those older units lacking some sort of signal for too high a temp?

    Either way, i've increased the return a bit, and at least the stages help the zoning a bit (but don't by any means match perfectly).

    Thanks

    Steve

    The high temp switch is first and foremost to protect you home form having ductwork thats over 160F and can catch fire, then it's to protect the heat exchanger. Frequent cycling on the high temp switch causes the metal to expand and contract beyond it's intended design which causes cracks along with the metal degrading from extremely high internal tempratures. THe steel actually changes composition I believe and becomes more brittle and less ductile.

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by central nj View Post
    Motoguy128-thanks. It says air temp rise range 40-70.

    It ran for 2 years kicking into second stage. I guess it would have shut down if the limit was reached, so it isn't a safety issue, but just efficiency. Is there a somewhat easy way for me to know if the unit has enough return air on second stage?
    Thanks
    Steve
    Effciency is suppsed to be fairly constant within that range if your near the rated CFM. Operating below hte lwoer end of hte range can cause flue gases to condense in hte primary heat exchanger which will cause it to rust out. This is also why you shouldn't operate a furnace below a 60F return temperature. Some mfg's may specify lower. My Carrier says 60-80F. Above 80F return temp, the heat exchanger can get too hot as well.

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