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  1. #1
    Join Date
    Nov 2007
    Location
    Tucson AZ
    Posts
    166

    Two stage furnace - rating question

    I have a Trane XV80i (TUD2B100AFV32A) two stage furnace and I have some questions about it.

    1/ It has a nominal rating of 100,000 BThU/Hr. Is this only when the second stage is in use?

    2/ What would the output be on the first stage only?

    3/ Can the output of the furnace be reduced? If so, how (lower gas pressure, different jets??)

    I am finding that questions like this are outside the knowledge of either my contractors salesman or the installation technicians, and Trane themselves clearly do not wish to talk to end customers.

    Any general advice on where to get answers without bothering the experienced professionals here?

  2. #2
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    100,000 btu's is the input rating at normal gas pressure.the out put would be 79,000 btu's on 2nd stage and 52,000btu's on 1 st stage. the only way to reduce capacity would be to replace the orfices. I would recommend to NOT reduce gas pressure at the valve, you may cause delayed ignition. (BOOM!)
    Just curious, why do you want to reduce capacity?
    Genius = The guy who can do anything...except make a living!

  3. #3
    Join Date
    Nov 2007
    Location
    Tucson AZ
    Posts
    166

    Here is my reasoning

    I suspect the furnace may be oversized for the heating needs of the house. I base this on posts I have read here where people report that even with their maximum heating they are unable to maintain or reach a desired temperature when it gets cold outside. I have the opposite situation - in the morning my furnace recovers from four degrees of setback in about ten minutes!

    I am convinced of the value of tending toward longer run times to very slowly change temperatures (that's what "comfort" is all about), but not to the extent of an infinite run time that never reaches the desired temperature.

    So, I'm thinking that a reduction in the furnace output, say enough to double the morning recovery time to twenty minutes, might be worth considering if it is simple to do and easy to undo if it does not work out.

    Would a change to the orifices cut down on both the first and second stage?

    Also, why 79,000 and 52,000? What happened to 100,000?

    P.S. Of course! It is only 80% efficient, so 100,000 fuel going in equals 79,000 coming out as heat. Duh! as they say....
    Last edited by jeff520; 01-02-2008 at 02:09 PM. Reason: Add postscript

  4. #4
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    What Tstat are you using?

  5. #5
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    did you have the furnace installed or did you buy the home with the furnace already installed? If you had the furnace installed did your contrctor do a load calc? You should have a two stage stat on your system to maximize the features of your furnace. You could pay a contractor to do a MAN J load calc to check and see what your homes loss is at the winter out door design temp is for your region. then you will know how to move forward. THe three stats I would recommend are the tcont802 or 803 and the honeywell pro9000 IAQ stat.
    Genius = The guy who can do anything...except make a living!

  6. #6
    Join Date
    Nov 2007
    Location
    Tucson AZ
    Posts
    166

    Stat is a TC803

    System has just been installed - vendors quote references a manual J heat gain calculation to maintain 75 degrees indoor with outdoor temp of 105. The answer was 3 tons of cooling - no mention of the heating need.

    I think the furnace was sized to meet the air handler needs of the AC system, since 79,000 BThU's of heat would be equivalent to about six and a half tons of cooling.

    Seems that there might be a danger point if the furnace is reduced too much and condensation forms in the heat exchanger. Would simply eliminating one of the five orifices be one way to cut output by 20%?

  7. #7
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    They can get you a smaller (60K or 80K) furnace with a blower to handle 3 tons.

    is this 10 min recovery in 2nd or 1st stage? That is SUPERSIZED! I'm in MN, have a 1,400 sq ft home, and have a 60k two stage, takes about 2 hours to recover at -8˚with a 60˚ set back to 68˚.


    Two stage stat being used?

  8. #8
    Join Date
    Dec 2007
    Location
    Carrollton,GA
    Posts
    39
    I just had a new gas furnace installed and the idea behind my "oversizing" was about the same: I was told --> That the 80K, 4-ton unit was recommended (for me)to handle the cooling needs (Georgia). Said it was standard practice to do load calc when replacing AC, and that furnaces in GA are usually oversized when comparing to a manual J Load, so in Georgia there is usually a higher btu furnace just to get the proper ton blower for the size AC needed. Standard practice is to size to the cooling load and install the manufacturers matched furnace with that AC Unit.

    Note: my furnace recovers from a 6-degree setback in about 50 minutes (one stage stat). Stage 1 for 10 minutes, then stage 2 ... with more duct-noise than I would like, although I can live with it...

  9. #9
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    Quote Originally Posted by jeff520 View Post
    System has just been installed - vendors quote references a manual J heat gain calculation to maintain 75 degrees indoor with outdoor temp of 105. The answer was 3 tons of cooling - no mention of the heating need.

    I think the furnace was sized to meet the air handler needs of the AC system, since 79,000 BThU's of heat would be equivalent to about six and a half tons of cooling.

    Seems that there might be a danger point if the furnace is reduced too much and condensation forms in the heat exchanger. Would simply eliminating one of the five orifices be one way to cut output by 20%?
    Have your dealer change the unit out to the correct size, they are feeding you a crock crap. You should not need to figure out a way to correct there mistakes.

  10. #10
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,614

    Confused

    a 2 stage furnace with a 2 stage tstat when it comes out of set back brings on high fire you could disconnect 2nd stage heat and only use low fire and see how it works

  11. #11
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    Quote Originally Posted by catmanacman View Post
    a 2 stage furnace with a 2 stage tstat when it comes out of set back brings on high fire you could disconnect 2nd stage heat and only use low fire and see how it works
    It is starting in first and satisfies before getting to second stage.

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,763
    Don't need a 4 ton drive for a 3 ton A/C.

    Not much you can do to derate that furnace, and not risk condensation in the heat exchanger.
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    How many times must one fix something before it is fixed?

  13. #13
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,150
    Quote Originally Posted by jefferyb View Post
    I just had a new gas furnace installed and the idea behind my "oversizing" was about the same: I was told --> That the 80K, 4-ton unit was recommended (for me)to handle the cooling needs (Georgia). Said it was standard practice to do load calc when replacing AC, and that furnaces in GA are usually oversized when comparing to a manual J Load, so in Georgia there is usually a higher btu furnace just to get the proper ton blower for the size AC needed. Standard practice is to size to the cooling load and install the manufacturers matched furnace with that AC Unit.

    Note: my furnace recovers from a 6-degree setback in about 50 minutes (one stage stat). Stage 1 for 10 minutes, then stage 2 ... with more duct-noise than I would like, although I can live with it...
    Horse crap. They make lower btuh furnaces with larger drives specifically for warmer climates.
    Perhaps you should have read the instructions before calling.

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