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  1. #1
    Join Date
    Aug 2008
    Posts
    54

    Is it possible to avoid an oversized furnace despite a need for more airflow?

    Here is my dilemma: Per the heat calcs done I need about 56K btu of cooling and a max of 75 or so of heating. To be safe, some suggested a slightly larger furnace.

    My current hot air furnace puts out a full 1 gph, or 139K btu gross and 111K net, after deducting 20% for efficiency loses. I also have a Lennox 5 ton heat pump

    On the surface it would seem that I could use a far smaller and right sized unit, perhaps one that yields about 110K gross and 85K net. But.... there may be a problem, of course, that eludes me.

    I have been told that a larger unit is needed to pump the airflow necessitated by a 5 ton heat pump/AC unit, which puts me n a quandary. To get the air volume I need I "must" install a too large unit and impliedly waste fuel, perhaps alot of it. If I go for a smaller unit then I might not get the air volume & flow req'd.

    So, what have I missed? Any suggestions or ideas? Thanks to all in advance.

  2. #2
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,450
    Call another company and have them do a manual j on your house. Sounds like they are sizing your system based on its existing sizing or just taking a wild stab at it. Get a couple different estimates than post their findings, we will need more than just equipment size if you see the manual j you will understand what I am saying.
    "It's always controls"

  3. #3
    Join Date
    Aug 2008
    Posts
    54

    Unfortunately, these only TWO of the 10-12 people I called would do an Man. J!!

    Quote Originally Posted by skwsproul View Post
    Call another company and have them do a manual j on your house. Sounds like they are sizing your system based on its existing sizing or just taking a wild stab at it. Get a couple different estimates than post their findings, we will need more than just equipment size if you see the manual j you will understand what I am saying.
    Nobody wants to do it. ALL wanted to do "ballparks", except one. He did a full computer analysis, which is where I got the figure from. It factors in duct loses, etc and appears to my untrained eyes quite complete. This assumes, of course, that it is accurate! The only other people who took a half stab at it gave numbers pretty much the same. That, in part, is how I concluded my existing furnace is too large.

    Maybe it is my area, but none of the contractors want to go near such a calculation. Wish I could explain it......

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    Different manufacturers, have the 5 ton blowers on their some what smaller furnaces.
    100,000 BTU furnaces.

    Out west, they have the same problem.
    Contractor locator map

    How-to-apply-for-Professional

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

  5. #5
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    We Have to Be Careful

    Like beenthere says some areas like ours have a bigger heat load then cooling load and we can get 2 different size blowers on most models. You have to be Careful when messing with max CFM numbers. 2000 CFMs
    Do it right the first time.

  6. #6
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,450
    Quote Originally Posted by jws3 View Post
    Nobody wants to do it. ALL wanted to do "ballparks", except one. He did a full computer analysis, which is where I got the figure from. It factors in duct loses, etc and appears to my untrained eyes quite complete. This assumes, of course, that it is accurate! The only other people who took a half stab at it gave numbers pretty much the same. That, in part, is how I concluded my existing furnace is too large.

    Maybe it is my area, but none of the contractors want to go near such a calculation. Wish I could explain it......
    That is a shame because that is the way it is supposed to be done. However a lot of times I do find systems that are over sized. Some of the things would be in heat mode the house would heat up quickly and the furnace would shut off after only a few moments,(contrary to how this sounds it is not efficient at all) in cooling mode the same rapid cycle and poor humidity removal. There are some occasions that a larger blower is required but I find usually that it is due to improper sizing or poor ductwork installation.

    Some of the cheaper family owned shops are not that technically inclined, it may be worth the investment if you can not find a reliable local company to go with a "big" named company. It would never hurt to let the others know that you asked this site for help and that they should be doing manual j's. It should never be assumed that a system is sized properly when replacing it with a new one as there are too many variables that have to be considered and the homeowner (no offense) may not know that his/her system was not running as good as he/she thought that it was. Good luck I believe you made a very wise choice coming here for help.
    "It's always controls"

  7. #7
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,450
    Quote Originally Posted by 21degrees View Post
    Like beenthere says some areas like ours have a bigger heat load then cooling load and we can get 2 different size blowers on most models. You have to be Careful when messing with max CFM numbers. 2000 CFMs
    Agreed.
    "It's always controls"

  8. #8
    Join Date
    Aug 2008
    Posts
    54

    Very interesting, beenthere. I am not alone, I see! Who makes such a furnace?

    Quote Originally Posted by beenthere View Post
    Different manufacturers, have the 5 ton blowers on their some what smaller furnaces.
    100,000 BTU furnaces.

    Out west, they have the same problem.
    I sensed I was not alone. There has to be others who need airflow, but not 50 kazillion btus as well.

    Might I ask what companies make hot air furnaces in the 100K btu range which an accommodate a 5 ton blower? Any ideas?

    Thanks for all. This board is a wealth of knowledge & helpful people. I've had several contractors ask me how I know so much. "Know" is a relative term. I am familiar with terms based on what I've learned here). I've already learned quite a bit and avoided getting a raw deal several times over from this board!

  9. #9
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Texas Furnace Co

    ABA080ni5r

    80kbtu input five ton drive http://www.texasfurnace.com/

    Furnace may be derated t by 20% although the out put is not grossely oversized to meet your needs as it is equipped.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    York. Has 5 ton blowers on their 100,000 BTU units.
    And I think Trane does also.
    Contractor locator map

    How-to-apply-for-Professional

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

  11. #11
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Have the installer plug off the farthest orifice from the valve. 1/4 inch pipe plug. Have to move the flame sensor.

  12. #12
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    Not sure where you are out of?

    Trane has a XV80 two stage that is rated 100k input/81k output with a 5 ton blower. If you go that route, be sure to get a two stage t-stat.

  13. #13
    Join Date
    Aug 2003
    Location
    Lincoln, Nebraska
    Posts
    1,051
    Quote Originally Posted by ar_hvac_man View Post
    Have the installer plug off the farthest orifice from the valve. 1/4 inch pipe plug. Have to move the flame sensor.

    I would not have anyone change the furnace from its specified design. This could cause more problems and the changes could cause problems with insurance, warranty and such.

    Lennox has 60 blowers on some 90,000 models.
    Its a good Life!

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