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Thread: Furnace sizing question

  1. #1
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    Furnace sizing question

    Im doing a side jobby for family that requires replacing a 47000 BTU OUTPUT furnace.
    Below are outputs...
    My supplier can give me. 41000 hi /29000 lo
    Or a 56 hi / 40 lo
    The house is about 2500 ft sq, half that is basement. Its got 2 warm walls because its a condo.
    Ductwork should handle either unit.
    Word is that the old unit short cycled a bit.
    Anyone got input on these outputs?

  2. #2
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    Whats the TESP on the existing system?

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  4. #3
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    Depends on the load. In my experience I cant buy gas furnaces small enough to match the load. Maybe look into doing a blowerdoor test and load calc then going with an appropriate sized heatpump instead?

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  6. #4
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    If the old one short cycled and it is 47k out and the smallest new one it 41k out on high fire then put in the 41k. Otherwise you just as well put in a single stage 40k out as the 56/40 will never run on high fire anyway. Just make sure you pull wires etc so the thermostat can control the staging.

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  8. #5
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    Quote Originally Posted by TannerD View Post
    Depends on the load. In my experience I cant buy gas furnaces small enough to match the load. Maybe look into doing a blowerdoor test and load calc then going with an appropriate sized heatpump instead?

    Sent from my VOG-L29 using Tapatalk
    you are in Alabama, he is in Canada....
    I r the king of the world!...or at least I get to stand on the roof and look down on the rest of yall

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  10. #6
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    is the existing 80% b vent or 90% plus with the pvc..
    you did not say if your numbers are output Or input...
    a load calculation needs to be done...and the neighbors might keep their temps higher so you have a radiant gain..got a thermal camera to check?
    you also might have a geothermal gain with a basement but it is not mentioned how much of it is in the ground..seen some that are only 1/4, others 1/2 a few 3/4 in the earth..
    the short cycle issue should also be looked into..it should not be hard to look into it...it will give you a good insight if the existing is oversized

  11. #7
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    Quote Originally Posted by Refrig1234 View Post
    I’m doing a side jobby for family that requires replacing a 47000 BTU OUTPUT furnace.
    Below are outputs...
    My supplier can give me. 41000 hi /29000 lo
    Or a 56 hi / 40 lo
    The house is about 2500 ft sq, half that is basement. It’s got 2 warm walls because it’s a condo.
    Ductwork should handle either unit.
    Word is that the old unit short cycled a bit.
    Anyone got input on these outputs?
    Ive got a 10 X 10 walk in box what size condensing unit do I need?

  12. #8
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    Short cycling? By t'stat or limit sw?

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  14. #9
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    Quote Originally Posted by pecmsg View Post
    Ive got a 10 X 10 walk in box what size condensing unit do I need?
    You gave too much info.

    I could easily make some educated guesses from that.

    You should have said:

    I have a walk in cooler. What size condenser should I use? Larkin has like 29 sizes available.


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  16. #10
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    Thread Starter
    I don’t have an esp reading.
    Or an existing temp rise.
    Or a heat load.
    There was no word of any past hi limit lockouts so the system is likely cycling on the stat. And there’s no word of it ever not being able to keep up to the load.
    I could only do a very rough existing esp estimation that might make everyone here sick. But I don’t believe air flow to be an issue.

    I know my questions full of holes.
    I guess I’m looking for an opinion on whether having a furnace 6000 btus less than the original really has the potential to give me comfort problems.
    Or if I should put in the larger one which is going to be oversized for the space on the high end... if it ever even gets there.

    There’s no way everyone here is going out and getting a heat load done every time you end up in the middle of two pieces of equiptment...

  17. #11
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    Quote Originally Posted by heatingman View Post
    You gave too much info.

    I could easily make some educated guesses from that.

    You should have said:

    I have a walk in cooler. What size condenser should I use? Larkin has like 29 sizes available.


    Sent from my iPhone using Tapatalk
    Exactly and you gave the same information!

  18. #12
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    Auto reset on hi limit.

  19. #13
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    Quote Originally Posted by Refrig1234 View Post
    I’m doing a side jobby for family...
    What's a jobby?


    "You never know what others don't know." -

    If I can't laugh at myself...then I'll laugh at YOU! -

  20. #14
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    I'd go with the smaller one.
    Contractor locator map

    How-to-apply-for-Professional

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

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  22. #15
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    If the current unit has worked for years but short cycles on the stat I'd go smaller.
    In my entire life I've upsized a system exactly TWICE. Both times were for a tag mahal type Renovation that more than doubled the square footage of the conditioned space.
    When replacing equipment I usually do a fairly quick basic manual j
    And that usually leaves me still installing larger equipment than is necessary. But almost every single output unit is 20% less than the one I removed.
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

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  24. #16
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    Quote Originally Posted by Restaurant mech View Post
    If the current unit has worked for years but short cycles on the stat I'd go smaller.
    In my entire life I've upsized a system exactly TWICE. Both times were for a tag mahal type Renovation that more than doubled the square footage of the conditioned space.
    When replacing equipment I usually do a fairly quick basic manual j
    And that usually leaves me still installing larger equipment than is necessary. But almost every single output unit is 20% less than the one I removed.
    I am not understanding this line. To me what it is saying is that you replaced a 2T with a 2.5 ton. Depending on brand/model/combination the actual output will vary but how do you end up with a 30,000 unit putting out less than 20,000 (24k x .2 = 4800 off the 24000 or 19,200). Or you take out a 60k 95% furnace and replace it with a 80k 95% and end up with an output of under 46k.

    I don't understand. What am I missing?

  25. #17
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    Quote Originally Posted by BNME8EZ View Post
    I am not understanding this line. To me what it is saying is that you replaced a 2T with a 2.5 ton. Depending on brand/model/combination the actual output will vary but how do you end up with a 30,000 unit putting out less than 20,000 (24k x .2 = 4800 off the 24000 or 19,200). Or you take out a 60k 95% furnace and replace it with a 80k 95% and end up with an output of under 46k.

    I don't understand. What am I missing?
    I think what he is saying is that the rough manual J is not as accurate as a full blown room by room, or maybe some assumptions are made for unknowns that end up adding a little CYA to the sizing.

    But still winds up selecting a smaller unit then what was already there.

    For example.

    He does not no the exact R value of the walls so maybe he uses R-11 where their might actually be R-13

    So that adds some extra btu.

    So maybe a totally accurate man J would come up with 40,000 BTU and his rough calc says 50,000 btu

    And he's removing a 70,000 btu furnace.

    He's still downizing, just maybe not as much as he would if he had a totally in depth Man J with room by room calcs etc..

    Sent from my SM-G930V using Tapatalk

  26. #18
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    Quote Originally Posted by heatingman View Post
    I think what he is saying is that the rough manual J is not as accurate as a full blown room by room, or maybe some assumptions are made for unknowns that end up adding a little CYA to the sizing.

    But still winds up selecting a smaller unit then what was already there.

    For example.

    He does not no the exact R value of the walls so maybe he uses R-11 where their might actually be R-13

    So that adds some extra btu.

    So maybe a totally accurate man J would come up with 40,000 BTU and his rough calc says 50,000 btu

    And he's removing a 70,000 btu furnace.

    He's still downizing, just maybe not as much as he would if he had a totally in depth Man J with room by room calcs etc..

    Sent from my SM-G930V using Tapatalk
    Thanks I think I got it.

    He is installing smaller equipment than was there but not a small as Man J would suggest.

  27. #19
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    Quote Originally Posted by Refrig1234 View Post

    There’s no way everyone here is going out and getting a heat load done every time you end up in the middle of two pieces of equiptment...
    If you don't do a load calculation, who is responsible if the size proves to be incorrect after the installation?
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  29. #20
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    Quote Originally Posted by heatingman View Post
    I think what he is saying is that the rough manual J is not as accurate as a full blown room by room, or maybe some assumptions are made for unknowns that end up adding a little CYA to the sizing.

    But still winds up selecting a smaller unit then what was already there.

    For example.

    He does not no the exact R value of the walls so maybe he uses R-11 where their might actually be R-13

    So that adds some extra btu.

    So maybe a totally accurate man J would come up with 40,000 BTU and his rough calc says 50,000 btu

    And he's removing a 70,000 btu furnace.

    He's still downizing, just maybe not as much as he would if he had a totally in depth Man J with room by room calcs etc..

    Sent from my SM-G930V using Tapatalk
    That's exactly correct. For a quick manual J I'm still going room by room and measuring windows etc.
    But unless I'm going to spend an entire day in a house I have to make some assumptions......
    So on average I think the quick manual J gives me a load around 10% higher than ACTUAL load.
    Then on top of that when looking at heating options if the manual J calls for 47500 btu output......
    I'm not installing a 45000 btu output furnace.
    I'd go one size larger which for argument sake would be 52000btu.
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

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