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  1. #1
    Join Date
    Sep 2012
    Posts
    37

    Oversized, Undersized or Right-Sized?

    Sitting here looking over proposals for my two new units, getting ready to pull the rope. I got to thinking (which can be dangerous) about the downstairs furnace sizing.

    The existing downstairs furnace is an OLD 80,000 BTU Lennox (Natural Gas) from the early to mid 1970's. My understanding is that it's probably between 65-70% efficient, based on its age. Of you split that range (67.5), the old beast is outputting 54000 BTU. We had no complaints about the heat level from the old furnace, so for the sake of discussion let's assume that 54,000BTU output is the reasonably correct size.

    All three of the bidding contractors specified an 80,000 BTU replacement furnace. I've settled on the Trane XV95, which according to AHRI is 97% AFUE and outputs 78,000 BTU.

    Looking at these in terms of BTU Output (and for the moment ignoring that the XV95 is 2-stage), isn't the 80,000 BTU XV95 seriously oversized? The first stage in the 80K model is rated at 49,500BTU out. It seems to me that the 80,000BTU m0odel would rarely (if ever) use the second stage.

    The 60,000 BTU XV95, on the other hand, is rated for 37,7000/58,000 BTU output. Much closer match.

    And before the question is asked, I've already asked my guy to go back to his calculations and tell me what size the math thinks the furnace should be. On the other hand, since the 60,000 BTU is the smallest in the line, there's nowhere else to go.

    Am I missing something here? Does my logic make sense? Should I downsize this to the 60K model?

    Thanks!

    D.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    First, the old furnace capacity is typically 80% of input. So figure it is 64K output. Being inefficient, when starting & stopping it is wasting a lot of gas so it gets the 60% efficiency rating. But if ran non-stop, the 64K is accurate.

    Now, it is likely rather oversized, most 70s vintage units were plus you've likely improved the house too. So anyone putting an 80K 95% where an 80K relic was is probably REALLY oversizing.

    The math is: only buy from a dealer who will run a load calc and see EXACTLY what the loss is. No point in paying for a 95% furnace that short cycles or never goes to high. That's inefficient and dumb. And it sounds like all 3 are cutting corners by not calculating and sizing right.

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,034
    Just curious as to what climate area you live in.

    Also, concerning some data on your home...

    Ever had an energy efficiency audit performed on your home?

    Did the load calcs expose areas where heat loss/gain could be reduced?

    Usually you can make those cost effective home efficiency changes that will make the smaller heating & cooling equipment work much more efficiently with the existing duct system...

  4. #4
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    Quote Originally Posted by dancaselr View Post
    Sitting here looking over proposals for my two new units, getting ready to pull the rope. I got to thinking (which can be dangerous) about the downstairs furnace sizing.

    The existing downstairs furnace is an OLD 80,000 BTU Lennox (Natural Gas) from the early to mid 1970's. My understanding is that it's probably between 65-70% efficient, based on its age. Of you split that range (67.5), the old beast is outputting 54000 BTU. We had no complaints about the heat level from the old furnace, so for the sake of discussion let's assume that 54,000BTU output is the reasonably correct size.

    All three of the bidding contractors specified an 80,000 BTU replacement furnace. I've settled on the Trane XV95, which according to AHRI is 97% AFUE and outputs 78,000 BTU.

    Looking at these in terms of BTU Output (and for the moment ignoring that the XV95 is 2-stage), isn't the 80,000 BTU XV95 seriously oversized? The first stage in the 80K model is rated at 49,500BTU out. It seems to me that the 80,000BTU m0odel would rarely (if ever) use the second stage.

    The 60,000 BTU XV95, on the other hand, is rated for 37,7000/58,000 BTU output. Much closer match.

    And before the question is asked, I've already asked my guy to go back to his calculations and tell me what size the math thinks the furnace should be. On the other hand, since the 60,000 BTU is the smallest in the line, there's nowhere else to go.

    Am I missing something here? Does my logic make sense? Should I downsize this to the 60K model?

    Thanks!

    D.
    There is another thread about oversizing 2-stage equipment. It happens all the time. You are right on with your concerns! Second stage will never be needed, except, it's on a timer (assuming you don't use a 2-stage stat) and it'll "kick" into high fire (and more blower noise) automatically.
    Bummer.
    Contractors size off off the older furnaces quite often.

  5. #5
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,778
    Sometimes you have to oversize the furnace to get enough airflow for a larger air conditioner. Happens all the time in the southeast bc we need little heat and lots of ac.

  6. #6
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,508
    Anywhere south of the Mason-Dixon line buy the smallest furnace possible that has enough blower capacity of handle the AC.

  7. #7
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,034
    Quote Originally Posted by 54regcab View Post
    Anywhere south of the Mason-Dixon line buy the smallest furnace possible that has enough blower capacity to handle the AC.
    Right-on> 54 & jtrammel.

    This is a critical reason to make the home & duct system more efficient so the cooling tonnage can be reduced, therefore, the air hander & duct system will move more airflow CFM at lower static pressure.

    Count the many ways to optimize/maximize airflow CFM...oversizing return air filter areas, using low pressure drop filters, performing a Manual D with the many ways to reduce Total Equivalent Length (TEL), thus increasing Available Static Pressure (ASP), & much more...

  8. #8
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    One challenge we encounter in the south with gas heating systems is that they are definitely over-sized, as already mentioned. As utility companies and other entities offer incentives on AHRI rated matches, a .5 SEER rating reduction equals less incentive $ paid, as well as reduced efficiency. Many homeowners we deal with don't want anything less than a 16 SEER match. The entities that provide the incentives audit most installs with a post-job visit, to verify the equipment numbers and AHRI matches, as it is "their $" they have given away.

    A lower BTU furnace that meets load criteria usually results in a narrower unit, sometimes reducing the SEER rating by .5, as the air volume must remain constant for the cooling BTU's. Does this have to do with lower EER due to increased TESP through a skinnier furnace? I hope someone who knows answers that.

    It is quite the quandary to satisfy both the load and AHRI requirements, even with the multitude of equipment variations available on the market these days.

  9. #9
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,034
    THERMOJOHN: Providing there is sufficient room above the furnace, you install a long enough transition to spread an even airflow distribution through the evaporator coil.

    Then when you do all the things that are possible to improve ductwork airflow efficiency, the system gains SEER sufficient for the rebates; a few of those things to do were listed in prior post 47.

    That is providing they credit the system with those SEER Gains; which they should do!

    Additionally, using a temperature SWING room thermostat when set for longer more SEER efficient run-time cycles, increases the SEER performance rating.

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