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  1. #1
    Join Date
    Dec 2010
    Posts
    20

    Why oversize the indoor coil?

    Just curious why a 4 ton coil was installed with my 3 ton heat pump. Installer said it was the only way the system would qualify for the tax rebate. I was hoping for more of a technical answer.

  2. #2
    Join Date
    Dec 2010
    Location
    Chillicothe, Ohio
    Posts
    19
    Larger coils = more surface area = greater heat transfer = higher efficiency

  3. #3
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,970
    Quote Originally Posted by chillitech71 View Post
    Larger coils = more surface area = greater heat transfer = higher efficiency
    Maybe, but he has a HP and and that may not be true, I have "always" heard on a HP system, the coil and condenser should match tonnage wise.

    These Tax credit systems are sometimes really strange. We just installed a Ruud system, 3ton that required the 100,000 btu furnace with the Ruud coil, and a 75,000 furnace with the Aspen coil, go figure. In Ohio this might be a good thing, but in Texas, I am sure the furnace will short cycle.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  4. #4
    Join Date
    Dec 2010
    Posts
    20
    Quote Originally Posted by chillitech71 View Post
    Larger coils = more surface area = greater heat transfer = higher efficiency
    So should the air handler be sized for 3 tons (1200 cfm) or 4 tons (1600 cfm)?

  5. #5
    Join Date
    Dec 2010
    Location
    Chillicothe, Ohio
    Posts
    19
    I agree that the matchups are very screwy with the tax thing ! That's our wonderful Government at work, but as for oversizing the evap, it's common practice and recommended by many manufacturers. Our company installs more heat pumps than anything in the residential market. We are a Lennox dealer, and all of the higher efficient units (HP or AC) are matched up by Lennox with larger evaps than condensers. TXV's need to match the condenser though..

    And that was post #15 for me .. I can apply for pro now !!

  6. #6
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    5,014
    Quote Originally Posted by chillitech71 View Post
    Larger coils = more surface area = greater heat transfer = higher efficiency
    Basically.

    It's technically not an "oversized" coil, as some people think. If your looking for a piece of equitment that would have a seer rating of 16, then to accomplish this you need to find two pieces of equitment, that when paired, produce 16 seer.

    Your outdoor unit, when paired with your coil, produce your desired seer rating.

    If you wanted a lower seer rating, then you could pair a different outdoor unit, with a different coil, and get a lower seer.

    These ratings can be found on your manufacturers website.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  7. #7
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,970
    Quote Originally Posted by Ellbaker View Post
    So should the air handler be sized for 3 tons (1200 cfm) or 4 tons (1600 cfm)?

    I guess not, read what I posted up yonder. ^

    Quote Originally Posted by Mr Bill View Post
    We just installed a Ruud system, 3ton that required the 100,000 btu furnace with the Ruud coil, and a 75,000 furnace with the Aspen coil, go figure.
    The 100,000 is a 5-ton furnace.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  8. #8
    Join Date
    Jul 2010
    Location
    North Richland Hills, Texas
    Posts
    6
    If you want the seer rating you have to "over size" the coil. That is the proper way to do it.

  9. #9
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,970
    Quote Originally Posted by tincup59 View Post
    If you want the seer rating you have to "over size" the coil. That is the proper way to do it.

    I don't know about "proper" way, but it is the "only" way.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  10. #10
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by Ellbaker View Post
    So should the air handler be sized for 3 tons (1200 cfm) or 4 tons (1600 cfm)?
    The AH needs to be sized for the matched coil and condenser. And the tax credit for AC depends on what it's matched up with.

    Some contractors give bogus info to their customer WRT the tax credit. Then they find out later they don't qualify.

    Post model numbers for the condenser, coil, and air handler and we'll look up the AHRI match and tax credit info for you.

    My 2.5 ton condenser has a 3.5 ton coil, and qualified for the credit. My air handler (furnace) has a 4 ton blower. I could not have used a furnace with a 3 ton blower, unless it could do the 1400 CFM (some 3 ton blowers can) required for the 3.5 ton coil.

  11. #11
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    If designed by the manufacturer to work together to accomplish a specific SEER or HSPF rating, I would still say they are a "matched" pair.

    To EllBaker's question about the appropriate CFM, I'm going to hazard a guess that setting the CFM to match the condenser's capacity is the way to go, regardless of size of the coil in the air handler (unless the coil is undersized I would assume). Regardless of indoor coil surface area, there is only so much "heat" to be extracted from the coils and too much air across them means each cubic foot absorbs less energy.

    But this is one where I am out of my league. Anyone know the physics behind this and whether it should be Condenser size or indoor coil size that should drive the CFM setting?

  12. #12
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    Dang Gary - you type faster than I do, and your answer is better than mine!

  13. #13
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by Tom Campbell View Post
    If designed by the manufacturer to work together to accomplish a specific SEER or HSPF rating, I would still say they are a "matched" pair.

    To EllBaker's question about the appropriate CFM, I'm going to hazard a guess that setting the CFM to match the condenser's capacity is the way to go, regardless of size of the coil in the air handler (unless the coil is undersized I would assume). Regardless of indoor coil surface area, there is only so much "heat" to be extracted from the coils and too much air across them means each cubic foot absorbs less energy.

    But this is one where I am out of my league. Anyone know the physics behind this and whether it should be Condenser size or indoor coil size that should drive the CFM setting?
    CFM should be set according to the manufacturer's specs in the product data or installation guides for the coil and condenser, for each air handler it's rated with. However, the ones I have seen are almost always rated at around 400 CFM/ton, for the coil size (tons). Sometimes this has to be adjusted for altitude though.

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