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  1. #1
    Join Date
    Jun 2012
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    Gulf South Region
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    2

    Combining a 4 ton a/c condenser with a 3 ton evaporator

    Hi all,
    My name is George and I am a Home Inspector. I need conformation for a question that was posed to me by a client recently.
    What she wanted to know is, whether or not she could install a 4 year old 4 ton Trane a/c condenser unit in place of a 15 year old 3 ton Ruud heat pump while using the original 3 ton Ruud evaporator coil that is already installed in her 1550 sq. ft. home.
    Any input would be greatly appreciated.
    Thanks

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,483
    I would NOT do that for a number of reasons.

    First, you're trying to force 1600-cfm of air volume through a coil rated for 1200-cfm; static pressure drop will skyrocket.

    Second, it would need a properly size TXV flow meter with it installed you could reduce airflow & the TXV would reduce the flow, however, the condenser would have excess liquid in it.

    Therefore, you'd be greatly reducing the efficiency & Btuh Rating of the 4-Ton condenser... There is more but am quitting typing...

  3. #3
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,001
    yeah, there is lots more.
    but Udarrel has hit the crux of the matter.
    also duct sizing may not be able to handle the
    increased amount of air flow. return size would
    have to be increased.

    if she needs more cooling she would do better
    to determine why the unit isn't providing this.
    upsizing doesn't always solve problems it often
    creates a whole set of new ones.

    I realize some home inspectors don't get into the whole
    hvac thing, but to learn enough to be able to answer these
    types of questions would increase the value of your service.
    to be able to recommend a hvac company that would
    correct her problems rather than exacerbate them is a good
    thing imo.

    best of luck..to both of you.
    Last edited by energy_rater_La; 06-03-2012 at 01:12 PM. Reason: spelling
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,420
    Then there's the issue of refrigeration. A heat pump needs less gas in the heat mode when the smaller indoor coil becomes the condenser. So if you hack a 4 ton outdoor unit on an extra small indoor coil, you are about guaranteed to have a charge imbalance that will require a tech adding and removing gas everytime the unit is switched between heat & cool.

    And besides the forcing 4 tons of air through a small 3 ton coil, what about the 3 ton duct system. Most aren't big enough for what is there let alone hacking 33% more capacity without increasing supply, return and register capacity.

  5. #5
    Join Date
    Feb 2003
    Location
    Wichita Ks
    Posts
    1,408
    she can not install a 4 ton condenser on a 3 ton coil and expect it to work. as far as everyone saying trying to force 4 tons of air through a 3 ton coil, he didn't say anything about changing the air handler out, only the outdoor unit.
    also the wiring probably isn't large enough to handle the larger unit.

    if she wants to change the indoor unit to a 4 ton then like others replied the duct probably is not big enough.

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,483
    Excellent points by baldloonie & freonrick.

    It could be that her existing air handler may not be rated up to 4-Ton but just 3-Ton; I always make those comparisons so components get selected to achieve proper airflow at appropriate static pressures.

    Sometimes the air handlers are rated at a much higher tonnage than the A/C tonnage, especially in the cold north country where many way-oversize furnaces therefore, the furnace could have called for a 4-Ton air handler which also tended to be oversized too.

    Some of the old belt-drive Oil furnaces had blower set-ups that went from 2-ton to 4-Ton by changing pulley sizes & motor HP; in heating mode many needed +1500-CFM with a mere 1.5-Ton A/C requiring only 600-CFM.
    Those units should be switched to a direct drive blower covering the required CFMs.

    Still a good valid point, by freonrick.

    I like the pics of the rabbit & the pups.
    Last edited by udarrell; 06-03-2012 at 08:49 PM. Reason: I like the pics of the rabbit & the pups...

  7. #7
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    4 tons for 1550 sq' sounds waaay to big.
    FILL OUT YOUR PROFILE!!

  8. #8
    Join Date
    Jun 2012
    Location
    Gulf South Region
    Posts
    2
    I want to thank all of you for your input. In my line of work, the value of my service isn't in me knowing everything rather, in me being able to find the "right" answer for my client. As a home imspector I am legally bound by a code of ethics that prevents me from recommending any particular contractors for any type of work to be performed. As I stated in my o.p., I simply needed confirmation on what I had already advised her of which is in line with what most of you are conveying. She isn't attemptimg to upsize the amount of cooling, just needing a new condenser unit and someone is offering her a 4 ton for a substantially reduced price. All she wanted to know is if a 4 ton condenser would be compatible with her existing 3 ton evaperator. Now that all of you have chimed in I feel that I have presented her with some valid information.
    Again,
    Thank you

  9. #9
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,420
    Quote Originally Posted by InspectorG View Post
    Now that all of you have chimed in I feel that I have presented her with some valid information.
    Again,
    Thank you
    You're doing good, rare to get 100% in agreement on something

  10. #10
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,518
    Quote Originally Posted by BaldLoonie View Post
    You're doing good, rare to get 100% in agreement on something
    Really! maybe we should ask Dad to make this thread a sticky.
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  11. #11
    Join Date
    Jun 2011
    Location
    Houston area
    Posts
    1,493
    Am I missing something? How is is changing out an outdoor unit going to change anything about indoor airflow? Not saying it's a good idea or bad (actually it's a bad idea) but it will work half-a$$ed. Will some of the pro's explain to this pro the concept of how swapping a condensing unit can affect indoor airflow?
    The picture in my avatar is of the Houston Ship Channel and was taken from my backyard. I like to sit outside and slap mosquitos while watching countless supertankers, barges and cargo ships of every shape and size carry all sorts of deadly toxins to and fro. It's really beautiful at times.....just don't eat the three eyed fish....

    ¯`·.¸¸ .·´¯`· .¸>÷÷(((°>

    `·.¸¸..· ´¯`·.¸ ¸.·´¯` ·.¸>÷÷(((°>

    .·´¯`· .¸>÷÷(((°>

    LMAOSHMSFOAIDMT

  12. #12
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,483
    Cooked: Will some of the pro's explain to this pro the concept of how swapping a condensing unit can affect indoor airflow?
    It doesn't, however, when you put a larger capacity condenser requiring more airflow, if the air handler has any additional CFM capacity, & some will, many will use it.

    Otherwise, of course, it would not affect it...

  13. #13
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,518
    Quote Originally Posted by Cooked View Post
    Am I missing something? How is is changing out an outdoor unit going to change anything about indoor airflow? Not saying it's a good idea or bad (actually it's a bad idea) but it will work half-a$$ed. Will some of the pro's explain to this pro the concept of how swapping a condensing unit can affect indoor airflow?
    Well what I saw here was, that with a 3-ton coil and a 4-ton condenser your suction will be lower, and you probably would not be able to overcome this low suction with a 3-ton blower, so freezing up might be an issue, especially without a valve. I do know that Dave Debian with Central City use to install larger condensers with smaller evaps here in Houston for latent control. He took a raw coil and built it special with a couple TXV's on the coil, I saw one before that he built, if I remember correctly he used two txv's somehow on these coils. You can "IF" you know what your doing make a smaller coil work with a larger condenser, but it's not the norm, especially in lower humidity climates. As for as air flow, the coils Dave used were smaller in tonnage capacity but were physically huge coils.
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

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