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  1. #1
    Join Date
    Aug 2006
    Location
    MA
    Posts
    2
    My old compressor in a 2 zone split system needs to be replaced. It's almost 20 years old, 2 1/2 ton unit that the tech says is shot. Tried a put a hard start kit on, and recharged, but it's just not blowing cool never mind cold.

    So it's a new unit I guess. If I get a 13 seer, do I need a new coil too. One tech said just change the orifice or valve on the coil, other said I have to change the whole coil inside. Or should I try to locate an older but new 10 seer to be compatible with the old coil...if can find one.

    I just want an honest opinion. Thanks.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,896

  3. #3
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    This current thread is almost the same question:
    http://hvac-talk.com/vbb/showthread.php?threadid=113336

    Changing the cooling coil's orifice to a thermal expansion valve (if that's possible) will allow a new 13 SEER AC to be functional with an old coil. And in some cases you don't even have to change the orifice in order for it to be functional. But you'll very likely have some loss of cooling capacity as well as efficiency.

  4. #4
    Join Date
    Jun 2003
    Location
    Florida
    Posts
    9,548
    At 20 years old, don't put a new condenser on that old coil. Do it right with a matched set. JMO
    If everything was always done "by the book"....the book would never change.

  5. #5
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,704
    Originally posted by smokin68
    At 20 years old, don't put a new condenser on that old coil. Do it right with a matched set. JMO
    the only RIGHT way to do it.

  6. #6
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    ...I have these 20 year old tires. i need a new car. should i put these 20 year old tires on the new car? will they work?




    don't be cheap now. get a new matched system.
    FILL OUT YOUR PROFILE!!

  7. #7
    Join Date
    Aug 2006
    Location
    MA
    Posts
    2
    I'm interested in why the coil should be changed--tires that are old get worn and are obviously a wear and tear item that should be replaced. But a coil would seem to be different if it is in fine condition-what makes it "match" the new compressor? If it's a question of orifice size, that is easily matched at lower expense.

  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,896
    As SEER goes up, compression ratio goes down. That means lower head, higher suction. A "10 SEER" coil may be looking for a liquid pressure say 250 psi on a 95 degree day. If it gets that, it will deliver the rated capacity and EER as that is ARI's EER rating point. But say you put a 13 SEER outdoor unit on the same coil. To get the SEER up, they've used a slightly smaller comp and real big condenser. Leaving liquid pressure is now 220 psi on a 95 degree day. What happens when the lower pressure liquid hits the coil? Lower refrigerant flow means less cooling happens. The liquid refrigerant boils off quicker, superheat is higher, coil is starved for refrigerant. High superheat could mean poor compressor cooling, lower capacity and insufficient cooling. The way around it is overcharge the system to get head up like a lower SEER unit. So we've defeated the purpose of having a high SEER outdoor unit. And the overcharge means more strain on the compressor and more electrical usage.

    You can play with piston sizing but that won't compensate for physical coil size. As far as I know, every brand came out with a new coil line for 2006 and most have phyically larger coils, some quite a bit larger.

  9. #9
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    Originally posted by f2benny
    I'm interested in why the coil should be changed--
    BL and I didn't link that other thread for our health. It explains exactly why the coil needs to be replaced, as does his second reply here.
    Originally posted by BaldLoonie
    The way around it is overcharge the system to get head up like a lower SEER unit. So we've defeated the purpose of having a high SEER outdoor unit.
    Is it really a way around it? In other words, do we really restore function at only the cost of electricity? It seems to me that a smaller compressor would be physically incapable of pumping the necessary amount of Freon to match the old compressor (without the higher backpressure provided by a new coil). Smaller horsepower means smaller displacement, no?

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