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  1. #1

    Is my unit oversized

    I replaced my old unit with a Rheem 4 ton 16 SEER unit, after some electrical problems the unit seems to be working fine. My concern how is the unit never needs to run on the 2nd stage to cool the house down. During the day when it's 97 degrees outside and high humidity. I will set the thermostat at 76 degrees and 45% humidity. The unit will run for around 25 mins on and 15 mins off. It stays on the 1st stage and never needs to go to the 2nd stage. It keeps the house at 76 degrees and 45% humidity. In the morning when its 80 degrees outside and very high humidity the unit will run for around 15 mins and be off for around 1 hour, and keeps the house at 76 degrees and 45% humidity. Everything seems to be great except for those short run times and long down times. What do you guys think, should I worry?

  2. #2
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,687
    the good thing is you have a back up compressor, did you put this in? what was in the house first?
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  3. #3
    Join Date
    Nov 2008
    Location
    Biloxi, Ms.
    Posts
    176
    Quote Originally Posted by jduggan View Post
    I replaced my old unit with a Rheem 4 ton 16 SEER unit, after some electrical problems the unit seems to be working fine. My concern how is the unit never needs to run on the 2nd stage to cool the house down. During the day when it's 97 degrees outside and high humidity. I will set the thermostat at 76 degrees and 45% humidity. The unit will run for around 25 mins on and 15 mins off. It stays on the 1st stage and never needs to go to the 2nd stage. It keeps the house at 76 degrees and 45% humidity. In the morning when its 80 degrees outside and very high humidity the unit will run for around 15 mins and be off for around 1 hour, and keeps the house at 76 degrees and 45% humidity. Everything seems to be great except for those short run times and long down times. What do you guys think, should I worry?
    I would hope that the installer ran a load on your house to determine the size needed. Not knowing the type of system you have, I'm wondering if the unit is staying in low speed to keep the humidity at 45%. Keep in mind that the length of down time is determine by how well the house is insulated. It does sound like the unit is oversized, but that maybe what you asked for. Sometime as dealers, we put in what the customer wants. Another thought, I wonder if the inside fan is staying in low speed, but the outside is in second stage. That could happen because of trying to control humidity. Just my thoughts.

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031

    Lightbulb

    Quote Originally Posted by jduggan View Post
    I replaced my old unit with a Rheem 4 ton 16 SEER unit, after some electrical problems the unit seems to be working fine. My concern how is the unit never needs to run on the 2nd stage to cool the house down. During the day when it's 97 degrees outside and high humidity. I will set the thermostat at 76 degrees and 45% humidity. The unit will run for around 25 mins on and 15 mins off. It stays on the 1st stage and never needs to go to the 2nd stage. It keeps the house at 76 degrees and 45% humidity. In the morning when its 80 degrees outside and very high humidity the unit will run for around 15 mins and be off for around 1 hour, and keeps the house at 76 degrees and 45% humidity. Everything seems to be great except for those short run times and long down times. What do you guys think, should I worry?
    The fan speeds ought to be set at 350-cfm per/ton of cooling.
    A 16-SEER R-410A system should have a TXV.

    It works okay on the low first stage
    ; however, U paid for more BTUH than it appears is needed... It appears, you might as well have installed a single stage with the tonnage of the first stage of your two stage system.

    Why have the cost of a second stage that is never used?

    It appears U have high humidity problems to deal with; a Manual J should have been performed with the idea of longer run cycles especially on the low first stage.

    Therefore, the high 2nd stage ought to be sized so on the hottest days it will run continuously.

    It is also important to do what is necessary to reduce high humidity air infiltration to help reduce the latent load factor.

    When it is extremely hot & humid outdoors even 78 or 80F indoors with low humidity with some air movement will usually feel very comfortable; therefore humidity is the determining factor.

    We should not turn into a bunch of sissies that demand 72F with low humidity, which is extremely costly to begin to achieve!

    All these options need to be thoroughly discussed & agreed on prior to any decisions or any work being performed. A final written/signed agreement before work starts is not a bad idea.

    An written/signed agreed-on consensus, whereby it is explained why this is the economical & energy efficiency choice, seems somewhat practical to me.

    They have to understand 'why' this is the best option, & fully agree; it's too late after it has been done the wrong way.
    What U say...

  5. #5
    Join Date
    Aug 2011
    Location
    Northwest Indiana
    Posts
    40
    Can you force it into 2nd stage? If you go over the to thermostat and kick it down 3 or 4 degrees it should (if the stat is capable and wired correctly) bring 2nd stage on right away. It's possible that your stat just isn't wired to the outside unit correctly. You mentioned electrical problems, maybe you've got control problems as well. I know some higher end stats allow you to specify that you want to end the cycle on 2nd stage - even if that means it only runs for a minute or two. If you can stay on first stage in 97 degrees and high humidity and the unit still cycles off for a while, and we assume first stage is about 34k BTUh between latent and sensible, I would say you've got about 1 ton too large a system (at least) and a very well insulated home (yeah!). If your contractor didn't perform a heat load to determine your sizing, shame on him. If you previously had a single stage condensor that was 4 tons and you insisted on replacing like for like, well....
    At this point you essentially have a 3 ton single stage condensor and aren't getting any of the benefits of 2 staging. I think that would be worth the effort to get that situation corrected.

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Soudns liek it does a great job dehumidifying despite it's size. Although it might not be operating in the most effeceint manner doing so. It might even just be short on airflow, which si why it dehumidifies so well.

    Off hand it sounds oversized, but only a load calculation would tell for sure. Not sure what your design temperature is.

    Keep in mind too that the newer 2 stage units only unload 35-35% of their capacity. So 1st stage is around 3-3.2 tons. .. not 2 tons like some older units which had 2 seperate compressors inside.

  7. #7
    Join Date
    Jul 2011
    Posts
    71
    I'm not expert, just a reader. But can you provide some info about the unit you replaced and some general information about the house i.e. sq. ft., style, location, age? When the old system worked, how did it work in comparison to the new system?

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,637
    Quote Originally Posted by jduggan View Post
    I replaced my old unit with a Rheem 4 ton 16 SEER unit, after some electrical problems the unit seems to be working fine. My concern how is the unit never needs to run on the 2nd stage to cool the house down. During the day when it's 97 degrees outside and high humidity. I will set the thermostat at 76 degrees and 45% humidity. The unit will run for around 25 mins on and 15 mins off. It stays on the 1st stage and never needs to go to the 2nd stage. It keeps the house at 76 degrees and 45% humidity. In the morning when its 80 degrees outside and very high humidity the unit will run for around 15 mins and be off for around 1 hour, and keeps the house at 76 degrees and 45% humidity. Everything seems to be great except for those short run times and long down times. What do you guys think, should I worry?
    Oversized and working well. The fun starts during low/no load conditions when the a/c does not run enough to remove the moisture from the occupants and fresh air ventilation/infiltration. What your estimate of the fresh air ventilation that you are getting to purge indoor pollutants and renew the oxygen. Would be interesting to see the CO2 ppm when occupied. The only thing wrong with the info above is that we do not know if the second stage works. Use the t-stat to setup the temperature when the home is not occupied 4-5 ^F and set down the temperature an hour prior to occupancy. That should test the second stage and save some cooling cost. I do this and fine it improves dehumidification when the cooling load is low. Keep us posted.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  9. #9
    The unit is a rheem. The old unit was a 4 ton, but I told the contractor that I had put open cell foam on the underside of the roof deck and we needed to do a load calculation. He said he had done a load calculation although I never saw it. That is my fault for not pressing to see it. The unit will go to the 2nd stage if I lower the temp on the thermostat. The compressor is changing from 1st stage to 2nd stage and back again when the thermostat calls for it. The indoor fan is running on the low speed of 960 cfms when on stage 1 and calling for dehum. mode. The thing that confuses me is at night when the temp outside is low and the humidity is very high, I live in south Louisiana, I have the unit set at 76 degrees and it will only run about 10 or 15 mins every hour and the indoor humidity will not go up.

  10. #10
    Join Date
    Jul 2011
    Posts
    71
    I try to tell my wife my unit is oversized but she doesn't believe me!

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by jduggan View Post
    The unit is a rheem. The old unit was a 4 ton, but I told the contractor that I had put open cell foam on the underside of the roof deck and we needed to do a load calculation. He said he had done a load calculation although I never saw it. That is my fault for not pressing to see it. The unit will go to the 2nd stage if I lower the temp on the thermostat. The compressor is changing from 1st stage to 2nd stage and back again when the thermostat calls for it. The indoor fan is running on the low speed of 960 cfms when on stage 1 and calling for dehum. mode. The thing that confuses me is at night when the temp outside is low and the humidity is very high, I live in south Louisiana, I have the unit set at 76 degrees and it will only run about 10 or 15 mins every hour and the indoor humidity will not go up.

    Well, if it's working. You may have a well sealed house. The spray foam probably went a long way to sealing where most of the air leaks were located before.

    960CFM is fairly low for 1st stage on a 4 ton scroll compressor, so that would explain in part, why it dehumidifies well. its' only getting about 350CFM/ton or just a shade less even.

    A 3 ton might have been a little more efficient for you and might have gotten humidity level down to 40%, which would have allowed you to raise the setpoint another 1-2F higher and be just as comfortable.

    Having a 4 ton instead of a 3 ton isn't as bad as having a 3 ton instead of a 2 ton.... whcih is often the case for many homeowner that go with 2 stage units.

  12. #12
    Join Date
    Mar 2010
    Posts
    195

    SO, what is the problem again?

    Quote Originally Posted by jduggan View Post
    I replaced my old unit with a Rheem 4 ton 16 SEER unit, after some electrical problems the unit seems to be working fine. My concern how is the unit never needs to run on the 2nd stage to cool the house down. During the day when it's 97 degrees outside and high humidity. I will set the thermostat at 76 degrees and 45% humidity. The unit will run for around 25 mins on and 15 mins off. It stays on the 1st stage and never needs to go to the 2nd stage. It keeps the house at 76 degrees and 45% humidity. In the morning when its 80 degrees outside and very high humidity the unit will run for around 15 mins and be off for around 1 hour, and keeps the house at 76 degrees and 45% humidity. Everything seems to be great except for those short run times and long down times. What do you guys think, should I worry?
    OK,

    Your is system is "working fine", is keeping your house at temperature and humidity levels you find comfortable in varying conditions, and seems to be "great".

    I think we are done here.

    Your first stage is running at around 2.5 tons and that looks like all you need most of time.

    Maybe someday it will be 100 outside and you'll be eating them swamp bugs, drinking a little beer, dancing to some of that sweet Zydeco music with your friends and that puppy will kick into second stage!

    Best of Luck,

    AC BAD DOG

  13. #13
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,423
    Quote Originally Posted by AC Bad Dog View Post
    OK,

    Your is system is "working fine", is keeping your house at temperature and humidity levels you find comfortable in varying conditions, and seems to be "great".

    I think we are done here.

    Your first stage is running at around 2.5 tons and that looks like all you need most of time.

    Maybe someday it will be 100 outside and you'll be eating them swamp bugs, drinking a little beer, dancing to some of that sweet Zydeco music with your friends and that puppy will kick into second stage!

    Best of Luck,

    AC BAD DOG
    Be happy!
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    Make your influence uniquely far-reaching.

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