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  1. #1

    Are My Carrier Air Conditioners Oversized ?

    This is DEFINITELY not a DIY. I want to know what my reasonable options are. But first, the details.

    We have a 3100 sq ft single story house (9 ft ceilings) in east central Fla. The house was built in 2008. White Aluminum roof, double pane single hung "hurricane proof" vinyl windows, sealed (unconditioned) attic. The house faces north. The east wall has no direct sun exposure (massive sea grapes). The north and south walls get no direct sun in the summer ( 2 foot eaves). The west wall gets direct sun for about 3 hours.

    We have two identical Carrier Air Conditioners. The Outdoor Units are Model 25HCA336A0030010. The Indoor Units are Model FX4CNF042. The thermostats are Honeywell TH6110D. About 3/8 of the house is on the "North System". The remaining 5/8 of the house is on the "South System" (where most of the heat generating stuff is located). The contractor who designed and installed the two systems ASSURED me ( on several occasions) that the units were properly sized (I did not know about load calcs and never saw any). The original ducts were so poorly installed that my builder forced the ducts to be removed and "properly" installed. During the "dry-in" phase, I noticed that the compressors only ran ~5 minutes and one unit "rattled". The tech who came out to fix the rattle told me "all Carrier's run short on-cycles".

    We moved in in late 2008. In the summer of 2009 we kept the house open most of the time so it was hard to evaluate how the AC was running. In 2010 my wife said the air from the ducts was "cold" and the humidity stayed in the high 50% region. Several times we turned down the thermostats to 60 (we normally run the temp at 78) and left the house for several hours. Humidity dropped for a while.

    Last year I decided to try and determine how our AC's were working. On 7/12/2011 I measured the run times of both AC Systems. The outdoor temperature varied from 75 to 94 degrees. The outdoor humidity varied from 65% to 94%.

    The North AC ran 45 times for a total of 3 hours and 24 minutes. The minimum compressor on time was 4 minutes (5 times) and the maximum run time was 5 minutes 30 seconds (2 times).

    The South AC ran 52 times for a total of 4 hours and 44 minutes. The minimum compressor on time was 4 minutes 30 seconds (4 times) and the maximum time was 6 minutes and 30 seconds (3 times).

    Both thermostats were set at 78 degrees. The indoor humidity was 52% to 55%.

    I talked to my contractor and told him that I wondered if we were "over air conditioned" because the cycle times were short and the humidity was high. My contractor talked to the AC company (they won't talk to me) and was informed that in no way were we "over air conditioned" and if we didn't like the humidity, they could install a dehumidifier.

    This spring we bought a Frigidaire FAD504DUD 50-Pint Dehumidifier. It fills up twice a day (100 pints of water) and the indoor humidity varies from 44% to 47% regardless of the outdoor humidity. Other than it being noisy and having to empty the bucket, we are satisfied with the comfort.

    Now that you are "bored to tears", I have several questions.

    1. Are the compressor cycle times typical for these Carrier Air Conditioners?
    2. If we are "over sized" is there any way to "de-tune" these Air Conditioners ?
    3. Would a humidity assisted thermostat help?
    4. What criteria would I use to select a new air-conditioner contractor ?

    Thanks for any suggestions.

    DaveCoolFl

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    Last year I decided to try and determine how our AC's were working. On 7/12/2011 I measured the run times of both AC Systems. The outdoor temperature varied from 75 to 94 degrees. The outdoor humidity varied from 65% to 94%.

    The North AC ran 45 times for a total of 3 hours and 24 minutes. The minimum compressor on time was 4 minutes (5 times) and the maximum run time was 5 minutes 30 seconds (2 times).

    The South AC ran 52 times for a total of 4 hours and 44 minutes. The minimum compressor on-time was 4 minutes 30 seconds (4 times) and the maximum on-time was 6 minutes and 30 seconds (3 times).

    Both thermostats were set at 78 degrees. The indoor humidity was 52% to 55%.


    I talked to my contractor and told him that I wondered if we were "over air conditioned" because the cycle times were short and the humidity was high. My contractor talked to the AC company (they won't talk to me) and was informed that in no way were we "over air conditioned" and if we didn't like the humidity, they could install a dehumidifier.

    This spring we bought a Frigidaire FAD504DUD 50-Pint Dehumidifier. It fills up twice a day (100 pints of water) and the indoor humidity varies from 44% to 47% regardless of the outdoor humidity. Other than it being noisy and having to empty the bucket, we are satisfied with the comfort.

    Now that you are "bored to tears", I have several questions.

    1. Are the compressor cycle times typical for these Carrier Air Conditioners?
    2. If we are "over sized" is there any way to "de-tune" these Air Conditioners ?
    3. Would a humidity assisted thermostat help?
    4. What criteria would I use to select a new air-conditioner contractor ?
    Those runtimes are way too short!
    There isn't even enough runtime to reach an optimally cold coil; & virtually no runtime for dehumidification...

    Is the RM-TH where supply air could be affecting it?

    You need a SWING or cycles per hour adjustable setting RM-TH so you can achieve longer on-time run-times.

    The blower may already be set for 350-CFM per/ton of cooling; if not, I would try that setting for a colder coil & longer runtimes; for better humidity control.

  3. #3
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,664
    I am gonna just throw this out there, you could most likely cool your house on just 1 of your systems.

    Or cut all the shade down and add some thermal load to the house.

    Contractor probably did the load calculation pretty close, but I would be willing to bet shade was not a factor.

  4. #4
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,188
    cut down the shade and add thermal load..

    udarrel...what the heck is RM-TH?? room thermostat??

    six tons for 3100 sq ft is 500 sq ft per ton. whacked load calc IMO.

    set fan speeds to low instead of med or high. air moving over
    the coil at slower speeds removes humidity.
    something is up if you are removing 200 pts of water daily, but
    at those RH levels the dehumidifier is doing its job.

    what do you mean by sealed attic?
    ductwork is in attic?
    original install must have been bad if contractor had them
    change it..wonder what it looks like now & if anything is
    actually sealed..

    new contractor..check site map.

    hope you dodged the Ivan bullet.
    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    udarrell...what the heck is RM-TH?? room thermostat??
    Well, I thought he might understand that better than T-stat or some other jargon...

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,123
    Check the CPH setting of your thermostats, they may be set to 4 or higher. Set them to 3 or 2.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,347
    Let's suppose a load calculation WAS done for this house - a stretch, possibly, but bear with me a moment. Most heat load calcs for cooling select the local outdoor "design day" criteria, plus a 75 degree indoor temperature target to maintain during those summer design days.

    You are running your thermostat setting at 78 degrees consistently. This has effectively oversized your two systems, assuming they are properly sized at a 75 degree setting.

    Try running your systems set for 75 or 76 degrees and see if your humidity levels improve. Where I am located has felt more like Florida lately thanks to an influx of moisture from Isaac. I run my system at 76 degrees and my indoor humidity is in the low forties.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    +1. You were right at design conditions. You total run time should be somewhere around 60% for the whole 24 hour period and you longest runtime upstairs should be around 30 to as long as 200 minutes and maybe 20-60 minutes downstairs depending on activities. Meaning it shoudl run almost continously at some point in thsoe conditions. Total cycles isn;t too far off if you're not using setbacks. I'm wondering if you thermostat is too close to a supply register.

    The large amount of shade and high setting as mentioend are really throwing you off too. With that shade and just looking at your run times, I bet you only need 2 tons units, not 3 tons. You might evne be able to go down to 1.5 tons possibly, but you need a proper load calculation for that. Going to 350CFM/ton will help as suggested. Although "standard" airflow for a carrier I think is already at I think 380 CFM/ton, so you mght enve go to 320 CFM/ton with your so oversized. No risk of freesing a coil if it only runs for 5-7 minutes at a time. With those short run times you'll struggle ot pull any humidity out.

    "all carrier run on short cycles". I like that. I should show him my Infinity 2 stage unit upstairs that will run for 4-6 hours straight in design conditions and still only has 70 cycles into high stage since late June.... basically one time a day in hot weather. Its' all abotu proper sizing, air dstribution, thermostat location and it helps a LOT if you have a lot of thermal mass.

    Just looking at the raw numbers. Hes' at 15-20% duty cycle. On a single stage in design conditions he should be 2X that. Wit ha 2 stage unit 3X that. He might be oversized by as much as 100% thanks to the cool roof and shade. Going from 78 to 75F will increase that load by maybe 25%, so that should help when you factor in Latent load. With the longer run times, I bet it will hardly use any more energy either.

  9. #9
    Join Date
    Jan 2009
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    Keokuk, IA
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    Just ot add one more comment. If you had a normal roof and no shade and se the thermostat to 75F, you're equipment would still be a little big, but not too far off.

  10. #10
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,188
    got a smile out of the carrier/short cycle thing myself.

    and I need to correct TS name..guess I was focusing on
    what I needed to do here before tomorrow morning...Isaac..not Ivan.
    wasn't Ivan a few years ago??

    gotta go get the grass cut..rain is going to make it
    grow like crazy.

    best of luck OP.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    We'll take a little of that rain. We got 0.7" this weekend. That was significant, since the total for August & July combined was a whopping 1.9". Normally we should average 8-9" over that period. You'll be getting that tomorrow in 30 minutes.

  12. #12
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by Southern Mech View Post
    I am gonna just throw this out there, you could most likely cool your house on just 1 of your systems.

    Or cut all the shade down and add some thermal load to the house.

    Contractor probably did the load calculation pretty close, but I would be willing to bet shade was not a factor.
    Here's a thought, tie the 2 units together and install powered dampers on hte supply and return of each unit and install a lead-lag controller. These can also stage the equipment. Then if you want, add soem zone dampers and a zone controller too.

  13. #13
    Quote Originally Posted by energy_rater_La View Post
    cut down the shade and add thermal load..

    udarrel...what the heck is RM-TH?? room thermostat??

    six tons for 3100 sq ft is 500 sq ft per ton. whacked load calc IMO.

    set fan speeds to low instead of med or high. air moving over
    the coil at slower speeds removes humidity.
    something is up if you are removing 200 pts of water daily, but
    at those RH levels the dehumidifier is doing its job.

    what do you mean by sealed attic?
    ductwork is in attic?
    original install must have been bad if contractor had them
    change it..wonder what it looks like now & if anything is
    actually sealed..

    new contractor..check site map.

    hope you dodged the Ivan bullet.
    best of luck.
    A sealed attic is an attic which has no air circulation. Spray 4-6" of Isocyanine foam on the underside of the roof. Effectively moves the thermal barrier from the ceiling to the roof. My attic temp's are 5-10 degrees above/below the inside room temp. The duct work in the attic is not in 120+ attic temps.

    According to site maps, no contractor within 90 miles.

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