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  1. #1
    Join Date
    Aug 2003
    Posts
    907

    Oversized A/C: replace all or what

    A/C equipment is in good condition (no fins bent on condenser, fairly clean evaporating coil, works well, SEER 13, etc...) but is now seriously oversized after putting in better insulation (yes there's enough fresh air ). It's currently 3 tons, but I'm fairly sure that 1.5 would now be enough, just from the running times. What should I ask for in bids for the work to correct this? In an oversized system, should everything get replaced or is it possible to replace only the coil or condenser and still operate at a reasonable efficiency? I'm looking forward to a quieter system that controls humidity better.

    If the condenser should be replaced, I have a preference for easy to clean condensers with sturdy and well protected fins. Any suggestions on how to ask for this?

    Also, I'd like to be able to run the fan continuously on my current ECM furnace, but without re-evaporating the water condensed on the evaporating coil, after the condenser stops running. I'm wondering how to ask for this without being sold extravagant solutions, or being misunderstood. Should I ask for something specific like positive closure motorized dampers that open an alternate air path while blocking air through the coil, or leave it to the HVAC company to figure out something? If I ask for something specific, it should be easier to compare bids.

    Thanks to your help, I hope to do the bidding process only once and correctly, and not waste anyone's time.
    -If you won't turn it on then nothing else matters.

  2. #2
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,818
    Depending on the furnace and coil it might be possible to change the condenser and adjust the airflow.m

  3. #3
    Join Date
    Aug 2003
    Posts
    907
    Quote Originally Posted by catmanacman View Post
    Depending on the furnace and coil it might be possible to change the condenser and adjust the airflow.m
    The lowest speed the furnace can be set at when cooling is 680 cfm, but on continuous fan (what I want) it can be set to 600 cfm. It sounds like I need to have people inspect the coil before asking for bids. Thanks for your reply!
    -If you won't turn it on then nothing else matters.

  4. #4
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,443
    It's not uncommon for systems to match large indoor coils with smaller condensers. Got to get the airflow right though.

    I too have an oversized system and have found that the newer White Rodgers 90 series thermostats have a neat little feature called "cool savings". It turns the temperature up when the condenser runs a lot. Also the 1f95-1291 has a dehum terminal that will work with your VS blower to run at lower speed when it's humid in your house (you set the humidity level to switch over).

  5. #5
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    Get a proper heatload analysis done before getting bids on a new system. I doubt very much adding insulation cut your heating requirements in half, but the proper analysis will give you the exact numbers. That's quite an expense to put yourself to for a slight increase in indoor comfort.

  6. #6
    Join Date
    Jul 2010
    Location
    dfw tx
    Posts
    102
    I also have a oversized system. Is your system getting old, or are you replacing it just because its oversized? Since I have a TXV, I ended up slowing the blower down to somewhere around 330 cfm per ton. It made a huge difference in humidity. You may have already tried everything already, but I just wanted to throw that out there. good luck

  7. #7
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,757
    2 ton 2 stage heat pump would be my preference if your cooling load is 1.5 and you can save the furnace.

    Furnace model would get you better responses.

    How big is the house? Blower door leakage?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  8. #8
    Join Date
    Aug 2003
    Posts
    907
    The furnace is an 8 year-old Thermopride CHX1-75N high efficiency 2-stage natural gas. House is 2000 sq. feet, Indiana, with CO2 around 650 ppm with 70 cfm of mechanically brought in fresh air and 2 people + cat. It used to be very leaky, but over the years, closed cell foam, new windows, insulation and some caulk took care of that. Without the fresh air, CO2 easily rises to 1000 ppm, but I never let it go above that just to find out how high it can get . No blower test data available.

    I want to be able to leave the fan on continuously without getting the humidity back in the air.
    -If you won't turn it on then nothing else matters.

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,597
    Quote Originally Posted by pmeunier View Post
    The furnace is an 8 year-old Thermopride CHX1-75N high efficiency 2-stage natural gas. House is 2000 sq. feet, Indiana, with CO2 around 650 ppm with 70 cfm of mechanically brought in fresh air and 2 people + cat. It used to be very leaky, but over the years, closed cell foam, new windows, insulation and some caulk took care of that. Without the fresh air, CO2 easily rises to 1000 ppm, but I never let it go above that just to find out how high it can get . No blower test data available.

    I want to be able to leave the fan on continuously without getting the humidity back in the air.
    I also have an oversized a/c 90% of the time. And so will you. Running the blower on low speed dries the coil in 45 mins after the a/c stops cooling. There is currently no way out of this problem. A wet coil dries in about 1.5 hours without the fan operating. By passing the coil will not help much because small leakage will dry the coil. Drying the coil is a good thing because mold would grow in the ducts surrounding the coil if left wet. Also as the cooling load declines overnite and as weather cools, even a small a/c will short cycle.
    I operate my fan "on" low with wide dead band t-stat and us temp set-up during the day to 78^F until 5:00 PM. Drop to 75^F until 8:00 AM. Than back 78^F. The 3^F drop plus the 3^F dead band provides a couple hours of operating which removes a lot of moisture without any re-evaporation. My a/c removes 25-30 lbs.of moisture per day. My whole house dehu removes about the same amount of moisture.
    During light cooling loads like now, the a/c removes very little moisture while the dehu removes about the same. With the fan "auto" mode the a/c removes 5-6 lbs. of moisture more per day and the dehu less. My experience is that you may improve the moisture the a/c removes but only slightly. I have monitored the importanted totally varible speed a/c which are only slightly better during light loads.
    Will be monitoring a new model varible speed down to 50% speed. I expect the results to be similar during lite load conditions. A compressor at 50% speed on a 100% coil with a low speed fan (Poor circulation) will take up to an hour to load the coil/pan with moisture before starting to drain. At the end of the cycle, the moisture will slowly re-evaporate back to the home.
    More practical to make sure your coil is very cold removing as much moisture as possible without making your ducts sweat. Max the length of the cooling cyle to minimize the short cycles. Run you fan as you like. Some controllers allow the fan to run during specific times of the day. Operate your dehumidifier to get <50%RH throughout the home. Like fresh air when the home is occupied.
    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"

  10. #10
    Join Date
    Aug 2003
    Posts
    907
    Thanks Teddy Bear for your response, especially the tips in programming the thermostat for more dehumidification. Is it possible that your coil dries so quickly because your dehu discharges hot dry air not too far from the coil, as your company manuals suggest that it be connected? My dehu discharges directly in the basement and that makes it quite comfortable. I have trouble believing that positive closure dampers would not be effective in delaying the drying of the coil.

    Regarding mold, I think you need dust and spores for it to grow. There's not a trace of it in my system, and I think that's because I caulked all the leaks that would diminish the efficiency of the MERV 13 filter. No dust, no mold food, no mold (unless it was pre-existing).

    Cheers
    -If you won't turn it on then nothing else matters.

  11. #11
    Join Date
    Jun 2011
    Location
    Wake Forest, NC
    Posts
    352
    I don't know all the specs for your home, but a 1.5 TON on a 2000 sq/ft house raises my eyebrows a little... Make sure to verify that information before making a purchasing decision. I agree that a 2 stage sounds like a great option for you. If the load is verified a 2 ton 2-stage would do the trick, if it is higher than a 3 ton 2-stage would work as it would run on low stage most of the time, only operating essentially as a 2 ton unit but reserving the 3 ton capacity for those 100 degree days.
    It's not rocket-science...

    It's electromechanical thermodynamic engineering

  12. #12
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,115
    Quote Originally Posted by pmeunier View Post
    A/C equipment is in good condition (no fins bent on condenser, fairly clean evaporating coil, works well, SEER 13, etc...) but is now seriously oversized after putting in better insulation (yes there's enough fresh air ). It's currently 3 tons, but I'm fairly sure that 1.5 would now be enough, just from the running times.
    Take and stick you a 4 or 5-ton coil on there, that should increase run times.
    __________________________________________________ _______________________
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