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  1. #1
    Join Date
    Aug 2012
    Location
    Arlington, VA
    Posts
    3

    Nervous about 2-ton AC for a 3400 sq ft zone in a new house

    Having trouble getting subs to pay attention to what my new house needs. Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit. I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows. I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain. I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.

    I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.

    But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.

    Anyone have any experience with this sort of situation? All advice gratefully appreciated.

    Mark

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,795
    Quote Originally Posted by mxarey View Post
    Having trouble getting subs to pay attention to what my new house needs. Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit. I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows. I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain. I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.

    I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.

    But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.

    Anyone have any experience with this sort of situation? All advice gratefully appreciated.

    Mark
    I would look into an IQ drive system from Nordyne (maytag, broan, fridgidaire) they are completely variable from 30%-118% of size equipment and are very good at keeping a constant temp and rh with any outdoor temp. Also the trane xl20i would be another option. I think the 3 ton would short cycle too much and not dehumidify at all because it would satisfy the temp before the coil even got loaded and began removing humidity.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,903
    I'm surprised that a 3400 sq ft foamed house comes out to 2.5 tons. A member here "paul42" has a 4000 sq ft house in Texas that came out to 2 tons. He report that when it hit 104 outside it still maintained 74 degrees inside.

    http://hvac-talk.com/vbb/showthread....C-is-oversized
    Contractor locator map

    How-to-apply-for-Professional

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

  4. #4
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,627
    Its my opinion that all foamed houses need a dehumidifier and fresh air ventilation. I woud think the guy saying 4 ton just does not believe a foamed house is that efficient and is worried it wont do the job .go with a2 ton 2 stage unit
    We really need change now

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,273
    Quote Originally Posted by mxarey View Post
    Having trouble getting subs to pay attention to what my new house needs.
    Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit.

    I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows.

    I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain.
    I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.

    Anyone have any experience with this sort of situation? All advice gratefully appreciated. Mark
    BOTH recommendations are Really Suspect.
    Where does one get a 2.5-Ton 2 stage?
    (Or is 2.5 typo, meant 2.0)

    18,500 BTU/HR Sensible might be a bit low in my opinion.
    Your calc - How many total square feet of windows?
    Square feet facing each direction?
    SHGC for 'good' windows means 0._ _?
    U-value 0._ _

    EXCELLENT WINDOWS
    SHGC < 0.52
    U-VALUE < 0.35
    If sensible load is < 20,000 BTU/hr with typical 350 square footage window area (3,400 sq foot residence),
    the windows must be excellent.
    Last edited by dan sw fl; 09-23-2012 at 09:29 AM. Reason: Good versus Excellent Windows
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,721
    Quote Originally Posted by catmanacman View Post
    Its my opinion that all foamed houses need a dehumidifier and fresh air ventilation. I woud think the guy saying 4 ton just does not believe a foamed house is that efficient and is worried it wont do the job .go with a2 ton 2 stage unit
    X2

    Or iaq per JT, or carrier GREENSPEED.

    Or ducted mini splits. How are you managing fresh air? How are you insuring qc on the foam work?

    Btw, nice job uncovering bs work.
    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.

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,361
    Quote Originally Posted by mxarey View Post

    I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.

    But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.

    Anyone have any experience with this sort of situation? All advice gratefully appreciated.

    Mark
    You have figured out the great mystry of comfort in modern homes. Most a/c designers do not understand this problem.
    The senisble cooling load of homes in green grass climates varies from 80% on a design day to 0% during wet, cool day. On the otherhand, the latent load is 20% of the total cooling load on a desigh day to 100% during wet cool weather. Yet our a/c systems operate with 75% sensible/25% latent cooling ratio. The tools used are slowing capacity and over-cooling. The new varible speed systems, while they work better are not the solution. You will end up with a cold damp home during cool wet weather. Also a/c designers are avoiding the necessary fresh air ventilation that all homes need to purge indoor pollutants, renew oxygen, and remove odors. In addition to the latent load in infiltrating/ventilation fresh air, the latent cooling from the occupants occurs throughout the cooling load range. 100 cfm of 70^F dew point fresh air plus the moisture from 4 occupants is 7,000 btus per hour of latent cooling with very little sensible cooling.
    I will attach data from the a home with a variable speed a/c slow the inability to maintain <50%RH. The owner spared no expense to get have the best, yet had high humidity in the home and cold temps during certain weather.
    I will follow later with data.
    The solution is simple, add a whole house dehumidifier that compliments the home's load profile and a/c preformance to maintain the customer's desires. Typically, occupants want 70-78^F, <50%RH. A properly setup single speed a/c with a sized WH Dehu will maintain <50%RH during all of the ranges including no a/c during unoocupied times.
    The total installed cost is comparable to the VS a/cs. In addition all quality homes should have mechanics to provide the amount of fresh filtered make-up air to purge indoor polutants and renew oxygen at a minimum when occupied. Whole house dehus like the Ultra-Aire unit provide fresh air ventilation as an option.
    Again congrats on figuring out the variableness of the sensible cooling loads, while having a steady latient load.
    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"

  8. #8
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,115
    have you considered a Heatpump water heater in the home to provide humidity control when design conditions are low? or if you have one, are you factoring it's cooling/dehumidification in the calc?
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  9. #9
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,215
    When figuring the latent capacity on the 2 ton unit did you use low speed?

  10. #10
    Join Date
    Jun 2004
    Location
    Kathleen GA
    Posts
    236
    What Teddybear said.

  11. #11
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,215

  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,361
    VS AC RH Control .pdf

    The varible speed a/c do better at removing moisture than a single a/cbut are unable to maintain <50%RH during low/no sensible cooling loads. The attached data illistrates the problem of not being able to maintaining <50%RH during should seasons and wet cool weather.
    While adding a small whole house dehu even with fresh air ventilation easily maintains <50%RH throughout the home.
    Regards TB


    Sept WI Basement Ultra Aire 70 .pdf
    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"

  13. #13
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,795
    Quote Originally Posted by teddy bear View Post
    VS AC RH Control .pdf

    The varible speed a/c do better at removing moisture than a single a/cbut are unable to maintain <50%RH during low/no sensible cooling loads. The attached data illistrates the problem of not being able to maintaining <50%RH during should seasons and wet cool weather.
    While adding a small whole house dehu even with fresh air ventilation easily maintains <50%RH throughout the home.
    Regards TB


    Sept WI Basement Ultra Aire 70 .pdf
    I agree that fresh air is needed in any home and controlled ventilation is a must in a tight home. I also agree that dehus are great. Although when it is cooler out outside rh is lower and internal loads alone in milder weather will produce enough gain in a typical house for a true variable capacity hvac system such as the iq drive or mini split to run on low almost constantly thus removing enough humidity to keep comfortable at a reasonable temp setting. These higher end systems also have dehumidify functions that will run the unit long enough to maintain rh.

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