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  1. #1
    Join Date
    Aug 2006
    Posts
    28

    What is adequate cooling?

    Without getting too philosophical or going too far into the nuts and bolts of system sizing, etc., hypothetically, when the right system is in the right home, what is the degree of cooling a homeowner should expect in the current heat wave on the East Coast?

    Last spring we had installed a new Trane system - I think the XL14i, 2.5 tons. It does great until these heat waves. It will maintain 74 degrees normally, but in the heat wave, by mid-afternoons, the temps rise on the first floor to 78 and a little higher on the 2nd floor. Overnight, the system "catches up," but it isn't until 7 a.m. that it hits back down to 74 degrees. Now that we are in the 4th day of this thing, last night the temp on the first floor of the house went to 79 and this morning it was only down to 76.

    I don't want to bombard a professional for a service call right now if it's not warranted. When temps are 95+ and heat indexes are higher, is 78 degrees what I really should settle for? Or does our new system maybe need some tweaking?

    Thanks for any thoughts on the issue.

    K. in MD

  2. #2
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Typically, there should be about a 20 degree "split" difference in temperature between air entering the indoor coil and air leaving the indoor coil at the air handler. If the temp of the air entering the coil is 74F, the air leaving the air handler should be about 54F. High humidity air will increase the exit air temp which decreases the split temp.

    The "split" temp is NOT the difference between outside air and indoor air.

    If you have ductwork running in the attic, it needs to be checked for leaks.

    Reducing the cooling load on the house should also be looked at: adding attic insulation, caulking, storm doors, good windows, etc.

  3. #3
    Join Date
    Apr 2008
    Posts
    65
    Design specifications should be discussed before any system is installed. It is common to design a system to hold 78*F in high heat. If a system is sized to hold 72*F, but is actually set to 78*F to save electrcity, then humidity may climb too high.

    If you are getting a reduction in temperature between the return air and supply air of about 20*F, then you need to reduce the cooling load on the house. Adding insualtion, thermal barriers on windows that let the heat in and sealing air leaks will be the most cost effective solution at this point.

    Bill

  4. #4
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    For example, ACCA Manual J recommends Baltimore MD have a summer outdoor design temperature of 91F. (Indoor design temp is chosen by the contractor, probably is about 75F). One principle of Manual J is to aim for running continuously at design temperature, therefore if this is followed then interior temperature is *supposed* to drift upward when actual outdoors exceeds design.

    That's the theory anyway. I am a homeowner in Texas and have done my best to understand this part. What people do in real houses often deviates considerably from theory.

    Part of an answer, hope this helps -- Pstu

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    Those (Three) replies were excellent.

    You can go by the indoor SA/RA split providing you know that the duct system & blower are delivering the required airflow, I like 400 to 450-CFM per a nominal rated ton of cooling.

    The first thing we all must know is how many CFM of airflow is going through the coil & how much of it is latent condensation heatload & how much is sensible heatload.

    All else being optimal, insulation, windows, radiant heat barriers, etc., if the ductwork is sealed & properly insulated, & the system is does not appear to be performing to its nominal tonnage rating. It could be inadequate airflow trough the evaporator.

    Provide me with an accurate through the E-coil CFM; Return Air dry bulb & wet bulb & The Supply Air dry & wet bulbs, or Relative Humidity percentages instead of the wet bulbs & I will provide you with the psychometric data for those conditions.

    Total Cooling Btu/hr; Sensible Cooling Btu/hr; Latent Cooling Btu/hr.
    Condensation Rate; dry bulb temp drop.

    I like to have the outdoor dry bulb temp, % Relative humidity & the temp rise of the condenser discharge air. If you know the condenser CFM you can get a ballpark on the Btu/hr it is delivering out doors, that should verify the indoor results.

    I know of a setup where the old gravity flow SA registers at floor level, dump the cooler high %humidity air on the floor & it recycles through the floor level Return grille.

    Coupled with very low airflow & throw, that makes it impossible to get anywhere near the required heatload through the Evap-Coil.


    Modern test instruments (if you can afford them) make it a lot easier to get all that data & get it a lot faster! - udarrell
    Last edited by udarrell; 06-10-2008 at 04:01 PM. Reason: All (Three) Replys

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    Those three replies were excellent.

    You can go by the indoor SA/RA split providing you know that the duct system & blower are delivering the required airflow, I like 400 to 450-CFM per a nominal rated ton of cooling.

    The first thing we all must know is how many CFM of airflow is going through the coil & how much of it is latent condensation heatload & how much is sensible heatload.

    All else being optimal, insulation, windows, radiant heat barriers, etc., if the ductwork is sealed & properly insulated, & the system is does not appear to be performing to its nominal tonnage rating. It could be inadequate airflow trough the evaporator.

    Provide me with an accurate through the E-coil CFM; Return Air dry bulb & wet bulb & The Supply Air dry & wet bulbs, or Relative Humidity percentages instead of the wet bulbs & I will provide you with the psychometric data for those conditions.

    Total Cooling Btu/hr; Sensible Cooling Btu/hr; Latent Cooling Btu/hr.
    Condensation Rate; dry bulb temp drop.

    I like to have the outdoor dry bulb temp, % Relative humidity & the temp rise of the condenser discharge air. If you know the condenser CFM you can get a ballpark on the Btu/hr it is delivering out doors, that should verify the indoor results.

    I know of a setup where the old gravity flow SA registers at floor level, dump the cooler high %humidity air on the floor & it recycles through the floor level Return grille.

    Coupled with very low airflow & throw, that makes it impossible to get anywhere near the required heatload through the Evap-Coil.


    Modern test instruments (if you can afford them) make it a lot easier to get all that data & get it a lot faster! - udarrell

  7. #7
    Join Date
    Mar 2003
    Posts
    7,326
    In many cases, home systems are designed for a specific indoor air temp at design outdoor temp. This is the right way. In other cases, systems are ballparked with no load calcs done. At any rate, lets say your system was designed for 76 indoor when its 91 outdoor. Most of the time you will be fine. when the outdoor conditions go beyond the design temps, you lose the ability to hit that indoor setpoint, and it will drift upward. If your system is oversized, you will stay at setpoint, but may suffer the rest of the season. Ideally we could have a system that modulates and is able to hit setpoint all year long, and do proper dehumidification. Not quite there yet with residential.

  8. #8
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    Quote Originally Posted by citypaws8 View Post
    when the right system is in the right home, what is the degree of cooling a homeowner should expect in the current heat wave on the East Coast?
    i would not consider my system "right" unless it kept my house at 70 degrees even on record hot days "over 100 degrees"

    that being said i like it cooler in my house than most people!

    the way i obtained my "right" system is i oversized my system by 3/4 of a ton, "its a 2 speeder"

    i would not attempt this oversizing with a single speed system

    i figured out my prefered temp on the hottest day

    you need to be specific with your HVAC contractor about how cold you "need" it on the hottest day!

    with my system (heat pump) and its being "oversized" an additional benefit for me is

    in the winter, i dont require backup (electric heat) until below 15 degrees!

    i am not saying its the "right" system for anyone else, but it is for "me"

    some of the pro's on this website wont agree with me and "my" sizing, but it works for me!

    and if i had a humidity problem "which i dont" i would purchase an IAQ t-stat to help dehumidify!



    .

  9. #9
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    in short, the right HVAC unit is that which satisfies your family's comfort.
    [as I type, my office meter= 85F, 55% RH -- with 12" circulating fan & ceiling fan blowing on me = ok]

    in long, what was the HVAC installer given for criteria?
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  10. #10
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    "as I type, my office meter= 85F, 55% RH -- with 12" circulating fan & ceiling fan blowing on me = ok"

    Wow.

    No way (unless there's a power outage).

    Take care.

  11. #11
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,452
    Quote Originally Posted by flange View Post
    In many cases, home systems are designed for a specific indoor air temp at design outdoor temp. This is the right way. In other cases, systems are ballparked with no load calcs done. At any rate, lets say your system was designed for 76 indoor when its 91 outdoor. Most of the time you will be fine. when the outdoor conditions go beyond the design temps, you lose the ability to hit that indoor setpoint, and it will drift upward. If your system is oversized, you will stay at setpoint, but may suffer the rest of the season. Ideally we could have a system that modulates and is able to hit setpoint all year long, and do proper dehumidification. Not quite there yet with residential.
    What a mess. We have been sold on the "over-size excuss" for an a/c being unable to maintain <50%RH during cool wet weather. The a/c that is unable to maintain 76^F during extreme hot weather, is unable to maintain <50%RH, with minimum fresh air ventilation during cool wet weather. All the homes that are <50%RH during wet cool weather are not getting the IAQ necessary 50cfm-75 cfm of fresh air ventilation. The exception is dehumidification systems or reheat a/c.
    We should be sizing the a/c to handle +10^F above design. This takes care of extra occupants or one hour temperature pulldown from t-stat setup. Provide fresh air ventilation via a whole house ventilating dehumidifier, maintaining <50%RH regardless of the cool load or a/c "off" conditions. Remember, providing the suggested ASHRAE, AM Lung ASSN fresh air ventilation has 60-70 pint(lb.)(60,000-70,000 BTU) per day moisture load. These moisture loads can occur with 70^F-80^F outdoor temperatures. Living in poorly ventilated space is said to have long term health effects. 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"

  12. #12
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    >>We should be sizing the a/c to handle +10^F above design.
    >>This takes care of extra occupants or one hour temperature pulldown from t-stat setup.

    Wow. Pulldown capability *is* a good value in the customer's eyes. But it seems to me to be a very interesting disagreement with the principles of ACCA Manual J sizing. First time I have heard it mentioned in such a professional manner. If there is any professional discussion of this heresy, sure would make interesting reading.

    Best wishes -- Pstu

  13. #13
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    For most houses, Manual J is the right answer - but, it is not the right answer for every house.

    Manual J assumes that there is no central dehumidifier in the equation.

    Based on my very limited personal experience - with the one house I have built - Manual J does not work all that well for houses that are "built right" e.e. very well insulated and sealed.

    Adequate control of humidity in a well sealed and insulated house requires a dehumidifier - even when it is 98 degrees outside.

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