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  1. #27
    Join Date
    Dec 2005
    Location
    Vero Beach, Florida, United States
    Posts
    2,669
    400 CFM per ton I believe is where the number comes from. BTW 800 sq ft per ton? Never heard of that. I have heard folks use 400 per ton and 500 per ton though.

  2. #28
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,242

    Thumbs down Limited Use

    Originally posted by richardkletty
    400 CFM per ton I believe is where the number comes from. BTW 800 sq ft per ton? Never heard of that. I have heard folks use 400 per ton and 500 per ton though.
    800 Sq.Ft per ton originates from NE Ohio.

    AND, it only applies to houses without 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

  3. #29
    Join Date
    Nov 2005
    Location
    Northern VA
    Posts
    512
    (whoops, wrong thread)

    [Edited by perel on 08-12-2006 at 12:07 AM]

  4. #30
    Join Date
    Jul 2006
    Posts
    169
    800 per ton is madness. in phx commercial is 250 per ton. res is 400 max per ton. this is all meaningless without a load calc and proper airflow. for goodness sake do it right or dont do it at all. call a prossesional

  5. #31
    Join Date
    Feb 2006
    Posts
    468
    The 400 CFM per ton is an average (rule of thumb!?!?) for air handler output. It can and should be different in some cases - humid conditions may need down around 350 CFM per ton, increasing the temp split across the evaporator and increasing latent capacity at the expense of both sensible AND overall capacity.

    OTOH hot dry climates can and should use higher airflows per ton for greater sensible capacity and efficiency.

    Smart systems such as Carrier's Infinity allow the user to enter a humidity setpoint and the system will vary the airflow to emphasize / deemphasize dehumidification.

    Key to all of this is ensuring adequate duct capacity to quietly and efficiently handle airflows projected in all conditions. I don't think I've ever come across a post or complaint of ducts too big. Even though the newer VS blowers can, to an extent, stuff the air into inadequate ducts, the cost is noise, extra energy, increased duct leakage, and likely decreased blower motor life.

  6. #32
    Join Date
    Aug 2006
    Location
    Marco Island, Fl
    Posts
    729
    First of all, I cannot tell you how important a proper load calc is.
    I would recommend spay foam if it properly applied.
    A Carrier 5 ton Infinity system will run at approximatly 50% capacity (2.5 tons)in stage one cooling. The indoor fan will run anywhere from 250 cfm/ton to 400 cfm/ton. The indoor fan actually modulates the fan speed and adjusts the compressor capacity to the operating conditions at hand. It uses temperature and humidity trending, indoor humidity and temperature set points, and outdoor ambient temperature and condensing coil temperature to monitor operations, among other things.
    Properly sized duct work is a must.
    If you purchas an INFINITY heat pump with a Infinity Furnace, You will benefit from thier Duel Fuel Technology.
    The comfort cannot be beat. You will have all the cooling for the extreeme hot days, or parties, and comfort on the average days. I would recommend the Infinity Air Purifier too. Nothing works like it.

    The addition of a Thermastore dehumidification system is icing on top of the cake. 85% of the systems we install are like the one described above, and I have it in my own house. Customers call us for more product info so they can have this system installed in their home up north.
    Get it right the first time. Even the best systems will not work if they are not properly installed.(By the way, SEER's are 15 and above)

    [Edited by davo on 08-19-2006 at 06:04 PM]

  7. #33
    Join Date
    Aug 2006
    Location
    Carrollton, TX
    Posts
    44
    I dunno, but from what I have been researching the last 3 months including talking to building science engineers all over the country, with a properly Isonene foamed house with a sealed, non vented attic, 750 to 1,000 square feet per ton is the norm. I live in the Dallas area and there is a 3,100 sq ft 2 story that is doing nicely in 105 degree temps @75 degrees inside with a single 4 ton unit. No complaints from the homeowner regarding electric bills or comfort. In fact, their electric bills are roughly half of their neighbors that have conventional fiberglass insulation and a 140 degree attic with conventional soffit vents and ridge vents with the A/C unit in the attic.

    My biggest concern is the lack of knowledge and experience with HVAC contractors regarding this new building science. I have heard answeres to my questions that make me shake my head in shame! I agree it's a whole new deal but I found it amazing how cool the attics are even with temps of 105 outside. Not much more than 5 - 10 degrees warmer than the living space temp below. Good news is I think I have found an HVAC contractor who understands that using foam that creates an envelope leakage rate (air infiltration rate) of .1 ACH at natural pressure is perfectly acheviable using the new building science techniques. I have seen it first hand.


  8. #34
    Join Date
    Aug 2006
    Location
    ridgecrest ca
    Posts
    9
    LETS FORGET ABOUT LOAD CALCS FOR ONE MINUTE AND ALSO CONSIDER CFM PRODUCED FROM ONE SYSTEM INTO A 4000 SQFT HOME???

  9. #35
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,741
    Originally posted by calmat
    LETS FORGET ABOUT LOAD CALCS FOR ONE MINUTE AND ALSO CONSIDER CFM PRODUCED FROM ONE SYSTEM INTO A 4000 SQFT HOME???
    Its the load calc that will tell you how many CFM that one system must move.

    A bedroom that would need 86cmf with fiberglass insulation now might only need 50cfm.
    Contractor locator map

    How-to-apply-for-Professional

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

  10. #36
    i live in NE ohio who is the builder and who is thehvac contractor why not go with amana or trane stay away from heil bad rep up here

  11. #37
    Join Date
    Aug 2002
    Posts
    4
    Originally posted by ohioac123
    As a matter of fact, the ones who quoted 5 tons said they'd have me sign off on a release stating that they wouldn't be responsible if I chose to go with a lower tonnage unit.
    Thanks,
    OH
    I agree with him. I'd demand a release too.

    You are grossly undersizing for a run of 90 degree days (and hot nights). Your house guests will hate you next summer.

    If you have a need to grossly undersize your system for some strange reason, then at least consider a 2 speed compressor.








  12. #38
    Join Date
    Jul 2004
    Posts
    253
    I have 4 tons for around 3100 sq. ft. It is a Bryant Evolution 2-speed system with 4 zones. About 500 square feet is the bonus room and we have that zone shut when we are not using the room. We are typically cooling 2600 sq. ft. (main level) with 2 tons (1/2 speed) 90% of the time according to the service menu. On hot days (93+) it may kick into high speed in the late afternoon. It can maintain 76F for the entire house (turning on the bonus room zone) on hot days but will run at high-speed much longer. We maintain the RH at 46%. Walls are ICF and the attic is foamed.

    Originally posted by txborn
    I dunno, but from what I have been researching the last 3 months including talking to building science engineers all over the country, with a properly Isonene foamed house with a sealed, non vented attic, 750 to 1,000 square feet per ton is the norm. I live in the Dallas area and there is a 3,100 sq ft 2 story that is doing nicely in 105 degree temps @75 degrees inside with a single 4 ton unit. No complaints from the homeowner regarding electric bills or comfort. In fact, their electric bills are roughly half of their neighbors that have conventional fiberglass insulation and a 140 degree attic with conventional soffit vents and ridge vents with the A/C unit in the attic.

    My biggest concern is the lack of knowledge and experience with HVAC contractors regarding this new building science. I have heard answeres to my questions that make me shake my head in shame! I agree it's a whole new deal but I found it amazing how cool the attics are even with temps of 105 outside. Not much more than 5 - 10 degrees warmer than the living space temp below. Good news is I think I have found an HVAC contractor who understands that using foam that creates an envelope leakage rate (air infiltration rate) of .1 ACH at natural pressure is perfectly acheviable using the new building science techniques. I have seen it first hand.


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