Parts difference between 3.5T and 4T AC? (and associated problem)
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  1. #1

    Parts difference between 3.5T and 4T AC? (and associated problem)

    I have a 3.5T AC. If I had a 4T AC, what critical parts would be different?

    The reason I ask ...

    I bought a new 2300 sq. ft. standard stick home from a national builder about 10 years ago (in the Pacific NW). The AC was added at the time, and it has never worked well (having moved here from the SW, we were surprised at how small the AC was). It used to hold about 10 degrees below the outside. The builder has had AC folks come out and work on it every year, and they have gone over the whole house - moved ducting around, changed the vents, etc. etc. Last year, things got a little better, but it still doesn't hold much more than 15 degrees below outside - and it can't ever cool the house. If we turn it on during the day, we have to wait until the house cools naturally overnight and then see if it can hold it the next day.
    So - this year, the builder has brought out a new AC group. They are going to be replacing the fan blades in the outside unit. They are hoping to increase air flow for the system this way.
    I have always suspected that a larger unit is required. So - I was wondering what actually changes if we moved to a larger unit. I assume it is more than the fan blades :-)

  2. #2
    Join Date
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    chibulls21

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  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    Where are you located?

    A larger unit will just use more energy. I can cool my whole 3200sqft home on two 2 tons systems and keep it at or below 75 in 100F humid weather which is above design conditions. Oh... and it's 86 years old with original windows with good storms and only attic insulation improved.

    It's just a matter of home construction, shading, insulation, and how tight it is. I would look at the insulation and tightness of the home along wiht attic ventilation. Tract home builders are known to cut corners and not always follow best practices in sealing up a home. The more corners they cut, the more money they make. You can build a 2300sqft home with spray foam insulation or ICF that only needs 2 tons or less.

    Is this 2 story? Do you have a zone system? IF not, air distribution may be the biggest problem. You not getting cool air where it needs to be.

  4. #4
    I am located in the Portland Oregon area. We have a 2 story home and do not have a zoned system. As to the insulation, all I can really add is that the heating system works extremely well. It is gas heat, but uses the same blower and ducts, and seems to do a very efficient job heating the house.

    When the A/C system was not working well before, you could put your hand above a vent and feel some movement. The lace curtains hanging over it wouldn't move. With the work they did last year, moving the ducts around in the attic and under the floor, and adding another air intake on the second floor, things improved to where the curtain ripples a little bit now. But still, nothing near the air flow that I have been used to feeling in other homes in Arizona and Maryland.

  5. #5
    Join Date
    Jan 2009
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    Holy cow... Portland and 3.5tons isn't big enough??? IF you haven't noticed Protland is a mild climate without a winter and a very mild summer ... by midwest and Sotueast standards at least. Is this an all glass house without insulation? Something must be wrong with your AC.

    IF it's not zoned and it heat evenly, then I can almost gurantee the ductwork is too small upstairs to cool properly. Believe it or not, it might even cool better with a smaller AC, or at least one that's working properly.

    It's also possible you have a problem and something is restricting your airflow.

    I'd ditch the builder (they frequently use low bid low quality HVAC contractors) and find a smaller outfit that "speaks the right language" and can properly evaluate and actually fix your system. The builders HVAC company probleby has no idea what's wrogn because they install them all like this and lack the proper training and knowledge top correctly size and install a system.

  6. #6
    Join Date
    May 2012
    Location
    San Diego, CA
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    There is a good chance you are looking at low air flow... usually even if an a/c is sized correctly but your ducts aren't designed right then you might have problems delivering enough cool air.

    To answer your question... a 4 ton a/c will mean a new outside unit and indoor coil (above the furnace). The blower fan in the furnace still might be too small so you will probably need a bigger fan, but then your ducts will be too small... so everything just kinda adds on...

    I would get a load calculation done to make sure your 3.5 ton isn't sized right. Is the builder paying for all these repairs? If so, have them put in the larger blower... but do the load calculation to make sure your a/c is sized right.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  7. #7
    Join Date
    Feb 2009
    Location
    Portland OR
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    While you may not have a zone system I do often see many upstairs supply ducts with a manually adjusted damper on them. The damper going to the upstairs may manually be set to be closed off to push the heat to the downstairs in the winter to let it naturally rise to the second floor. Otherwise you do have a ductwork issue. If anything your AC is oversized, I am located here in Portland and usually would put a 2.5 ton AC with that size house and possibly even a two ton.

    You said you get good airflow in winter but not summer so you could have an issue with freezing your coil which reduces/eliminates any airflow while your coil is frozen. The coil could freeze from plugged filters, improperly charged AC, mismatched fan and AC capacities etc.
    If you would like us to take a look you can google my username or e-mail which is my name Travis at my username .com

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  8. #8
    There is a damper, but it is open to allow air up to the second floor. They have played around with dampers in the ducts. Yesterday, the HVAC guy took out the damper that was put in last year, and put a scoop in. All for naught. They are convinced that the A/C is working properly - the splits and pressure are correct. They add more refrigerant, cleaned out the filters outside, replaced the fan blades - but yesterday when they left (around noon) it was 75 in the house, 85 outside, and the A/C was set for 72. The inside temperature stayed at 75 until much later in the evening (when the outside temp had dropped below the inside temp). On a vent or 2 downstairs, you can stand next to them and feel the air coming out. Upstairs, you need to get within a foot or two to actually feel the airflow.
    So - yeah - it sure looks like some sort of airflow problem - but what we are still trying to figure out.
    Thanks for the responses - I will see what else they plan on and possibly check myself with some local vendors as suggested.

  9. #9
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