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  1. #1
    Join Date
    Mar 2006
    Location
    Los Angeles, CA
    Posts
    5
    Hi. One of my highly recommended contracters - who also at this time has the lowest bid - is trying to sell me on a 4 ton AC. Three other contracters are pushing 3ton and 3.5 ton units. My house is only 1300sq feet 3bd/2ba; new windows, R30 attic ONLY, hardwood floors 17 windows (2 east facing, 4 west facing), built in 1956. I just don't think such a big unit would be benificial based on other posts I've read here. The 4 ton contracter however, says it's better to have the extra cooling capacity and just manipulate the fan to a slower speed so that the house doesn't cool too fast.

    This slower speed would PRESUMABLY reduce the short cycling of a larger unit and thus still remove moisture. The only reason I could even ponder at why this might be okay is that we live in a suburb around Los Angeles - it never really gets very humid so moisture removal is not such an issue. And after all is said and done does it cost substantially more to operate a 4 ton versus a 3.5 or 3 ton unit?

    Thank you guys for help

  2. #2
    Join Date
    Aug 2005
    Location
    Florida
    Posts
    326
    Have any of these contractors suggested or performed a load calculation on the home? 4 tons would appear to be grossly oversized for the desciption you have given. I live in a 1295 sq ft home of roughly the same desciption with a 2.5 ton sys in Florida. This is not to say that your home would require the same tonnage. The only way to know for sure is to have a load calculation done. If you are fairly handy, you can do your own calculation for cheap by clicking on the red tab at the top of the screen labeled hvac calc.

    Just try and be as educated as you can because this is a big investment and a big PITA if its not done correctly. After the contractor has gotten his money and is long gone, you are the one that will be stuck with either a great functioning sys or a badly functioning nightmare.........hope this helps......l8r

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,184
    Did ANYBODY do a heat gain calc? Around here, and we get 95 and very humid each summer, 1300 sq ft would be 2 ton, especially with that attic insulation. Chances are that little house doesn't have the ducts for 4 ton. If it did, it would be very drafty in there with 1600 CFM moving through such a small space

    And reread dachipsta's last paragraph. Well said!

  4. #4
    Join Date
    Mar 2006
    Location
    Los Angeles, CA
    Posts
    5

    Is fan speed adjustment even feasible?

    Thanks for the quick responses guys. I have two more contracters coming and I'll ask them about heat load - IF they do the heat load calculation I'll probably go with them.

    But, is it even feasible though to adjust the fan speed / CFM of a 4 ton unit to make it run "smaller"? Say for example if a future add-on was planned but over 5 years away - could it be made to work well (ie. not drafty, damp , or over cycling)?

    Thanks again for all your help,

  5. #5
    Join Date
    Oct 2004
    Posts
    3,400
    But, is it even feasible though to adjust the fan speed / CFM of a 4 ton unit to make it run "smaller"? Say for example if a future add-on was planned but over 5 years away - could it be made to work well (ie. not drafty, damp , or over cycling)?
    The answer is no, unless you spend a bunch of money on additional controls, including economizers, and makeup air.

    In five years, your system will be pretty long in the tooth anyway.
    The ductwork will need a redesign also.

    Go with a smaller system, or do the add-ons first.

    Another option is a separate, properly sized system, as part of the add-on.






    Why would any thinking person even consider buying a larger, more expensive unit, that will give a lower comfort level, run noisier, have a shorter life, all the while producing higher electric bills?






    Just leave three or four windows open.

    It will be fine.




  6. #6
    Join Date
    Aug 2005
    Location
    Florida
    Posts
    326
    The bottom line is that you are the one writing the check. After a contractor pleads his case, one of three things is going to occur. He will either convince you to do the job right given current circumstances (not future ones.), walk away from doing a job that he knows will come back to bite him in the a-- again and again, or do what you tell him to do because after all you are paying for the job and money talks. Ok, there are more than 3 possible outcomes but that sounded kual. You are in the beginning stages of what we fondly know and love as hackery. Some people will do anything if you pay them enough.

    To answer your question, you cant trick a 4 ton unit to run as a 3 ton. That is why there are 3 ton sys and 4 ton sys and 3.5 ton in between. Not trying to be a smart ass so please dont take offence. I will refrain to answer anymore questions on this situation in the likelyhood that my answers may incriminate me....hahahaha. Good luck whichever way you decide to go....l8r...

    Thanks Bald for the props. I'm out.

  7. #7
    Join Date
    Mar 2004
    Posts
    1,042
    There are a few tricks you can use to make a system act sort of like a smaller one. They don't work very well, though, and you can't trick the electric company either way. It will cost way more to run a four ton unit than a three ton.

    The proper setup for an arid climate (like yours) still demands using absolutely the smallest tonnage unit that will do the job. The only difference from other climates is that higher than normal airflow rates are desirable to maximize efficiency. By doing so, you keep the system from using a lot of energy trying to dehumidify air that doesn't need dehumidifying. Not only does that save energy, it also can let a system provide more cooling and energy efficiency than its ratings would suggest. How ridiculous for a contractor to try to sell the idea of oversizing a system to dehumidfy MORE in the Los Angeles area; that's like trying to get milk from an orange. The only times when dehumidification is desirable in that area are when it's too cold for running AC anyway- when it's 60 degrees, gloomy, and raining.

    Here's an outstanding reference for consumers written by someone who works in dry climate like yours:
    http://hphaa.com/services/installation/installation.htm

    You can't oversize a system now in anticipation of an addition that might get built years in the future. You'll get poor performance for the life of the system, regardless of whether the space is added or not. Also consider how you'd achieve temperature balance between a new addition, insulated to modern codes, and the existing space, which is poorly insulated. The right answer is to build for what is there now, and if an addition happens in the future, to deal with it later.

    I would be reluctant to even put a 3 ton unit in a house like yours. I have a hunch a heat load analysis would suggest more like two (nominal) tons, with airflow in the 900-1000 CFM range, and ideally the largest evaporator coil the manufacturer approves for the outdoor unit. Some oversizing won't hurt comfort as much in your area as in more humid areas, but as a former Angelino, I know how murderous the electric rates are. You can't afford to get the efficiency part wrong, and the first part of that is to get the utmost efficiency out of whatever equipment you do buy; that's not even expensive, in comparison to the price of buying fancy super-efficiency equipment.

    If you're farther into the hotter inland areas than your post suggests- ie San Bernardino County- then a load calc might suggest a little more tonnage. It just doesn't take that much to cool most houses in LA county, though.

    What indoor temperature would you typically set the system for during cooling season?

  8. #8
    Join Date
    Mar 2006
    Location
    Los Angeles, CA
    Posts
    5

    Talking

    Thank you all for your insight and help. It has confirmed what I thought is right. I just didn't get the idea of "tricking" such a huge machine into a much smaller one. I live near the ocean and now I see that such a unit would only benefit me if I were storing meat in my living room ha ha ha. Thanks again.

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836

    Re: Is fan speed adjustment even feasible?

    Originally posted by ranthony
    Thanks for the quick responses guys. I have two more contracters coming and I'll ask them about heat load - IF they do the heat load calculation I'll probably go with them.

    But, is it even feasible though to adjust the fan speed / CFM of a 4 ton unit to make it run "smaller"? Say for example if a future add-on was planned but over 5 years away - could it be made to work well (ie. not drafty, damp , or over cycling)?

    Thanks again for all your help,

    Are you planning to add-on??

  10. #10
    Join Date
    Mar 2006
    Location
    Los Angeles, CA
    Posts
    5
    No concrete plans to add on but we are starting a family - if any add-ons are performed it may just be a single bedroom - (200-250 sq. feet).

  11. #11
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by ranthony
    No concrete plans to add on but we are starting a family - if any add-ons are performed it may just be a single bedroom - (200-250 sq. feet).
    If you get an accurate load calc on your house ,you may find it needs a 2.5 ton,say 28,000 btus,because your load calc is 24,500 btus,so you have 2,500 extra btus to start with.

    Then when you build on one of your current exterior walls and it's windows/doors,will become an interior wall,so not part of the load.

    The addition can be built with superior insulation and windows to the current home,so the increase in size needed for the addition may only be the extra 2,500 btus,and the deduct for the current exterior wall becoming an interior wall.


    You could also add insulation or better windows to the existing home ,when you build the addition,which reduces the load even more.


    So,carefully planed ,the correct size today ,could be the correct size for the addition.

  12. #12
    Join Date
    Jan 2001
    Posts
    7,816
    To compensate for an oversized hvac system you could just crack a few windows...or possibly drill some 3 inch holes around the outside perimeter of the house...thru the exterior wall....nail up some window screen over the holes on the outside....stuff rags into them from the interior to increase or decrease the load.

  13. #13
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    running a blower continuously will REDUCE the amount of moisture removed -- such works fine in snow country -- in winter.

    read lots here !

    are you going to buy a 9 passenger van this year, just because you may have a big family?
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

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