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Sq Ft VS. Cu Ft / Tonnage

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  • Sq Ft VS. Cu Ft / Tonnage

    A quick question. I have an 1800 sq ft home in the Houston, TX area. The builder installed a 3 1/2 Ton A/C unit in the home. I have had it serviced and there are no leaks in the system. The filter and coils are clean but it can not keep up with the heat load on mild summer days, much less the hot ones. I believe that the problem may be the fact that over 1/3 of the house has 10 ft ceilings. That adds at least 1200 cu ft to the interior space of the house (from 14400 to 15600) over a standard 8 ft ceiling. Is this unit just too small to do the job? If it is, do I have any options other than replacing the whole system with a five ton?

    Thanks for the help,

  • #2
    Yes, you do have options, but it seems your problem is one shared by many homeowners. For whatever reason, your insistance, the contractors lack of due diligence, or maybe others factors it seems likely that the system installed for your home does not match its needs (I assume for cooling) This could be one of or a combination of problems.
    Let me set forth some possible solutions:

    1) Each home being cooled has a heat gain/loss. This is affected by many variables such as sq.ft, ceiling heigth, type of construction, orientation to the sun, local climate (obviously Houston has much different cooling needs than say St Louis or Clevland), insulation, number, type, and location of windows, etc.
    The first step here is to get a heat/cooling calculation load run on your home. This is done with exacting formulas and can be calculated by hand, but usually is much easier to do with a proper computer porgram. This is what shoud have been done before trying to size a system to your house.
    Get it done by a professional. Just using rule of thumb by square feet is not good, many times it works, but not always. Many companies should do it for no charge (or a small charge) if you will then allow them to try and solve your problems.

    2) Even if a system has been sized properly for your entire home the duct work (probably in the attic) may be improperly sized, or installed, or there may be other problems here. Again a professional can be of great help.

    Let me add something here after I finshed the initial post. I am sorry to say that many times in my experience the builder got the lowest price to get this installed from his sub-contractor, so lots of times corners are cut and you get what you pay for. Well, you may not have been paying the low ball price but you got the results. I hope this is not the case, but if so I would not be suprised.

    3) The way you use your home may be a factor. e.g. the typical home load allows 2 adults for each bedroom in the house. If, for instance, you rarely have fewer than 12-15 or more people in your home (say for a party) the typical system would be under a strain to keep you cool.

    4) Humidity has a huge factor to play. Obviously Houston has a much higher humidity factor than many other cities. The system needs to run for longer periods of time to lower humidity . Thus if a system is oversized for the space needed, it will not run long enough to lower the humidity and thus not seem to provide cooling. You would feel cool air blowing when the fan is on, but most of the time you would not be comfortable and that is what your goal is, comfort at a reasonable cost of operation. A simple trick here is to go to your thermostat and set the fan to on rather than automatic, this will override the fan control and it will run until switched off or back on auto. This will not increase you utility bill much and will help with comfort, but the trade offs include constant fan noise, etc.
    This, by no means, will solve all your problem, but many times its improves system efficiency slightly. Some of the newest systems, that have vairable speed controls, may not allow this to be done, but these systems are more expensive to install and probably don't apply here.

    I have not said much about insulation, and this can make signifcant changes on the heating/cooling load levels, but this should be factored in with the load calculation.

    I in no way have covered everything, but you can see there are many possible factors to consider and no one solution may solve your problem. Get a good company or two to come out and take time to let them explain some of this. If their staff can't or will not use some of these proceedures, continue to shop.
    A local community college instructor might be willing to take on some of this as a teaching tool for students. Maybe or maybe not. Make a few telephone calls where ever HVAC is taught and inquire.

    Hopefully I have not been too boring on this one. My general knowledge on this is better than my ability to fix it. I was in the business (sales) for a few years and I saw several folks with your kind of problem. Usually my company was able to provide a solution.

    Good luck

    [ 03-27-2003, 12:00 AM: Message edited by: thepapabear ]
    thepapabear<BR>When a bureaucrat has a hammer, everything begins to look like a nail.


    • #3
      Thanks PB,

      We run the fan in "on" for most of the summer unless the house is unoccupied. The fan noise is not bad unless you are right under the air return. The insulation of the house is, for contractor grade, fairly good. The house stays very warm in the winter without the furnace running very much even on the coldest (5 - 10 deg) days. Even on hot summer days, the temp stabilizes around 80 - 82 degrees with the condensing unit at a constant run. Since it is running constantly at 100%, the humidity is controlled fairly well. I have heard of homes that have too large of a unit installed getting mold and mildew due to the high humidity and that is why I asked the question. I did not know that I could get a survey/calculation done (cheap or free) with out actually contracting to have the job done. I will check into this with a few local contractors when I get home.

      Thanks for the help,


      • #4

        Sounds like your are on the way to getting this solved.

        The mold and mildew problem would be very likely with an oversized unit.

        I think the load calculation is still the best start, but just a guess here without looking at you specific site: You may need some duct work improvemnets. Here I am no expert, but let me give a little info (like I haven't run my mouth (keyboard) too much already.

        Part of the sizing of the system is airflow. Most AC units are designed to have 400 cfm airflow for each ton sized for the system. Thus your present 3.5 ton system shoud be putting out 1400 cfm total for the house. Part of the load calculation is to tell the intsaller/contrator how much cooling is needed in each room and thus how many cfm's are needed to overcome the heat in each room or space. A kitchen will typcially need more cooling than a bath and the large rooms will usually need more. Other obvious factors will be size and number of windows, sq feet of exposed (to outside) walls, insulation, so fort.

        Good luck gettting this solved.

        P.S. I always liked to oversize my return air grill(s). I learned this from a utility sponsored program that financed heat pumps for customers. They would not pay for an installation unless we had 1 sq in. of return grill for each 2 cfm. This is almost hearesy in the industry. Way larger than required by most manufacturers.
        You will not hurt the system by using this much return air grill and it will lessen the air noise.

        If you have a single return air grill in a central part of the house just measure the sq inches and see if you have somewhere near 700 sq inches. If not, don't make too much of a fuss, but let's say 250 sq inches would probably be inefficient. Again this is my personal view and not all good contractors will agree with me on this one.

        Obviously if you have more than 1 return grill this will require a little more measuring.

        Be sure there is nothing like furniture, drapes, etc restricting air flow near the returns.

        Do you have a high efficiency air filter like some of the electrostatic ones Paul Harvey promotes? I like these, but they can become clogged with pet hair etc. I am sort of taking stabs in the dark here. Trust a good contractor who is on site, better than my comments. Most of mine are just rules of thumb, in a way.

        [ 03-29-2003, 11:59 AM: Message edited by: thepapabear ]
        thepapabear<BR>When a bureaucrat has a hammer, everything begins to look like a nail.


        • #5
          Have you checked the system to see if it is properly charged? Find a technician who knows how to charge a system using the super heat method. Many techs simply charge to 70 PSI on the vapor side and leave. Super heat will charge the system to it's maximum efficiency point.

          Have you done anything to reduce the heat load in your attic?? An attic vent fan(s) can reduce the heat load in the attic and top floor rooms and in turn lower your need for more cooling capacity. Valted ceilings have a much smaller distance between the roof and the upper floor ceilings which inturn allows more heat energy through the ceilings. An addition layer of insulation in the attice if possible will also help.

          Since you say that your heat pump is heating efficiently in the winter I don't think your system is lacking in size. If it was you would also have a heating problem.

          These solutions will be a much less expensive than replacing your whole system. If you plan on upgrading to a 5 ton sytem you will be replacing both the inside and outside units.

          Good Luck