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  1. #27
    "I bought this house in June, 2.5 stories, 3500 sqft, built around 1998"

    Your rich,,, why do you care? Call anyone local and get a price. simple.

  2. #28
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Quote Originally Posted by Leo3006 View Post
    "I bought this house in June, 2.5 stories, 3500 sqft, built around 1998"

    Your rich,,, why do you care? Call anyone local and get a price. simple.
    Useless post.

  3. #29
    Join Date
    Nov 2007
    Location
    Tucson AZ
    Posts
    166

    Variable Speed

    This is my opinion as a homeowner, so it may cause upset to some of the professionals here. Sorry guys, feel welcome to comment on my opinion.

    The blower motors on higher end air handlers (the indoor part of the AC system) have the ability to adjust their speed and are rightly called variable speed. They typically demonstrate this feature by slowly ramping up to speed at the start of a cycle and ramping down at the end of a cycle. This adds to comfort by making the operation of the system less obvious - it seems quieter when it just creeps up on you instead of starting with a rush.

    Comfort is also obtained by the ability to run at reduced speeds to just circulate air since this will tend to eliminate hot or cold spots.

    However, the control of the fan speed is never given directly to the user because the majority of systems rely heavily on using all the cooling capacity being generated by the outdoor condensing unit, which has at best only two capacities it can operate with. Some two stage units do it by having two separate compressors, some by running a single compressor in a reduced power mode.

    The consequence of this is that the indoor unit has to handle the cooling capacity being supplied which means that the volume of air being passed has to meet a minimum. Here is where the variable speed motor shines because it is easy to set it up to move a given weight of air per second regardless of how many restrictions there are to the smooth flow of the air.

    Poor (undersized) ductwork, dirty filters, closed dampers can all impede airflow, and a variable speed blower motor will automatically increase its speed to overcome the restriction and so meet the need of the outdoor system to have the heat removed from indoors match the heat being dumped outdoors by the condensing unit.

    If the airflow is insufficient then bad things can happen like indoor units icing up or liquid refrigerant being returned to the compressor causing it to slug and eventually self destruct.

    Until recently the AC industry did a good job of making their products work over an increasingly wide range of operating parameters. Thermostatic Expansion Valves feeding evaporator coils, variable speed blower motors, multi stage compressors all allowed the systems to respond in a limited way to changes in ambient conditions but the user in the end was limited to either getting cooling when the thermostat called for it or no cooling when the thermostat found that the setpoint was reached. There was no way to tell the compressor to run in smaller steps than OFF/PARTIAL/FULL capacity, and the best the evaporator coil could do was to monitor pressure and superheat with a mechanical TXV so as to have some limited control over refrigerant flow.

    Recently the rest of the world looked at the problem and applied modern technology to allow the user to control the speed of their fan! A modern minisplit system can control the speed of its outdoor compressor precisely (usually by powering it with a variable frequency power source), and this allows the outdoor unit to match the cooling load presented by the indoor unit. The user has control of both the fan speed and the temperature setpoint of the indoor unit, so the cooling load is not dependent on only the indoor temperature. The user might ask for a small temperature drop at a high fan speed which would make for a high cooling load, or a huge temperature drop at a very low fan speed which would be only a small cooling load.

    I know my top of the line Trane XL19 system does not give me the option of making those choices. I'm going to get either 1.5 or 3.0 tons of cooling at 600 or 1200 cubic feet per minute, and which one I get will be decided by the system - my only input is to tell the system what temperature I want to get to. The system controls the blower motor (600 or 1200 cfm), and the blower motor is smart enough to work a little harder as my filter accumulates dust, but I get no control over how much cold air I want supplied.

    The floorplan of my (mid 1950's) house is such that the central AC system creates a zone covering most of the living space and is not effective for the kitchen or my office. The best way to cover those areas was to instal a minisplit, and this is why I now see the value in being able to control both temperature and airflow and have come to a belief that my expensive central system is positively agricultural in its simplicity compared to the microprocessor controlled minisplit.

    In my opinion to industry need to take a close look at their products - I see a distinct parallel with the way that Detroit marketed their V8s and ridiculed the funny little Honda boxes that started to appear once upon a time.

  4. #30
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,818
    If your contractor would have used the correct stat, and wired it in properly, then your 19i would run at 4 fan speeds.

    A VS blower does NOT over come all bad duct designs.
    Contractor locator map

    How-to-apply-for-Professional

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

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