Thermostat for Variable-Speed Blower - Page 3
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  1. #27
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,509
    Since there seems to be some confusion over what you have exactly. I would call a service company out to take a look at what you have. It sounds as if you have a fixed speed that is adjusted by moving motor speed taps to where you want the speed to be. If so, purchasing a thermostat to dehumidify will be a complete waste of money, it won't do anything except cost more money. If you have a true variable speed, which runs at a range of speeds and has a control module piggy backed on the actual motor, then have your service company check into what you need to make it work properly. I will say at 14 years old, I would also make sure it has the capability to run your new system. You could develop a coil leak or failed motor which fixing would be fairly expensive and a waste of money.

  2. #28
    Join Date
    Aug 2006
    Location
    Dallas (Plano), TX
    Posts
    168
    Actually, if it turns out to be a PSC blower, then this would make it straight forward for a technician to install a relay to switch between a normal speed and a slower speed.

    I remember this was how my Lennox 2-speed unit, circa-1992, was installed. The installer added a relay to switch the PSC blower from one speed for 1st stage cooling to a faster speed for 2nd stage cooling.

    I think it's the variable-speed blowers that don't already have a set of external contacts, or don't support a field-installable module, for slower speed that are the hardest to adapt to the IAQ's dehumid 'port' - in fact, maybe just not possible.

    Best regards,

    Bill

  3. #29
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,630
    On most VS blowers. Fan on speed is fixed at 50% of the selected high cool speed.
    Contractor locator map

    How-to-apply-for-Professional

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

  4. #30
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    Quote Originally Posted by wtw View Post

    Are most modern "variable-speed" blower motors continuously variable, and if so, how is this accomplished?
    Yes and no. Through jumpers and connections to the control board, a certain CFM is requested. The control board varies the speed of the blower motor attempting to maintain the requested CFM. If the duct work isn't too far off or the filter too clogged, it will supply the requested CFM.

    Common variable speed blower controls have at least 12 fixed CFM settings available.

    So, the blower motor is continuosly variable, but there are discrete CFM steps that can be set through the controller.

  5. #31
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    For more information - embedded in a sales pitch

    http://hcsupplyco.com/Product%20Pres...pt#377,1,Slide 1

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