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Modify HVAC air handler for variable speed fan

Post in 'The Green Room' started by semipro, Feb 27, 2012.

  1. semipro

    semipro Minister of Fire

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    We have a 25 year old Florida Heat Pump ground source system that we use for winter heat and some summer AC.

    I've wondered if I could convert the fan to variable speed operation for energy savings and comfort. I know that some air-source HP systems use variable speed fans for this.
    Would it be worth considering converting my unit?

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  2. basod

    basod Minister of Fire

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    I'd guess I'd question why upgrade to VFD fan and what economic gain you'll get from it.

    Have you ever checked the consumption rate of the fan? I'd doubt its much over 500 watts, and how much does it run?

    Most of what I've seen in installation analysis is a fan that is oversized for the application, basically for the HVAC system flow to prevent freezing the coil.

    If you have ground source system you don't need the high flow rate but maybe a smaller fan motor.
  3. Corey

    Corey Minister of Fire

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    Assuming this system is old enough to only have orifice tubes, dollar for dollar, your best savings is likely going to be to install a thermostatic expansion valve (or two - since you're heat-pumping)
  4. semipro

    semipro Minister of Fire

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    I have not checked power consumption at the fan but have a clamp on amp meter so that's a good idea. Your comments made we wonder if it would be best if the fan ran at one speed for cooling to prevent evaporator freezeup and another speed while heating to minimize the evaporative cooling effects (comfort) in the winter. Most of the air handler motors I've come across are multi-speed based upon which windings are energized.
  5. semipro

    semipro Minister of Fire

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    How will this improve things? I'm fairly familiar with refrigeration systems but wasn't aware of the any advantages of an expansion valve over an orifice tube. I can open the cabinet to see whether its orifice or expansion valve. I thought orifice tubes were a more recent trend and that they replace expansion valves?
  6. begreen

    begreen Mooderator Staff Member

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    Isn't there usually there is a 2 stage compressor that works in conjunction with the variable speed fan? Variable speed fan systems need to be integrated with the systems to get best efficiency. I can't see this being much of an energy saver by itself. What would control the fan speed? FWIW, I think our air-handler's motors are DC.
  7. semipro

    semipro Minister of Fire

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    I remember reading about an afftermarket fan controller that varied fan speed according to temp differential across the inside coil (condenser in heating mode) or something like that. The idea was to minimize the the evaporative cooling effect associated with heat pumps that makes occupants feel like cold air is coming out of the ducts even thought the house is getting warmer. I guess the thinking is that if you move air more slowly it has time to pull more heat off the coil and that lower velocity air doesn't create as much evaporative cooling on skin.

    I don't recall that these were necessarily paired with dual speed compressors but I've no doubt that's how manufacturers do it.
  8. buddylee

    buddylee Member

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    Seems like most units have high speed for the ac and lower speed for the heat. The new 410a units have expansion valves.
  9. semipro

    semipro Minister of Fire

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    I'm thinking now that I'll check my fan motor and see if its a multi-speed unit. If it is I may try to rewire it to run at a lower speed during heating.

    Thanks to all for your feedback.

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