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  1. #1
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    Should you keep the fan on low all the time in the summer?

    While there seems to be a consensus for keeping the fan on low all the time in the winter, what about during the summer months?

  2. #2
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    Smile

    By keeping the fan on all the time you will increase your air quality. As long as the furnace/blower is running you will be constantly filtering your air. If quality of air is important then I would say to leave it on all the time. You did say low, however most furnaces will run on high speed in the fan on position.

  3. #3
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    Quote Originally Posted by jeff32470 View Post
    By keeping the fan on all the time you will increase your air quality.
    You will alo increase your humidity.

    Better to use a thermostat with a circ feature, then just running the fan 24/7 in the summer.
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  4. #4
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    Keeping the fan running does help maintain an overall more consistent temp in the home. But, during the summer it will re-evaporate the condensate on the coil and the drip tray and cause the humidity within the home to spike back up right after the compressor shuts off. Having the blower shut off immediately will keep that from happening.

    Depending on the t-stat you have, some can cycle the fan on periodically to help with keeping a more consistent temp. If you have that feature, that would be the best compromise.

  5. #5
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    The typical PSC motor uses 300+ watts when running even on low. (You can add on another 100-150 since the heat produced has to be removed)
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  6. #6
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    Quote Originally Posted by amd View Post
    The typical PSC motor uses 300+ watts when running even on low. (You can add on another 100-150 since the heat produced has to be removed)
    No.

    The motor can't produce more heat then it draws amps.

    If it uses 300 watts, its still only 1024BTUs.
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  7. #7
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    Quote Originally Posted by beenthere View Post
    No.

    The motor can't produce more heat then it draws amps.

    If it uses 300 watts, its still only 1024BTUs.
    Not quite correct. As far as power output of the motor and the power shown on a wattmeter that is correct, but a lightly loaded motor may have a power factor of easily as low as .50. Which means that the total load is really 600 watts of heat. The formula for power is Volts X Amps X power factor = watts. The power company charges you for watts, but the heat load is Volts X amps like for a DC circuit. This is the amp load for the system.
    The difference in VA and watts in AC is the heat losses and magnetizing losses.
    This is common to AC motors and makes calculations more difficult. This is one of the big reasons that the ECM GE variable speed motors are more efficient. They are Electronically Commutated DC Motors. The reactive effects are not involved and the power converted into mechanical power is like DC (Volts X Amps). The power factor is essentially 1 instead of some fraction.
    Induction AC motors have power factors from the high 80% range at full load to as low as 30% a no load. The power factor is very nonlinear, especially at the low end. Multi speed induction motors are less efficient than single speed motors.
    All that said the heat gain from the motor is relatively small, but is higher than the Watts that would be measured by a Wattmeter by the power factor of the motor at any particular loading.

  8. #8
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    A motor that shows 300 watts at 12 vols using amp X volts would have a amp draw of 2.5

    Using yor formula with power factor.
    volts X amps X power factor.

    120 X 2.5 X .5 = 150 watts.

    Are you trying to use motor efficiency.
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  9. #9
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    Quote Originally Posted by beenthere View Post
    No.

    The motor can't produce more heat then it draws amps.

    If it uses 300 watts, its still only 1024BTUs.
    I was referring to the energy an a/c would have to use to remove the extra heat produced by running a blower continuously.

    A unit would a COP of 3 would consume an additional 100 watts on average to remove 300 watts worth of heat; of course, this would be reflected by (slightly) longer condenser runtime, not higher instantaneous energy use.

    Total extra energy used = ((blower runtime in ON mode - runtime in auto mode)*(Blower wattage on low speed))/COP
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  10. #10
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    COP is a heat pump heat mode only rating, not an A/C rating.

    BTU for a blower is 1250BTU(an hour) per 1000CFM.
    Last edited by beenthere; 04-23-2009 at 06:15 AM.
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  11. #11
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    Quote Originally Posted by ron3637 View Post
    While there seems to be a consensus for keeping the fan on low all the time in the winter, what about during the summer months?
    I do not run my air handler fan all the time in winter. Why would I want cold air blowing from the vents in the winter when the heat pump condenser is not running?

    I do run the air handler fan in the summer because it helps to balance the temps between the 2 floors, it helps to drown out the street noise especially at night, and (most importantly) it keeps the Queen away from the thermostat because she can't hear the compressor cycling off

    Take care.

    Edit: I do not run the fan 24/7 in the summer but I do indeed run it a lot without the a/c on.
    Last edited by gary_g; 04-23-2009 at 12:39 PM. Reason: added edit

  12. #12
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    The heat from a fan and motor is tied into the static pressure.

    Constant fan in cooling mode elevates humidity. If your duct wok is all in the conditioned space it will probably raise your RH by 5 to 10 points due to the fact that moisture gets re-evaporated into the air.

    If your ductwork is in the attic and leaks a bit, constant fan could elevate your RH by 20points. Same is true if you have a fresh air intake connected to your return air ducts.

    So if you find that your RH is excessive like always 60% plus, switch back to auto fan and enjoy the comfort of lower RH.

    Maybe you live somehwere that is not overly humid and it will not matter.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  13. #13
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    Quote Originally Posted by gary_g View Post
    I do not run my air handler fan all the time in winter. Why would I want cold air blowing from the vents in the winter when the heat pump condenser is not running?

    I do run the air handler fan in the summer because it helps to balance the temps between the 2 floors, it helps to drown out the street noise especially at night, and (most importantly) it keeps the Queen away from the thermostat because she can't hear the compressor cycling off

    Take care.
    I can only assume you don't have a low speed fan??? On a low fan, air is kept circulated in the winter to help keep the temperatures balanced and circulate air. On my system, the fan is virtually silent at the approx. 500CFM it's running at. It's also too low to create the feeling ofdrafts, especailly with the house at about 30-45% RH (depending on outside temp). I don't hear the fan until it ramps up to at least about 900 CFM on 2nd stage of the furnace or 1050CFM on 2nd stage of the heat pump or A/C.

    I also allow the humidifier to run with low speed fan as well as some positive outside ventilation for fresh air and to reduce infiltration.

    Register placment, velocity and other factors affect a "drafty" feel with a constant fan. If you can hear the fan or air flowing out your registers, I can only assume the velocity is fairly high so I agree, it would likely have a cold feel to it.

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