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  1. #1
    Join Date
    Nov 2011
    Posts
    22

    Should indoor fan shut down when Heat Pump on defrost?

    Hi everyone,
    I realize this question has come up before,but I am seeing conflicting answers.
    My Heat Pump is a Carrier 3 1/2 ton air to air split. It has been operating 10 years.
    I get cold air for about ten minutes during defrost because the indoor fan is running.
    I was told by a VIP poster on the Refrigeration Engineer.com forum that the indoor fan should not be running during defrost and that he has never run his.
    A poster on this forum says a definite yes, it shuould be running.
    Because of these conflicting answers I was nervous to try a defrost with the fan stopped,so I did a defrost with a normal fan speed and a cfm of 960 and with a reduced fan speed and a CFM of 530.
    The results are attached.
    So--- Should indoor fan shut down when Heat Pump on defrost????

    Regards

    pilko
    Attached Images Attached Images   

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,104
    It should be running. Most heat pumps have aux heaters that come on to temper the air so that you don't get the cool/cold air blowing out while in defrost.
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  3. #3
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,574
    The defrosting effect comes from the heat being gathered into the indoor coil as the system runs in defrost.

    So, the fan must run in order to gather sufficient heat from the indoors, to melt the frost or ice outdoors.
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  4. #4
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    fan needs to be on durning defrost to prevant the evap coil from icing up
    looks like you may need to have the unit serviced your aux heat isnt coming on during defrost to temper the air

  5. #5
    Join Date
    Sep 2009
    Location
    S.C.
    Posts
    1,401
    The VIP on the Refer Engineering is Wrong. Excellent answers above from our "Pro's"!

    And as a "Guest" you are not the Average Guest!
    Yes, I know I Shouldn't But I Just Can't Help Myself...

  6. #6
    Join Date
    Nov 2011
    Posts
    22
    Thank you beenthere, timebuilder, linknocker service tech and Home&Marine.

    @ Home&Marine --- what do you mean "And as a "Guest" you are not the Average Guest!"

    regards

    pilko

  7. #7
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,665
    I'm thinking someone is confused the indoor fan must run or it won't defrost ,the outdoor fan should not run for a quicker more complete defrost .it sounds like the heat strips need some diagnoses as to why they are not tempering the air

  8. #8
    Join Date
    Nov 2011
    Posts
    22
    Hi catmanacman,
    I use a 15KW plenum heater to temper the air, but with a COP of 1.0 I prefer not to use it. That is why I posted my original question.
    Attached is a snapshot of this mornings performance.

    Green = Discharge air temp
    Dark blue = Return air temp
    Green = Outdoor temp
    Light blue = Frost level on outdoor coil
    Attached Images Attached Images  

  9. #9
    Join Date
    Nov 2011
    Posts
    22
    @ Homes & Marine,
    "The VIP on the Refer Engineering is Wrong. Excellent answers above from our "Pro's"!"

    Here is his response:-

    "Right time to get technical (copy onto the other site)

    Rough examples

    In normal heating mode ; (R22 I guess)
    1;ambient = -2C
    2: return air = 20C
    3;Estimated SST -8C
    4;Estimated SCT 45C
    Basic COP = 2.85 (heat)

    When defrost with fan on!
    SST 0C
    SCT 27C (min limit for comp, but would expect lower in real world)
    COP 5.51 (heat) Remember that the energy for defrost comes from inside your house, and has to replaced.The cooling COP is much higher

    So what does this mean, you require twice as much energy to replace the heat you loose when on defrost when the fan is running.

    Now with out fan running (no cold draft and minimum energy drawn from the house)
    SST -20C
    SCT 27C
    COP 3.39 (heat)

    So as you can see, the lower COP in this defrost cycle means less energy is removed form your house.

    Also remember that on this nominal 10Kw the comp we still draw 2.3Kw, which is defrost the evap coil (which becomes a condenser during defrost) What does this mean in defrost terms, the comp power draw alone will defrost 54.5lb of water an hour. 1/2 the weight for 1/2hour and so on. You are still getting some energy from the mass of the indoor coil and surroundings.

    Defrost will be slower, as the total amount of heat rejected to the defrosting coil will be less when the fan is not running.

    Well I hope that makes sense"

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,104
    If a coil has 10 pounds of moisture to defrost. it takes the same amount of energy/heat to thaw/melt that 10 pounds weather or not the indoor fan is running.

    However. Defrost boards have timers that only allow them to be in defrost mode so long(14 minutes max time by IECC), after that they switch back to heating mode weather or not the coil is defrosted. So without the fan running,, many coils won't be fully defrosted, and will provide less heat when they come back out of defrost, then if the fan had been running.
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  11. #11
    Join Date
    Nov 2011
    Posts
    22
    I agree that it will take extra time if the indoor fan is stopped during defrost(I could overcome that problem by increasing the timer span).
    My big question is can it cause any harm to defrost with the indoor fan stopped?

    regards
    pilko

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,104
    Quote Originally Posted by pilko View Post
    I agree that it will take extra time if the indoor fan is stopped during defrost(I could overcome that problem by increasing the timer span).
    My big question is can it cause any harm to defrost with the indoor fan stopped?

    regards
    pilko
    Unless you install a different timer on the defrost board, there is no way tom increase the defrost time.

    It could harm the compressor, since the head pressure during defrost would be low, it may not return as much oil from the outside coil as it should, leaving your compressor low on oil.
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  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,104
    barbar.

    This is the Ask Our Pro's forum. In order to post advice here, you must have verified qualifications and have been approved by the AOP Committee.

    You can find the rules for posting and qualifications here.
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