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Thread: defrosting

  1. #1
    what are the sequence of events during the defrost cycle on a heat pump? ive got this question on a test tomorrow,ive read the book five times but was looking for a more better answer. thanks

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,338
    What do you understand about the sequence of events so far?

    And are you talking about a heat pump with time/temp defrost, pressure differential defrost, or demand defrost?

    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    i dont understand what tells the system it is defrosted

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    When its defrosted, the coil becomes warm. Changing the state of the frost is the hard part and the rest of the coil should not become warm as long as there is still frost. The defrost sensor (temperature sensor connected to the coil)determines the coil has reached a specific temperature. It will end on time if the defrost sensor has not sensed a complet defrost in a specific time frame.

  5. #5
    Defrost cycle usually terminates and is initiated by means of a defrost sensor that is nothing more than a switch that opens and closes as needed.

  6. #6
    Join Date
    Sep 2004
    Location
    Glendale AZ
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    299
    Time AND tempture acuated---- Time OR tempture terminated. You need the time( 30-60-90 min depending on the setting )and a outdoor coil temp. of 27 degrees to close defrost sensor to start the defrost cycle. After 10 min. OR 70 degrees( maybe 65)outdoor coil temp. to terminate defrost.


    This is how most residential units work.

    [Edited by danj on 04-14-2005 at 09:25 PM]
    The bitterness of poor quality remains long after the sweetness of low price has gone.

  7. #7
    Join Date
    Apr 2004
    Posts
    7,680
    I suspect most systems produced nowadays use demand defrost, not time/temperature. The sensor used in a demamd defrost is an actual sensor, not a snap acting switch. A demand defrost board compaires the outdoor temperature to the coil temperature to determine when to go into defrost. It used the set temperature to halt the defrost. The defrost board also has a timing function to prevent the defrost from occruing too often. It is also generally timed in the event the defrost does not complete in a specific time period.

  8. #8
    Join Date
    Sep 2004
    Location
    Glendale AZ
    Posts
    299
    Well, I think most all package equipment(carrier 50JX 50JZ and 50XZ)--still use time and tempt. as do the carrier split pumps 38yra's 38bco's 38yxa's(puron). The heil--tempstar-- tappans--frigidare-- rheem---ruud-even goettle (package equipment) still use time and tempt. for defrost. ( In AZ we use a lot of package equipment)

    I'm sure there's a lot more that still use time and temp. so it's still good to understand how it works.

    reverunbilly-- You might ask your instructor what type of defrost cycle he wants. (Doc explained the demand defrost -- and you know the time and temp.)

    Also it's 80 degree coil temp-- not 70 to end cycle!!

    [Edited by danj on 04-14-2005 at 10:33 PM]
    The bitterness of poor quality remains long after the sweetness of low price has gone.

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