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Air to Water Heat Pump Defrosting

  • 19-08-2018 4:23pm
    #1
    Registered Users Posts: 3,017 ✭✭✭ John.G


    Re the above: I understand that in Reverse Cycle Defrosting (RCD) that the outside unit (Evaporator) fan is stopped, the reversing valve changes over, the hot gas from the compressor discharge is directed to the evaporator (to melt the accumulated ice) and the liquid refrigerant is then expanded through another? expansion valve into the condenser? and returns to the compressor suction.
    My question is: Where does the heat come from that is required to evaporate that liquid refrigerant?, is it from the residual heat in the heat exchanger and is the circulating pump left running to the UFH (or rads) and is a auxiliary heating device like a electric heating element switched in while the defrost cycle is on.?. Thanks.


Comments

  • Registered Users Posts: 237 ✭✭ ShanE90


    The heat source for defrost comes from the heating circuit as there wouldn’t be enough heat in the indoor heat exchanger alone to perform a successful defrost of the outdoor coil. The circulating pump is left running to prevent the heat exchanger from freezing and to supply the needed heat for defrost. There is a minimum water volume that is Always required be in circuit to be available to “defrost into”. Systems with low water volume may require a volume tank for defrost and short cycling purposes. The immersion heater isn’t normally used for defrost it’s purpose it to provide backup heat for DHW and to bring the DHW tank past 60degC for sanitization.


  • Registered Users Posts: 3,017 ✭✭✭ John.G


    ShanE90 wrote: »
    The heat source for defrost comes from the heating circuit as there wouldn’t be enough heat in the indoor heat exchanger alone to perform a successful defrost of the outdoor coil. The circulating pump is left running to prevent the heat exchanger from freezing and to supply the needed heat for defrost. There is a minimum water volume that is Always required be in circuit to be available to “defrost into”. Systems with low water volume may require a volume tank for defrost and short cycling purposes. The immersion heater isn’t normally used for defrost it’s purpose it to provide backup heat for DHW and to bring the DHW tank past 60degC for sanitization.

    Thanks, what kind of temperature drop might you expect to see in a circulating system at the end of a defrost cycle from say heating mode conditions of 35C/30C flow/return.


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