Heat pump defrost cycle and startup/shutdown sequence

[Acuario]

Hi,

can someone explain exactly what happens during a heat pump defrost cycle and in a heat pump start up/shut down cycle when used in heating mode.

Obviously the reversing valve operates and hot refrigerant is cycled in reverse to defrost the heat exchanger but is the compressor stopped first and a time allowed for the pressures to equalise or does it just reverse?

Also, it seems the reversing valve (in the unit I have) is set in the cooling mode when de-energised. When energised it changes to the heating mode. Should there be a delay in the valve changing and the compressor starting (in startup) and a delay between the compressor stopping and the valve de-energising in shut down?

Thanks,

Nigel

[haiko17]

Hello Nigel.
Depending on the manufacturer there are small differences about the way heat pumps operate.
For example, a Trane heat pump which works with demand defrost; upon initiation of a defrost cycle will energize the reversing valve switching it to cooling mode and simultaneously de-energize the compressor contactor cutting out the fan-motor and compressor. Then, after a 30 second delay it will energize the compressor until the defrost cycle ends. Trane uses this method because most of their heat pumps have no accumulator (Climatuff comps have a built-in one) and the reversing valve is capable of switching easily.
In Canada every manufacturer's reversing valves are energized in cooling mode so in their natural, unenergized state they are in the heating mode position save for RUUD units. Their reversing valves are energized in heating mode.
Hope that clarifies things.

Haig