said by pandora:
Second, the coils heat and defrost the unit. In defrost mode the coils become very hot fairly quickly. I don't understand why the fan turns off in defrost mode, but it does.
The coils become hot the same way your indoor coils heat the house. When in defrost mode though, you want the coils to heat up to melt the frost/ice, instead of heating up the outside air in general. The outdoor fan turns off so that the coils heat faster and the heat isn't wasted warming the surrounding air.
The indoor fan may
turn off initially in defrost mode in an attempt to defrost the coils without having to use previously conditioned air, emergency heat strips, and/or other fuel as a heat source. If there isn't a call for heat, but a call for defrost, it may run until the indoor coil reaches a temperature cutoff and the blower turns on. This is when some report that they feel cold air from their vents, as the air isn't heated as hot as when in heating mode. If there isn't an auxiliary source of heat like heat strips, there may be no choice but to blow cold air during the defrost period.
I assume if a unit is frozen (in need of defrosting) the frost will cool the vapor without any need to visit the air handler.
Yes the frost would cool the vapor, if the unit was in cooling mode. However in heating mode, refrigerant returning from the air handler is in a cooled, condensed form. It needs to be EXPANDED back into a relatively warm gas absorbing more heat. The outdoor coil acts as a heat sponge of sorts. If the coils are frosty or iced up, it act like an insulator preventing air from circulating around and the coils absorbing heat from the air.
I'll hazard a pure guess here, the defrost sensor may determine if the refrigerant is being cooled when in defrost mode (indicating a need for defrosting). I have seen the units enter defrost mode and exit within seconds. Why run refrigerant to "cool" the air handler, when frozen coils are sufficient?
You're last statement is backwards. It's the outside coils that are "cooled" when in heating mode.
If the unit enters and exits defrost mode in a few seconds, something is wrong. You're units for instance has a "SmartShift" feature where the compressor shuts off for a short period before and after mode changes, as well as a short cycle protection which is hard on the compressor shortening it's life.