For All Climate Heat Pumps and Acadia CH&C Systems using the older
control it's essentially the same defrost mechanisms as most other
premium units on the market. There's 3 methods they enter defrost
(active, passive, and forcibly), and 2 methods they exit (active and
passive):
For entering:
The active system monitors outdoor temperature and the suction
temperature on one of the distributors of the coil on the outdoor
unit. There's a control algorithm that looks for a temperature
spread between the two at temperatures where condensate can freeze on
the coil. When the algorithm is satisfied (i.e. it is determined that
the likelihood of formed ice is very high) the unit will defrost.
The passive system uses timers built in to the control that will force
defrosts in certain operating modes after operation in that mode has
accumulated a set number of minutes below certain temperatures should
the algorithm not be keeping up.
The forced method (on the original - non-invensys controls) there is a
manual defrost jumper that will force a defrost every 45 minutes.
The defrost termination is determined by the temperature at the
distributor coil reaching a specified value for all defrosts for
active, or a maximum number of minutes being reached for passive.
Upon completion of a defrost the unit will go through the appropriate
sequence to continue operation in its last known operating mode,
assuming the thermostat is still not satisfied.
If the unit does not complete a successful defrost automatically
within the defrost time limit, it will attempt again in 45 minutes.
On the whole the defrost strategy is focused on keeping the coil clear
enough for safe, effective operation. It works independently of
thermostat calls and operating modes.
Hope this helps!