Most heat pumps (90% or so) use Time-Temp Defrost. When the unit runs, if the outside coil is below about 35 deg, a timer starts. If the timer times out (typically 30, 60 or 90 minutes) a defrost will start. It will end when the coil thermostat reaches about 45 deg. If the unit cycles off before the timer is reached, everything resets and it will start again when it runs again. Likewise if the coil is above the 35 deg point, the timer never starts. So when it is very cold and the heat pump runs long periods, it will defrost every time the timer is reached.
Demand defrost has been a feature of high end heat pumps, usually only on the premium models. It is, unfortunately, not a modern vs old heat pump, but an upgrade feature. Demand defrost uses temperature measuring sensors on the coil and for outside temps. When the outside temp is below a set value (typically about 35 deg), the coil and outside air temp should be within 5 deg or so. As the coil ices, the coil will get colder than the air, since air can not flow through the coil to warm it up. When this coil-ambient difference hits a threshold value a defrost is initiated. It is ended when the difference is again below 5 deg or so. There is also a timer to end defrost if the temps are never reached. Normally a defrost will occur only when needed due to icing.
If you read my post #24 on this thread you will see that a replacement/retrofit defrost controller is now available for most heat pumps. You can have one installed by a service tech, but consider the cost if you do not see an excessive number of defrosts in your local climate area.
paul
