The thermostat maintains the engine block at the desired temperature, cycling the heat pump on and off.
The existing block heaters only operate if the heat pump fails, and the temperature drifts down to the set point for the block heaters. At this point an alarm is activated on the heat pump, and an agent will repair or replace the heat pump within 48-hours. In the meantime, the engine is kept hot as it always has been kept, through the use of the existing block heaters.
Heat pumps also last a long time. A 2005 study reported in Air Conditioning/Heating/Refrigeration News, called Increasing Consumer Confidence in Heat Pumps, found that the observed median service life of a heat pump-when removed because of failure-was 26 years.