Heat pumps are really two system in one. The actual heat pump portion works much like a central air conditioning compressor, using refrigerant to cool air. The key difference is that a heat pump has the ability to reverse the refrigeration cycle allowing it to heat air in the winter.
However, a heat pump by itself will not perform well in extreme temperatures. That's why all heat pumps have a backup system to take over when the amount of heat that is called for exceeds the heat pump capacity. That backup is THE most expensive for of heat available, electric resistant heat.
The key to saving money with a heat pump is the set the thermostat at (1) either a steady number and leave it alone; or (2) to use a clock setback thermostat which is design specifically for a heat pump. These thermostats move the temperature up and down very slowly so as to not trigger the electric heat which usually comes on when the difference between the set temperature and the actual temperature in the room is more than 3 degrees.
I checked with the experts at Trane about your Series 15 and 16 units and they recommend the Trane XL803 Digital Thermostat. In reviewing the installation instructions, it seems important that this be installed by a qualified installer as the setting this thermostat up for heat pump operation requires specific steps.
In addition, Trane offered the following additional programmable thermostat operation tips:
Turn the heat up (or down): About 47 percent of the average household's annual energy bills stem from heating and cooling, according to the U.S. Department of Energy. Adjusting your thermostat is free, easy and can yield big savings.
You will save about three percent of your heating costs for every degree you lower your thermostat during winter months
During summer, you'll cut cooling costs three to four percent for every degree you raise your thermostat
Install a programmable thermostat: Programmable thermostats are easy to install and will shave about 10 percent off of your heating and cooling bills, says the Alliance to Save Energy. The average annual savings is about $180. Programmable thermostats often pay for themselves in two years or less.
Programmable thermostats are designed to maximize the performance of your home comfort systems with reliable and accurate temperature control
Because you can program them to raise or lower the temperature when you're not home, they also save energy while still maintaining your standard of comfort