Final answer:
The two means of engaging backup heat using a thermostat are by setting it to emergency heat mode and adjusting the temperature setpoint higher to activate auxiliary heat if needed. Option A is correct.
Step-by-step explanation:
The question asks for two means of engaging backup heat using the thermostat (T-stat). The correct answer is a) Setting the thermostat to emergency heat mode, adjusting the temperature setpoint.
In emergency heat mode, the heat pump will shut off and the system will rely solely on auxiliary heat (usually electric resistance heaters) to keep the home warm, which can also be considered as backup heat. Adjusting the temperature setpoint higher will also engage the auxiliary heat if needed.
The hold feature on a thermostat can be used to maintain a certain temperature for an extended period of time, which might engage the backup heating if the standard heating cannot maintain the set temperature. However, activating the vacation mode typically lowers the temperature setpoint to save energy while you are away, rather than engaging backup heat.
On a side note, turning down the thermostat when away and not in need of immediate comfort, is an energy-efficient practice. A programmable thermostat can be set to return the home to a comfortable temperature by the time you get back. This can help save money on heating bills, as maintaining a high temperature when it's not needed is unnecessary.