In what situation would a heat pump switch to emergency heat mode?

When a heat pump switches to emergency heat mode, it typically indicates that the system is unable to maintain the desired indoor temperature by relying on its standard heating operation. This situation can arise due to various issues, such as a drop in outdoor temperatures that makes it difficult for the heat pump to extract enough heat from the outside air, a malfunction within the system, or significant heat loss from the structure itself. By engaging emergency heat mode, the system activates auxiliary heating elements—often electric resistance heaters—to provide warmth and maintain comfort levels indoors.

In contrast, the other scenarios do not justify the need for emergency heat mode. Extended cooling periods would not necessitate a switch to heating, and high outdoor temperatures do not strain the heat pump’s heating capacity. Furthermore, routine maintenance checks should not disrupt the system's operation to the extent that it requires emergency heating mode.