Why is it necessary to add a control for a heat pump indoor coil installed on a fossil fuel furnace?

The necessity of adding a control for a heat pump indoor coil installed on a fossil fuel furnace primarily revolves around the prevention of simultaneous operation of both heating systems. When a heat pump is paired with a fossil fuel furnace, it is crucial to ensure that only one heating source operates at a time to avoid conflicts between the two systems.

If both the heat pump and the furnace were to run together, it could lead to several issues. Firstly, energy efficiency would drastically decrease because the systems would essentially compete against each other, resulting in unnecessary energy consumption. Secondly, it could cause mechanical stress or damage in the systems due to potential overheating or strain caused by operating simultaneously under conditions for which they were not designed.

Control systems are therefore used to manage the transition between the heat pump and the furnace based on outdoor temperatures or heating needs, ensuring that the more efficient system (typically the heat pump in moderate temperatures) operates, and only engaging the furnace when outdoor conditions necessitate its use (usually in colder environments). This strategic approach not only maximizes efficiency but also enhances the lifespan of the equipment.