What is typically the consequence of running a heat pump and a fossil fuel furnace at the same time?

The consequence of running a heat pump and a fossil fuel furnace simultaneously is typically increased wear on both units. When both systems operate together, the heat pump can be subjected to stress from the additional load created by the furnace, potentially leading to operational inefficiencies and increased mechanical wear. Each system is designed to operate independently and efficiently based on specific loads; when they run together, this can cause a mismatch in operation cycles. Over time, the simultaneous operation may cause components to degrade more quickly, leading to increased maintenance needs or even system failures.

Running both units may also lead to issues with humidity control and temperature fluctuations within the space being conditioned, as the systems try to regulate the indoor climate simultaneously, with different parameters and goals. Hence, while some may think combining the two might enhance heating capabilities, in reality, it generally results in increased wear and potential operational issues for both systems.