Why are boilers with tankless coil domestic water heaters considered inefficient?

Boilers with tankless coil domestic water heaters are considered inefficient primarily because the boiler needs to stay heated throughout the summer months, even when there is little or no demand for heating. This continual heating leads to energy waste, as the boiler operates to maintain temperature for domestic water supply while the heating system is not actively needed.

In addition, during warmer months, the demand for space heating typically decreases significantly or even disappears, yet the boiler remains operational, consuming fuel and energy unnecessarily. This creates a situation where energy is being used without a corresponding need, ultimately leading to higher operational costs and lower overall efficiency.

Other considerations, such as contamination risks in the system, susceptibility to scaling in the coil, or challenges in sizing the boiler, do exist but are not the primary reasons for the inefficiency of tankless coil systems in the context of seasonal operation.