On cold days, this air handler uses an electric coil to preheat the outside air. Gas-fired hot water can be used to provide the preheat much less expensively.
The electric heater would be removed, and a new hydronic heating coil installed in its place. Design work is required to ensure that air handler capacity (both heating and air flow) are not significantly reduced. For safety, hot water cannot be used directly in this application, as there would be a significant risk of coil freezing. A separate glycol loop must be installed. A coil circulator would pump glycol through the coil when heat is needed. A brazed plate heat exchanger would be installed to transfer heat from the hot water heating plant to the new glycol loop.