This is a variation on the First Co. Aquatherm systems, often used in apartments. With the Aquatherm systems, the natural gas DHW tank does double-duty, both space and DHW heating.
The fuel efficiency is associated with a different device from the air handler; and, to model it properly, you need to capture the heating efficiency, the air handler consumption, and the pumping consumption. Create a boiler in the Space Heating library. Use the DHW recovery efficiency for the boiler AFUE, and similarly for the capacity and fuel type; then for the fan energy uncheck the Auxiliary Electric Use default and enter the real air handler Watts (the Default will be incorrect because it is based on the capacity, assuming a gas furnace, while you have a storage DHW serving multiple systems). Finally, capture the pump power associated with circulating the hot water through the coils. If it is a central pump that always runs (even when the air handler is not running) then calculate the seasonal consumption of the pump, and add it to the seasonal consumption of the air handler fan (instead of using Watts in this case it would be kWh/yr). If the pump runs only when the air handler does, then simply combine the air handler fan watts to the pump watts and let REM calculate the seasonal consumption.