System 6 in ASHRAE 90.1 1999 is a Water-Source Heat Pump system with
constant volume fan control, DX cooling and Electric Heat Pump and Boiler
for heating.

My question is what is happening on the source side of the water-source heat
pump and how do the heat pump and boiler interact? Is the boiler for backup
only or is it providing hot water to the heat pumps continuously. This
system doesn't seem to make sense. If it were air-source then it would.

Any thoughts?

Thanks in advance!

Chris Flood