How do I drain a boiler considering it’s connected to the hot water heater?

I have an oil boiler in a one story house that pushes hot water through radiant ceiling tubing in the attic & also heats the water in the winter & is connected by plumbing to an electric hot water heater that I use for hot water in the summer when the boiler is off. I have switched to a heat pump but want to keep the boiler as a selling point for optional heat when I sell the house. Question is how do I drain the boiler, expansion tank and radiant tubing, considering it’s connected to the hot water heater?