Sorry, the title may be confusing. I talk about my own model, I do want to have boiler to pick up the remaining heating that heat recovery cannot meet. The reason I mentioned "parallel" is because I have ten boilers parallel together to support heating

In that case I would probably put the use side the water tank on the loop's demand side outlet branch and use load based heating operation scheme to stage the 10 boilers. This way the supply side inlet will already be conditioned by the heat recovery and the load based dispatch can go ahead as if there was no preheat. But it may not be all that necessary to really have 10 boilers in parallel in the model, combining them into one large boiler for modeling would be simpler and probably produce about the same results for boiler energy consumption.