What happens when there is no heat demand? By heat demand I mean there is no demand for space heating or domestic hot water. Is the water inside the pipes still flowing? Is this why bypass valves exist?
I am trying to characterise the output of a condensing boiler(s) (connected in series) which are generating the heat demand required by a sum of apartment blocks. I have assumed that each apartment flat has a Heat Interface Unit. I know how to calculate the required flow rate to each HIU when there is heat demand. However, I don't know what happens when there is no demand to an apartment. Is there water still flowing to the HIU but bypassing inside the unit? What happens to the return temperature of the bypassing unit?