Boiler based circulating water systems for heating and potable hot water 

Basic Concept: https://youtu.be/dPHtFe_4nSQ?si=o4dQ1ACToYVCJN9D

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Please watch the following video over and over until you get a good understanding of what is going on.  Also get those professional plumbers to watch the video. 

https://www.youtube.com/watch?v=vd5ZO9DpHDA&feature=youtu.be

Recirculation is the key.  Also check valves help keep it flowing in the correct direction. 

Circuit setters are sometimes used to help economize (reduce heat loss in the return line) and to help balance the return flow in systems where there is a low amount of recirculation or the design is marginal.

Here is the link to the fan coil units as originally installed at Victorian Condominiums.

As you will see on page 8 in the document, the “aquastats” as originally included and wired will not allow the fan to run if there is no hot water.

… Fan coil (blower unit) unit at Victorian Condos