"Phil Again" wrote in message

Let us presume you have 4 room zones, plus one for the potable hot water
tank, for a total of 5. Do all 5 zones have check valves for one way hot
water flow?

Not a check valve, but they do have a flow valve that acts as one.
This is one of a few b rands that will work. They can be opened in case of
circulator failure for gravity flow.


Pretend that one room zone is in the attic, and has a much lower R-factor
insulation than the other 3 room zones. This thermostat will request hot
water more than the others. In some installations, without check valves,
when only one pump turns on, the output manifold of the boiler(s) that
feeds all 5 pumps will draw hot water from all connections on the
manifold. In short, one pump turns on, and the other zones are going to
have reverse flow (complex relationship to determine the percentage each
off-line pump feeds the flow to the on-line pump.)

That is called sloppy design and installation.