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Phantom circulation in forced hot water
Last summer I replaced all the zone valves in a 5 zone hydronic system with circulators. The zone valves had been a PITA and I thought I would do it "right" using dedicated circulators (1 per zone). Now, I'm not so sure.
Anyhow, the problem is that I'm getting "phantom" circulation in zones that are not calling for heat, when a circulator runs in another zone that is calling.
I have attached a schematic of the whole system. The piping is more or less proportional to the line lengths. Which is why a couple of loops zig-zag - to represent their actual length better. The triangles represent zone valves, each with a check valve. A circle with an X in it is a balancing valve. A plain line is 3/4" copper, a double line 1" and a triple line 1-1/4". "M" is a monoflo tee.
The loops on the right are the 2 first floor zones and on the left are a basement zone and 2 second floor zones. The phantom circulation is occurring in the basement and 2nd floor, especially the zone highlighted in red. That happens when either (?) 1st floor circulators is running.
My diagnosis is that there is a venturi effect in the tee where the 2 main returns come together just before the boiler (at the red arrow). There is about 8' of 1-1/4 pipe after they come together, until the boiler, if it makes a difference.
Do you think that's my problem? If so, how do I fix it? If not, then what?