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We just installed new gas combi boiler (Lochinvar NKC199N), it is working but there is one issue that I don't quite understand.

We have 4 zones (4 different circulation pumps, 4 thermostats), when zone 1 calls for heat the return line 1 gets hot first (as expected), but a few seconds later the other return lines 2, 3 & 4 get hot too (burning hot). In fact, the hot water pushes back through return lines all the way to upstairs bedrooms, I can feel the hot baseboards of other zones.

So the question is, we only turn on zone 1, but zones 2, 3 & 4 get warm too (though return lines), is this normal? Should there be any check-valves on all return lines to prevent back flow?

boiler area with supply & return lines identified

FreeMan
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ysok
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  • Check valves are quite commonly at or in the pump, but that's not a requirement - just one way it can be done without being "as obvious." Assuming you paid someone else for the design and install, call them back and make them fix it. – Ecnerwal Jan 17 '22 at 15:53
  • This is normal. Some leakage up the return lines is not worth the added expense and complexity of installing more valves. If the leakage is excessive you need to determine why and deal with that. Perhaps the long vertical lines coming off the shared return manifold cause more leakage of heat than if the manifold were vertical or perhaps if the manifold were at ceiling level. Whether that really matters is up to you. Are the radiators getting hot in zones not demanding heat? – jay613 Jan 17 '22 at 16:19
  • Also I don't see any zone valves at all in your photo, but perhaps they are below the pumps? – jay613 Jan 17 '22 at 16:22
  • So the installer came back and concluded that there are two bad check-valves inside IFC circulation pumps (Taco 00R model, they are old pumps). So the water slowly slip back to supply manifold causing Ghost flow/heating on the other zones. They propose to either change the circulation pumps or add check-valve on each return line. – ysok Jan 19 '22 at 03:35

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