External spring check leaking by in one zone

ConstantGHVAC

My brothers boiler has 2 zones with circulator pumps on the supply header. One of the 2 zones is overheating. The bedroom zone. There is no call for heat. Pulled the internal checks and installed external spring checks. Still having the issue on the bedroom zone. The returns tee together and come into the central return at the boiler. The bedroom zone is on the "run" line of the tee to the return. Should I install checks on the returns as well? Checks are arrow with the flow. I'm wondering if I am having a thermosiphoning event. Let me hear your thoughts. Thanks in advance.

hot_rod

yes, in some instances you need to protect both S&R

Move your hand along the return as far up as you can, is it warm after the zone has turned off for a few minutes? it could be ghost flow up the return.

I would highly recommend a dirt sep or Y strainer on systems with ICFs

The few specs of dirt on this paper towel were holding the checks open

Alan (California Radiant) Forbes

Caleffi makes serviceable check valves. So easy to clean if fouled.

https://www.supplyhouse.com/Caleffi-304756A-3-4-Press-Serviceable-Inline-Check-Valve-Low-Lead