heating zone stays hot even when the thermostat isn't calling

Question

we're trying to solve a very strange hot water heating problem (drawing below).

We just separated the the 2nd and 3rd floors in my house into two separate heating zones, and the 3rd floor is now staying at 75 even though its thermostat is set for 65 and isn't calling for heat.

There is a horizontal copper pipe loop in the basement, with all 2nd floor radiators using monoflow T's to take the hot water from the loop and return it back to the loop right after using regular T's.

the way how we did the separation is the following: we added a Y and placed two zone valves to control the flow for the 2nd floor and flow to the 3rd floor manifold. The returns are shared.

We also placed zone valves to 2nd floor radiator supply sides to prevent the 3rd floor return flow from going into the 2nd floor radiators via 2nd fl loop.

the circulator pump is sitting at the return side so it's "pulling" the water.

there are separate thermostats on each floor, and they connect to the 6 zone Taco controller.

when the 2nd floor is calling for heat, 3 things happen: 1) main 2nd floor main zone valve opens 2) zone valves open in front of all supply side of the 2nd floor radiators 3) circulator pump turns on

when the 3rd floor calls for heat 1) 3rd zone main valve opens 2) circulator pump comes on

It is an odd setup, but we had certain limitations (did not want to open plaster walls in the historic house) so we reused the 2nd floor returns.

So right now, 3rd floor is 75 and we can't get the temp down.

any ideas? we're all out...Thanks!!!

Original design before changes were made:

Answer 1

You need a check valve on all three return lines from the 3rd floor to prevent flow from the second floor loop going through 3rd floor radiators. Follow your diagram from the circulator through the first 2nd floor radiator to the Tee. From the tee water can go "north" to feed the third floor radiator and return to the loop through another 3rd floor radiator from the common manifold (path of least resistance). A check valve is not electric and allows flow in only one direction.

Answer 2

The problem could possibly be convection from the 2nd floor, heat will rise. Do you have valves on your radiators? Are they thermostatic?

If you have thermostatic valves turn them down to lowest setting (on level 3) and see if the temperature drops. If you don't have thermostatic valves they will just be open/closed, close them and see if room temp drops.

Get an infra red thermometer to check radiators, pipes, walls, floors for heat sources

Answer 3

You may have water flowing backward thru part of the 3rd floor system, for example from the outlet of 2nd floor radiator #1 backward through 3rd floor radiator #1 to the 3rd floor supply manifold, and then forward through #2 and/or #3. You might try Mono Ts on the 3rd floor supply manifold, or possibly where the returns meet on the 2nd floor.