reversed flow in a loop

SteveB

Here's the layout...
3 Taco 3/4" zone valves on the supply, 1" copper. Circulator after the air scoop. 4 3/4" gate valves with purgers above on a 1" return,lined up 4" apart on a horizontal manifold. The main floor loop leaves the boiler 3/4" and splits into 2 3/4" loops coming back to the boiler individually. The 2 remaining loops heat the 2nd floor and the basement. The house is 15 years old and the homeowners have lived with uneven heat on the main floor since moving in. No doubt due to the main loop not being 1" as it leaves the boiler. 1 month ago 1/2 the main floor (one loop) began heating whenever the 2nd floor loop called for heat. The return manifold has the first return as the basement, the 2nd and 3rd returns are the main floor, and the last return is the 2nd floor. The loop that overheats is the 3rd return and the flow will eventually make it all the way around to the 2nd return. How is this happening and why didn't it do it for 14 years? This problem initially began after one zone valve was replaced because it wouldn't close. I am 100% sure no zone valves are passing by. My solution, which worked, was to combine the 2 3/4" returns into one 3/4" pipe before they tie into the manifold.

bob_44

Ghost flow

Steve, you can feed approx. 70' of baseboard with a 3/4" pipe. Your ghost flow was caused by a piping error that you corrected. When ever you have a loop of pipe with both ends open and exposed to a flow of water there will be a difference in pressure between the two ends. This difference is the result of the friction in the manifold. It causes flow in the loop. The loop exists when the first floor zone is closed and is from the second to the third port. You eliminated the loop.