Working on a previously healthy 4 port manifold radiant system. The thermostatic part of the system seems to be working... the pump (Grundfos UP 15-42F) & boiler (shows 140) come on & the motoroized actuators respond correctly.
The mystery is that only 2 zones (closest to the boiler input end of the manifold) get flow (both input & return tubes heat up)... even when manual depressing the actuator valve, there is no discernable heat in either of the other 2 tubes. The manifold & actuators were new 6 months ago and it is reported that everything worked fine, until recently.
What could have happened that would cause these 2 zones to not flow????
Any thoughts greatly appreciated.
jr