I installed a radiant floor system with 4 zones. Three of the zones are working just fine, but the 4th has not flow whatsoever. The thermostat calls for heat, the controller kicks on the pump, the thermal actuator opens, but nothing. My first suspicion was that I may have reversed the supply return lines to the manifold. But I reversed the manifold supply and return and it still doesn't work. So my next suspicion is the zone valve isn't opening, but I'm not sure how to check this short of ripping in out of the system. Any suggestions?