Ive dealt with those.Also it should have a pump,problably inside the air handler.Make shure its working and pumping good.Its a direct drive pump and thats the only moving part to the system.What kind of valves does it have?If they are gate valves,if so verify they are open completly.On gate valves the gate can break loose from the shaft and fall closed or even partially closed.You may need to open the system to make shure you have no excessive pressure drop accross valves after you verify the pump is running.They are really effective and relaively cheap to operate.The only moving parts are the pump and the blower.Really simple and effective