the return pipe on the peerless water boiler system I have goes from hot to warm to cool sometimes for a little while, which in turn causes the forced hot air to be warm to less warm to coolish. The circulator lights are on and the circulator seems like it's running. However, when I tap lightly on the Taco 218 Flow valve with a small hammer all around...I can get the water to flow after a while and return to hot. This lasts a while then the process starts again. I don't ever remember this happening before. I tried backing out the thumb screw for constant 'open'...it helped a little but still things don't seem quite right. Could it be something else like the thermostat, or some other issue. The blower seems to be running more than normal.