This is New Yorker CG-A series gas boiler about 10-20 years old. I recently refill the system, bled all radiators, open all valves and see clear water coming out. The unit fires up normally, but it shuts down after 1-2 mins when the temperature reaches 80-100 C. The water pressure was around 12-15, just below the red arrow, but it didn't drop when water was drained from the unit. The pump continues to make sound after system shuts off, temperature drops slowly, but can't restarts for hours. The supply pipe above boiler is hot, but not too far away from the boiler. I took out the TACO cartridge circulator pump, it looks fine, and I test it by inserting wires into outlet. It spins, and air comes out from one end. So it seems that the pump works. I decided to put it back on to try my luck. i refill the system, drain a bucket of water out from under the pump, and turn on the unit again. One thing to note is that the water pressure indicator didn't drop when the water was drained! Maybe something wrong here. It fires up as before, the temperature goes up to about 80C and the system shuts down again. The temperature continues to rise to just above 100C/220F, the pump continues to make noise after the boiler stops, and I can hear water flowing thru the pipe. The return pipe remains cold thru this time, so I think the pump is working (but not sure how efficient). The supply pipe gets hot quickly, but not much beyond the expansion tank. I am going to wait for the system to cool down while letting pump continues. The pump feels hot again, and pipe around it are getting warm too. The temperature is dropping slowly from around 100C.
After about 20 mins, the pump continues to make sound, the temperature drops to about 80C, but pipe above the pump starts to get warm, and supply pipe doesn't get hotter, and it actually cools down. This seems to indicate that the hot water is not circulating out of the boiler. The pump also feels extremely hot. Perhaps the pump stops working when temp goes up?
It's about 30 mins past, the pump still makes sound and is hot. The return pipe above the pump is hot, while supply pipe above boiler feels cold already. It seems the pump isn't circulating water at all. I turn off the unit! So, is the pump the problem? What could be possible reasons and what can be excluded? I need your professional help. Thank you.