My daughter just purchased a house with a pool that has a Sta-Rite SR200NA natural gas heater.. Initially the blower and burner would not turn on and the Service Unit light would come on when we tried to fire it up.. I Checked the LED lights on the back of the control panel and got the blower and burner to come on.. Now, however.. about 15 - 20 seconds after the burner ignites, there is a knocking sound coming from the manifold and the temp gauge shows the temp shooting up rapidly..It sounds like a spring or something has let loose in the manifold.. Is it possible that the manifold bypass valve spring has let go..? Bad Bypass valve? or is it more likely that the spring on the thermal regulator ( thermostat ) is the culprit.. When the unit is not running, there is no knocking or water restriction to the pool.. It happens so quickly that I turned off the heater to prevent damage to the coils in case it was not getting water and did not check to see if the water flow back to the pool was restricted.. Anyone have this problem before?? Any suggestions?