My hayward heater fires up but after a few seconds there is a loud banging on one side of the heater. any ideas???

it could be a bad blower vac switch, and you may look at the rubber tubing going into the switch to ensure it hasnt dry rotted.

Mine too today!

https://sites.google.com/site/haywardpoolheater/

What you have is a water flow problem. The water should be going through the heater but it is by-passing it. The water inside the heater is boiling and trying to expand as steam that makes the banging sound. Their is a internal by pass in the water manifold on the side of the heater. If you take this apart you will find a disc on a spring contraption, if it is stuck open the water goes past the heater but still has pressure to kick the pressure switch and fire the heater. Either clean or replace this assembly. The other thing to check for is an external by-pass. This will be a gate valve bridging the inlet and outlet pipes. If you have one of these it should be 80-90% closed or the water goes through it and creates the same problem. This is not on all systems so don't be alarmed if you don't find it. Good luck.

You will need to pull your tray from the heater to check for Wasp nests or other problems. You need to be careful with the wasps if you smoke them out be very careful not to burn any wires or components on the heater. If you have access to a co2 fire extinguisher the cold and lack of oxigen will hold them at bay for a few minutes. Back to the heater...
Your dead on about the gas build up. This comes from something blocking your pilot so that your burner flame isn't lighting on the burner that is connected to the pilot. If you have a wasp nest on the burner this will do be the problem. I want to think that when the heater fires this would fix that problem. What is more common is on the gas pipe that feeds the burners you have a gas orface for each burner. Small spiders will sometimes build webs that resemble lint inside the orfaces. You can clean these with a bent wire.Other than that anything blocking the burners will be evedent. I hope this helps, good luck.

possibly poor pilot adjustment or insuficient mv output from thermopile

Stop using the heater immediately. You are boiling the water inside and this can lead to a dangerous situation if a limit switch fails to act in time.

You will likely find the diverter in the inlet side of the heater is broken. You are looking for a stainless steel plate in the water path. If you can move it by hand, it is broke. It is not replaceable and requires a new heat exchanger assembly to repair it.

Your part number is IDXHXA1101. We sell it as will most pool dealers. If you would like a quote, contact us at www.arrowmotor.net

try to replace the thermo sensor it is located on the left side of tghe unit and has 3 wires running to it.