Response to If you have an external by-pass it needs to be closed. This will be a valve between the pipes in and out of the heater. When you close it, it forces water through the heater. When it is open no water goes through the heater. So the water in the heater boils and turns to steam. With no where to go it starts banging and jumping.If you close this valve or it is closed and you still have trouble the internal by-pass has gone bad. The overheating from the first problem will often cause this. You remove the water header from the side of the heater and pull the pipes from it. In reality you will probably have to pull the gas pipe so you can move the heater from the pipes. You will find the assembly inside the manifold. It will probably come out in pieces but the new one goes back in as a unit. This will regulate your water flow and keep it heating at a constant temp.

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