Model h100id banging noise when turned on shuts off

Sounds like a possible lack of flow. I have heard that sound before when pump suddenly shuts off. Water is boiling in heat exchanger

Just solved this problem by doing alot of reading online. Best solution that i found that actually worked was, "remove the exit piping of the heat exchanger header (heater outlet) and jam a 3/8" or so stainless bolt or rod between the heat exchanger exit tubes and the top of the baffle plate. This should hold the plate down and force water flow through the heat exchanger properly. For extra security, glue (thickened epoxy, or JB weld) the baffle brace in place." I used about a 5-6" long 3/8" stainless steel bolt and screwed it in between baffle plate and the inlet copper pipe leading into the heat exchanger. I really did not expect it to work.... but it seems to have solved my problem. Pretty damn rigged if you ask me but no more banging noises or internal exploding steam. Costs $1 to fix and pretty easy to do. This is after I replaced heat exchanged with another used one (didnt work) and put a clamp around the diverter plate to hold it to the roof of the pipe (also didnt work).

The H100ID1 has a known design flaw that allows free flow past the heat exchanger causing localized boiling in the heater (banging), a no flow condition and then shutdown.
Simple free fix is here: https://sites.google.com/site/haywardpoolheater/
Worked fine until they removed the pool a decade or more later.

VERY low water flow!
Or... No water flow.
This heater as all heaters do, need to have sufficient water flow thru them, or else they Bang like this.
This then creates a problem, for future banging - due to lime up!
The minimal water flow gets so Overheated, that it turns to steam! The steam knocks and pushes the heater around, then the Minerals will precipitate out of the water and forcefully attach to the inside walls of the heat exchanger! Which will then drastically reduce the inner diameter of the exchanger making a much smaller passageway for water to travel thru! This is a major problem to remain this way. It will then need to be dismantled and de-limed to increase the holes once more, and allow a proper water flow.