Unfortunately I have to start by saying that this information is provided for education purposes only, and that I, nor anyone I am affiliated with is responsible in any way for any injuries, or damage as a result of anyone attempting these repairs for any reason.
The diaphragm is responsible for cutting off the fuel supply when the water supply is shut off. If it malfunctions it can fill your house with explosive gas with lethal results. You must have a good working knowledge of Oring and compressions sealing techniques.
I was given a small Chinese water heater made by a company that has a difficult time providing parts for it. I am a self reliant person and feel that many people are finding repairing their own devices increasingly difficult.
My water heater has frozen several times, and I have had to repair the diaphragm cover by resoldering the pieces several times.
In my heater, a thin rubber diaphragm is pushed by a plunger that is activated by water flow, into a small metal rod that opens up a gas port, and activates a micro switch that activates the microcontroller and high voltage output.
It sounds complicated friends, but rest assured, it really is not.
The good thing is that virtually every part on this heater can be fabricated or repaired. There is no need to trash your heater if it freezes and cracks.
So here we go.
If you are willing to make two small holes in the vent on the right hand of the heater you can use a long phillips screwdriver to access the two deep screws to remove the diaphragm cover without having to remove the entire assembly.
There are two small brass tubes that are attached to the diaphragm, be careful not to lose these. The heater will not work without them.
Firstly, I took a screw with a large head and cut the threaded part off, flush with the bottom of the head of the screw.
I used a micrometer to measure the diameter of the plunger shaft that contacts the diaphragm. Then, I used a Dremel tool to round out the screw as such: