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Replace new hot water cylinder

I have a leaky hot water cylinder and I fear it needs changing soon. I have been catching the leaks up to now which has been manageable but may be getting unmanageable. It is an old house (maybe 100years?) The cylinder maybe 30years old. It is lying on its side. It is connected to 3 solar hot wather panels(think these are about 30 years old). I do not think there is a pump/valve to control this water flow, but this works well. It has an overflow that overflows a good bit in summer due to the solar. It is also connected to a wetback, that I am not too sure if it works too well. I was also thinking of swaping out the existing wetback fire for a bigger one and still having the weback there also?