Take a look at the blend valves from Sparco or Honeywell. They are also listed as anti scald valves. You can dial the temp to what you want, say set it for 106? and that is what your shower will be. That way if your supply tank is 55? or 100? it will mix that water with what is heated by the HE to give you a constant temp at the shower head. There are a few different set ranges so select one that will give the output you desire.

These get plumbed in after the HE with cold going in one side and hot in the other and the mixed output to the shower.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.