I have solar H.W. system for more than 24 years. (2) 4x8' "Flat panel" to a Shell & tube Heat exchanger circulated by a Grumfus Variable speed. 1/12 hp pump.(115 V ac) fast in summer- slow winter, better control. closed Loop.
on the fresh water (heated) side a stainless Grumfus Pump (115 Vac) to a 40 gallon regular water heater (storage) (not electrically hooked up). that Output to the Input of my 40 gallon Electric water heater, thus almost, 80 gallon hot water if the weather cooperates.
there are (2) 11,000 ohm temperature sensors to a controller (115 Vac) which turns on or off the (2) pumps. they are located on the Panel output and the bottom on the storage tank. works great for me.
various valves to fill the solar panels & bypass the entire system if a Leak occurs. I drain sediment off the Tank Monthly.
Richie.