Quote:
Originally Posted by solarhotairpanels
...My storage tank is only 33 to 40 gallons so one shower and most of it is replaced with fresh city water. Doesn't sit around in that tank any longer then a regular electric hot water heater would....
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It's because of the Legionnaire's risk
Water in a hot water tank sits in the tank at a level above the risk ALL the time, no matter how long it sits.
Your solar pre-heated tank has large volumes of water sitting at risk level all night long, all vacation long, etc City chlorinated water cuts the risk for say overnight issues, but chlorine becomes less effective at warmer temps, so it loses effectiveness over a weekend away. The risk is low, but the consequences can be deadly and there is no easy or automatic detection. It's not a big selling point for retail, ie higher cost and some risk. Some do use a heater element to keep the preheat tank above risk but it's a big hit to efficiency.
Also the people with DIY go the other way because it's easier and cheaper, complimenting DIY panels setups. Proper solar preheat tanks are expensive and need replacement. If the $ is not an issue to you, then the discussion is moot.
And the small preheat tanks can act as a buffer for the initial warmup at start of hot water use, which helps solve another problem. Commonly people just use an electric heat based one to either preheat lines on a circ pump that's volume is too low to trigger the instant hot water heater, or keep a handwashing volume ready.