Water heat system optimization

We heard the water heater electrical element can be left off 3 to 4 months out of the year in our area, thus heating entirely by solar, so we've been giving that a try. [Corrected 7/11/09: not 8 months as originally posted.] We shut off its big 240V double-switch circuit breaker. (The solar pump station is still on, powered by a separate 120V circuit.)

The past few days, the weather has been sunny and clear, and each afternoon we noticed the SWH rooftop collector temperature got around 300 degrees F ... and higher!The tank was already at the (default) maximum 140 degrees F, and we weren't using much hot water, so the pump shut off and the caution icon flashed on the LCD panel. (We called Steve at home, and he reassured us this was normal, and the system is designed for hundreds of degrees more.)

Why leave the heat in the rooftop collector, which just gets lost in the evening? Seems like a "waste," right? Steve and Jeremy stopped by today to make a simple programming adjustment on the pump: The tank maximum was increased to 155 degrees F.

This approach seems smarter:

• The heat gets moved into the super-insulated tank (when idle, Steve says the tank located indoors will lose only half a degree F per hour).


• The tank can take the heat (less than boiling, 212 degrees F). This fact is further evidenced by the electric element thermostat which is adjustable up to 180 degrees F.


• The adjustable mixing valve on the tank hot water output makes a safe, consistent temperature. We've played with a thermometer at the tap and rotated the mixing valve so we're at about 125 degrees F. (We might temporarily adjust it up while running the dishwasher, which would shorten the dishwasher run cycle since it won't have to heat up the water.) [Clarification 7/11/09: With the tank temp @ 155 F, the resulting mixer valve range is about 130 to 150 F at the tap.]

We'll see how see how long we can go into the fall and winter before we notice the electric element needs to be turned on. [Addendum 7/11/09: On a couple recent fully-cloudy days, we noticed the tank temp got to around 90 to 100 F. So in non-summer weather, the electric element will definitely be needed to augment the solar pre-heat.]