Arduino thermostat

Earlier I finished the piping for the the two additional zones.  A quick recap, one zone will allow  a boiler to heat the tank that is normally heated by solar.  The second zone is really not a zone but the piping and circulator pump that will add heated water from the chip boiler.

The solar water tank, or solar thermal store has two coils in it, the first coil is heated by the solar loop on the roof.  This coil is physically lower in the tank so that the heated water will rise and the sensor controlling this coil is lower as well.  This allows the maximum storage from the tank from solar at all times.  The second coil and sensor that is intended to be heated from the boiler is mounted higher.  The water at the top is  hottest and therefore the  least amount of energy will be needed and the maximum amount of free energy is used first.  Of course solar is not free, it still requires a circulator pump but I assume it to be negligible and therefore in this context free.

Knowing this and assuming that I am going to use an Arduino microcomputer in the process anyway, I am intending to implement the solution in two steps.  The first step to wire and shrink wrap a thermistor  and run the thermistor back to a breadboard, and display the temperature on an LCD.  If the temperature is below a certain value, digital write to a relay to turn on the circulator pump.  If the temperature is good, do nothing.

In the second iteration of the solution my plan is to add a time chip to the system and make the process more efficient.    The logic is this:

8am, no real need for hot water, and there is a good chance the solar will heat the water.

4pm, heat water and maintain until 8pm if needed.

8pm-8am maintain a minimum, lukewarm temp.  We’re not a shower in the morning family.  Of course on the weekend the program will have to be different.

To implement this 2nd iteration a real time chip will be required.  I have found a few, one at sparkfun.com

http://www.sparkfun.com/products/10160, $19.95

and a second one at

http://macetech.com/store/index.php?main_page=product_info&cPath=5&products_id=8

I am inclined to the second link, the board is cheaper with a built in battery holder and battery shipped with the product for $14.99

And the yourduino option for $9.00

http://arduino-direct.com/sunshop/index.php?l=product_detail&p=48

http://adafruit.com/products/264

Also $9.00

Any opinions?