Thanks lehua13 and mike1059. I've read the installation and setup manual for the ESP-SMT and I agree there's a lot of information needed but I'm sure I could eventually gather it all. I decided to continue using our Hunter timer with Rain Bird WR2 rain sensor because we get very little rain here I'm guessing the ESP-SMT would want to run the irrigation a lot. Over watering here with our red clay is not the problem. If we were in an area where frequent rain was normal, then I think the unit would be more suited for monitoring and controlling the irrigation.

I believe that those irrigation companies estimate the maximum wire length between station and solenoid according to estimated resistance of the wire, and the estimated inductance of the wire. These will depend on the particular sort of wire/cable being used. One way to possibly figure out a maximum length is to take whatever system you have at the moment, and use an AC voltage measuring device (eg, multimeter) to measure the voltage at the solenoid when the controller is supplying AC voltage to it. The controller output will be about 24 V AC (RMS). But the voltage across the solenoid is expected to be smaller, due to resistance and reactance of the long cable. If you know roughly the length of the existing cable, then you can take the difference (call it "DIFF") between 24Volt AC (RMS) and the measured RMS voltage across the solenoid. This will be the estimated voltage drop for that length of cable you currently have. Then, the voltage drop per unit length will be equal to DIFF/existing_cable_length, ie. DIFF divided by existing cable length. Call the result of this something like dpu (drop per unit length). So, this should let you calculate (estimate) the voltage drop for a particular length wire/cable. As long as the drop is not more than say '4 Volt', then it should be ok. This means, the AC RMS voltage across the solenoid terminals themselves, should be at least 20V AC. The voltage drop calculation will be dpu times length_of_new_cable.

http://forums2.gardenweb.com/discussions/3811193/check-out-my-ks-campaign-smart-watering-without-a-sprinkler-system?n=2

UPDATE ON RACHIO. CYBER-RAIN COMPANY APPARENTLY CAME BACK TO LIFE... BUT I DISCARDED THAT 4 YEAR OLD IRRIGATION CONTROLLER ANYWAY. I HAVE HAD THE RACHIO FOR 6 WEKS NOW. IT IS UNBELIEVABLY GOOD AS A PRODUCT. INSTALLATION A BREEZE. SOFTWARE FANTASTIC. CALLED SUPPORT 2 TIMES JUST TO CONFIRM MY UNDERSTANDING OF BEST WAY TO ACCOMPLISH WHAT I WANTED. MORE POWERFUL THAN I THOUGHT. CUSTOMER SERVICE FANTASTIC. IF YOU NEED A NEW OR REPLACEMENT IRRIGATION CONTROLLER. READ THE HUNDREDS OF REVIEWS ON AMAZON. YOU CAN NOT GO WRONG. HIGHLY RECOMMEND EVEN MORE, AFTER RUNNING 6 WEEKS.