Have gotten an Arduino uno R3 starter pack as I want to:
read ambient greenhouse temperature
start evaporative cooler on low speed with temperature rise
turn water pump on with further temperature rise
turn cooler on high speed with still further temperature rise
Turn water Circulator pump on (to circulate 500 gallons of water) to warm water (help cool an additional benefit) when air temp is above say 70 when water is cooler than ambient temperature. (differential type thermostat)
Turn circulator pump on when temperature drops below say 55 (differential type thermostat)
to warm/help warm greenhouse.
Have it to operate dpdt relays to handle evaporative cooler 120VAC +/- 8 amp.
Hopefully there is a starter pact that will come close to these needs.
Maybe later operate vents jet another issue.
Where can I find stuff to build & write program for differential thermostat without reinventing the wheel?
All you need is the temperature sensors and relays. Everything else, including calculating the difference between two temperatures, is easily done in software.
How many temperature sensors do you need? How far from the Arduino is the most distant temperature sensor? Knowing the answers we can advise you on what temperature sensors to get.
How many relays do you need? They typically come in modules with 1, 2, 4, or 8 relays.
You will need to decide on more specific details than "on temperature rise" and "on further temperature rise". You need numbers. At what temperature (or difference in temperature) does each action happen?
I'd suggest you put some hardware together to act as the greenhouse so you can test your logic. Attach a potentiometer to an analog input for each thermometer. Then you can ensure that the system behaves as expected when the temperatures change.
LEDs (with appropriate resistors) can simulate the other hardware, or you can just use the serial monitor to output messages when they're turned on or off.