On the Rainbird site, I found the statement below. If you have an electric water heater, its not going to produce any interference. Your furnace motor might. Sounds like Rainbird prefers to print disclaimers rather than invest in proper filtering and shielding of their electronics.
To minimize electromagnetic
interference, select a location at least 15 feetcorrect operation of the controller.
(4,6 m) away from high-draw motors, such as air
conditioners or refrigerators.
Never share a 120 VAC circuit with pumps, motors,
shop tools, etc. as they may interfere with the