What liquid expands the most with a temperature change?

[MossRoad]

I'd ditch the frost-free hydrant, bury the 12v solenoid below the frost line in an upside down 5 gallon bucket, and install an automatic drain valve like they use in a underground sprinkler system between the solenoid and the spigot. Then I'd just install a button on a post on a box with the components I listed earlier.

Or, you could install a 12v linear actuator on the handle of the hydrant. Here's one that says it has 10" of stroke and 330# of force for $10 on ebay.

Linear Heavy Duty Actuator Stroke 15N 33 Pound Max Lift DC 12V Electric Motor | eBay

 

[John_Mc]

If you do go with solar (or some other electric system), I wonder if you could use something that is spring-loaded to close, so if the power goes out, the water shuts off. (Of course, that might mean a constant electrical drain to hold it open, so maybe that's a bad idea for a solar/battery based system.)

There are mixing valves made which adjust the flow based on temperature. These take no power. I wonder what is driving the mechanical motion in those?

 

[jenkinsph]

If you do go with solar (or some other electric system), I wonder if you could use something that is spring-loaded to close, so if the power goes out, the water shuts off. (Of course, that might mean a constant electrical drain to hold it open, so maybe that's a bad idea for a solar/battery based system.)

There are mixing valves made which adjust the flow based on temperature. These take no power. I wonder what is driving the mechanical motion in those?

Tempering valves use bimetal straps and diaphragms to adjust the temperatures.