Hey all -
actually - I do know a fair amount about solar hot water systems....in layman's terms:
The controller is looking at a pair of temps to determine when to run - the first is the temp sensor in the panel itself on the roof, the second is in the tank. Assuming normal conditions - the controller turns on the pump when the tank needs heat, turns it off when the need is satisfied.
Now, any number of things could be going wrong here - in no particular order they are:
1. glycol has boiled out of the system and the pump can run all day without anything to pump (but the hot pipes might indicate this is not the case)
2. the temp sensor on the roof (in the panel) has failed
3. the temp sensor in the tank has failed
4. the pump has failed
5. the controller has failed
6. a valve has failed
If you can find the manual for that model of controller on line - you should be able to take some readings to determine if the thing is getting signals or not. Can you take a pic of what the installation looks like and send it to me? On many systems, the cold from the street provides the pressure to push the water thru the solar tank into the gas tank (pre-heating the gas heater)- but yours must not be set up that way or you would not have any hot water! IF there is a pump between the tanks, that may be kapoot.
Best bet - I can put you in touch with an installer in your area and he can come check out the system. The controllers are pretty straightforward, but after a few years (like 30?) they might poop out. Probably a good idea to have someone check your system regardless.
If you want to PM me I can find a good installer for you.
as an aside - we make a monitoring system that is non-invasive and senses when the pump is on, what the supply and return temps are from the roof, and the tank temp. It feeds data to our servers and we push out web-views for you to evaluate. A number of installers are using our system on their installs to tune the system for optimum perf, and many rebates require a system like ours. you can see more at
www.sunreports.com
let me know if I can be of any more help -
best
td