First, based on questions and FYI, my controller is a Hayward Aquastar II solar controller. The pump is high speed and I use a Pentair CC 150 filter. I run my pump & filter from 10 AM to 4 PM in the winter and to 5 PM in summer, so that is my only heat opportunity time. Essentially, it's a set and forget system, but it is an electronic timer with 4 on/off cycles available each day, so I could program it many ways. My solar control has two temperature sensors (thermistors), one on the pipe flowing from the pool to the pump (is this water too cold?) and one up on the pipe flowing from the solar panels (is this water hot?). Both sensors have wires to the controller where the first temp can be set (eg 86F). If both sensors say "yes," the controller opens the valve so that water flows through the solar panels. When the pool water reaches 86F or the roof water gets too cool, it closes the valve. It works great from mid-March to early November. So far so good.
Now, to use the pool year round, if I put a second heat source, like a gas heater with a built in sensor, in line after the solar panel out-pipe, then the gas heater will never turn on when the sun is shining because it will always sense warm water, even though the pool water is <86F. To avoid this, I need to tell the gas heater the actual temperature of the pool water. This is what the first sensor in the system already provides. I could install a new one next to it in the pipe and connect it to the gas heater if the gas heater accepts an external sensor. That's the key question! The only thing I want to connect to the gas heater is an external thermistor installed on the inlet pipe. I do not want an automation system for pool, spa, lights, etc. I also don't want the gas heater in line first and then run hot water up through the solar panels.
I am assuming I could still set the temperature on the gas heater wherever the sensor is. This would make a fully automated system allowing solar heating when the sun is sufficient and secondary heat from the gas heater when the solar can't maintain the set temperature. It would also heat the pool a lot faster whenever the pool falls below the set temp. It would seem that if heaters can connect to automation systems, they should be able to do this, even if it's a post install bypass circuit done by the tech who installs it. Does this make sense, or am I missing something?
My alternative would be to have a manual valve to switch between solar and gas depending on the time of year. Sorry for being verbose, but I wanted to get it all out there.