Heat Exchangers are external devices used in coordination with your solar thermal system and heating system.  Water to water systems use double wall protection for hydronic heating.  Water to Air devices are available for forced air heating systems.  You should install the return air valve downstream of the air filter.  Always buy quality heat exchangers and bigger is better.

Anti-Freeze:  Use only propylene glycol.  Check the temperature rating for freeze protection (slush) and burst protection.  Anti-Freeze must contain inhibitors.  A 5 gallon container of solar thermal system anti-freeze will cost about $90.  It can last up to 20 years if not overheated to stagnation.  At 400 degrees F propylene glycol will turn into ethanol and will require replacement.

Differential Temperature Controls turn the system on and off.  This uses standard sensor wire 22-2 or larger guage. The wire should be shielded, grounded, and have U.V. protection.  There are bolt on and probe type sensors.  The probe type sensor is located in pipe fitting.  It measures the temperature in the pipe or tank.