Two thermostats in one casing: 

Thermostat (contact breaker, normally closed) for regulating heaters. The contact opens when temperature is rising. 
Thermostat (contact maker, normally open) for regulating filter fans and heat exchangers or switching signal devices when temperature limit has been exceeded. The contact closes when temperature is rising. 

Heaters and cooling equipment can be switched independently from each other with a temperature offset as opposed to the usual change-over contacts.