Found this on BETTATALK...
If kept in less than 5 gal tanks you cannot use heaters, they might heat up the water too fast and cook your fish.
My bettas seem to do fine without heaters. They are in my kitchen, the warmest room in the house all year round.My Zackie, who I recently lost was well over 3 and who knows how old when I got him. He was fully grown when I got him. For a fish whose lifespan is an average of 3 years, that's not bad.