Without a heat source the max temperature the beer can get to is the ambient temperature plus what ever heat is generated by fermentation at that temperature.
If your ambient is warm enough no heat source is needed. I can't remember the last time I needed to apply heat to my fermentation chamber as it sits in a climate controlled room in my house in Texas.
If your chamber is in a garage in Minnesota this would pose a very different set of thermal challenges to solve.
Ambient temperatures will prevent any yeast activity. Heat generated by fermentation will be zero.
Likewise, some folks don't need a source of cooling. A basement with an ambient of 55F maight be ideally suited to brewing ales if a little heat can be applied.
Just depends on your circumstances and your requirements in a chamber.
