Can I use the FTSs to warm my beer when it's cold in my fermenting area?

Indeed!!

The FTSs is built around the idea of having a nice 304 stainless immersion chiller coil in your fermenter along with a digital temp controller. So none of that hardware particularly cares if you are pumping 50-degree water through its plumbing or 85-degree water if you wanted to keep beer warm in a colder basement or something.

There are a number of cheap/easy ways one could do this - including just sticking an aquarium heater inside your chill water box (say a camping or boating cooler that you normally would fill with ice water, you just drop a fish tank heater in there, job done!) The temp controller doesn't care about anything other than monitoring a temp and then switching a pump on / off based on that set point.

So you can chill and you can warm. Either way! And remember also, the actual temperature of the chill/ warm water source isn't determinate of the temp the beer will be. It just has to be "different enough" (sometimes as little as 10 degrees F whether that is colder or warmer, but certainly more efficient when the difference is more like 15 or 20 degrees F or more). In other words, your FTSs is switching a pump to maintain or hit a temperature.

So if you have 60-degree beer and a 50-degree ambient, and you want to WARM it to say 65 degrees F, then you set a fish tank heater in your icebox/water to like say 80 degrees F and the pump will switch on long enough to get the beer to 65F. Then over time, since it is 60 in the garage, the ambient temp will try to drop. When it does (and this takes some time indeed because of the neoprene insulation doing it's job!) , then the pump will kick back on and cycle a little more of the 80F warm water through the coil and get your beer back to its set temp of 65.

The whole system is predicated on having an external chill / warm water source that the digital controller switches a pump into the coil (in your beer) when the temp gets away from your set temp. So your external water can be cool or warm, the system doesn't care either way!

And for those of you wondering about temp gradients because of convection (warmed coil sits on top, and heat rises) - what we can say for sure (we use thermocouples and data logging) is that during the first 48 hours of primary ferment where the majority of the flavors are set up in your beer anyway, the yeast moves so much you really don't have any gradients of temp from top to bottom.  Over time as things settle down of course, you can start to see that.  Really depends mainly on the ambient outside and your target temps.