The way I work things is to not have to thaw a system out. Thawing ice takes a lot of time and energy. I have a holding tank where I produce water and my van tank. My holding tank is in an insulated shed with a small heater so there for will not freeze. To stop the van from freezing I transfer any excess water back to the holding tank at the end of the day.
I believe that in sub zero temperatures you don't need hot water but need warm. Cold will just freeze in sub zero temperatures. An L5 in line will give you hot or warm water as needed and very efficiently. At the moment I'm just cycling the water around my van tank through the L5 before I go to work, I've yet to plumb it in line.
If you can get flow through the L5 and back to the tank it would easily defrost a system but I think it's better to not have to defrost the system in the first place.
An immersion is good but I feel the L5 is far better, as I have used both.
Just a word of warning to immersion users, if your going to use it on a timer make sure that the timer can handle the amps. I had one melt on me, the socket timer and plug were all melted together, because of this I feel that a Gas fired L5 is safer.
Simon.