I use a 9Kw webasto hotbox which heats up my 850 litre Ionic Pro 5 system, which runs on diesel off the van fuel system.
I would say you need to have a battery which is tip top condition, as it can drain the battery with continual use, so I tend to charge the battery on occasions and also run the engine if doing a job for an extended period of time. It does recirculate into the tank, but takes ages to heat up, but I guess it would be dependent on the size of your tank and how long you operate the system for.
You can buy a secondhand Hotbox from a few places, including Ionic, but don't have anything less than a 9kw model.