It was diesel fired and tended to soot up very quickly. Cleaning the glow plug was pain and required removing the whole unit for service disassembly. It was not designed for serviceability, IMHO. I suspect the hydronic units would be similar but with a hydro jacket added.
Anyway, my experience was limited but I would not get another Espar. There are also Japanese and Russian-made diesel heaters on the market that look interesting. Do some research.