A few years ago I was looking at the specific heat of common substances and water is pretty much off the charts compared with everything else (http://hyperphysi

A few years ago I was looking at the specific heat of common substances and water is pretty much off the charts compared with everything else (http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/sphtt.html http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/sphtt.html). But the heat of vaporization is also huge: it takes about the same energy to boil a quantity of water as it does to take it from 0°C to 100°C. The main downside is that the vaporization/condensation has to take place at 100°C, which is arguably a bit hotter than is ideal for safe space heating. Also the water returning to the boiler might not be cold enough to condense out the water vapor from the fuel (more important in fuels like natural gas with a big difference between HHV and LHV). I've sometimes wondered if it would be practical to operate a steam heating system at a partial vacuum so that the condensation/vaporization could occur at a lower temperature, but I haven't seen that done anywhere so I'm guessing the answer is no.