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In the UK, we use water radiator systems when the boiler heats the water to about 70°C, this allows the boiler to get most of the energy out of the gas (or oil) by condensing the steam you get from burning gas.

I can’t understand why anyone would design a system that could not operate with a condensing boiler – what am I missing?

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    Jeff Ferland gave a great answer, so I'll just leave this as a comment: systems evolve over time, and builders install whatever is current. My parents' 1906 house has steam, my 1937 house has hot water, and I don't remember seeing anything other than hot air systems in houses built in the 1960s and 1970s. In England, I suspect that the housing stock is either relatively new (post-1945) or very old; in the latter case, it would use whatever was popular when central heat was added.
    – kdgregory
    Dec 10, 2011 at 13:54
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    @kdgregory, I think a big factor is that we don't have many basements in the UK, so a single pipe stream system will never have worked. Hot air come in the UK for a few years, but it then want back to all being water based.
    – Walker
    Dec 11, 2011 at 19:13

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The US has a mixture of systems. I'm not sure what the prevalence is, but I've lived in homes with both steam and hot water heating.

Steam offers the following advantages:

  • One-pipe systems
  • More heat transfer for a given radiator surface
  • No distribution pumps

Steam offers the following drawbacks:

  • Furnace needs to be a low point
  • Corrosion
  • Finicky distribution

Modern installations tend to favor hot water. The New England region has a legacy of steam heat that stems from coal furnaces and basements. Those systems were selected a century ago, and refit would require replacing the entire system. Steam was the leading technology in New England during its greatest period of growth.

Further, New York City had a substantial influence in the area with its district steam heat operations. Also, another good article that discusses steam legacy, why replacement is a bear, and how steam works in general.

Finally, in large distribution environments, there's the high pressure factor. If you need to move a lot of heat energy, you can do so space-efficiently with high pressure steam (smaller pipes). Harvard's Blackstone Steam Plant heats over 160 buildings and drives a 5 megawatt turbine. At larger scales like this, the thermal loss at the chimney is far lower than the combined loss that would be had from running at sub-boiling temperatures.

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