> The radiators transfer heat to the ambient air which cools the heated
 > water. This can create an appreciable flow which depends on the
 > temperature difference of the heated and radiator cooled water.
 > 
 > It can be a 1 pipe or 2 pipe system. The 1 pipe type must have a larger
 > gauge because the hot and cold water create opposing flows in the same
 > pipe. A 2 pipe type only has flow in 1 direction so the gauge can be
 > smaller.
Mike,
 I've heard of single-pipe STEAM heating systems, but never a single-pipe HOT 
WATER operation. The steam is pretty simple, they use fairly large pipe, 
sloped down to the boiler. The hot steam rises, condenses in the radiator, 
and the condensate (a small fraction of the volume of the steam) runs back 
down the pipe to the boiler.
Are you SURE you've seen or heard of single-pipe hot water heating systems?
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