On Tue, 03 Feb 2009 19:54:53 +0000, David Hansen wrote:

On Tue, 3 Feb 2009 10:44:34 -0800 (PST) someone who may be
wrote this:-

I have an oil-fired boiler for heating the house. The hot water output
goes to conventional radiators, plus a radiant floor system. The odd
thing, to me, about this system is that the boiler output, besides
branching off to the radiators and the radiant floor system, loops back
directly into the cold-water return. This loop-back takes place
immediately above the boiler, so the distance that some of the output
hot water takes is only about 8 feets before it returns to the boiler
via the cool-water return pipe.

Two likely reasons for this pipe. Thermostatic valves have already been
mentioned. Do you have any?

Secondly, the pipe is raising the return water temperature in order to
minimise corrosion of the boiler. Thermostatic valves can be fitted to
the pipe to minimise heat losses.

I would suspect all those are true. Probably the valve shouldn't be all
the way open, though - it should make a high resistance path compared to
the main heating loop, otherwise you probably won't get much hot going
round that way..