On Tue, 3 Feb 2009 10:44:34 -0800 (PST),
wrote:

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. Thus the "cold-water" return
pipe is just a hot as the hot-water pipe leaving the boiler (i.e., you
can't touch it with your bare hands). There is a shut-off valve which
can break this loop. I have asked some plumbers and some say that
that is an efficiency issue, needed to prevent the boiler from having
to heat up the fully heat-released return water; i.e., since the
return water is hot, the boiler doesn't have to work as hard. This
sounds true but nuts to me (and to some other plumbers), since much of
the water is bypassing heating the house and is just cycling directly
(and to me, pointlessly) back into the boiler.

Can anyone clarify if this makes sense (to have this immediate
feedback), or can I safely eliminate this loop by using the shutoff
valve?


This is a bypass to prevent the pump from working against to higher
resistance if all of the radiators are shut off or restricted by TRVs.
If I remember correctly it should be set to allow about 10% of the
maximum flow rate through it. The shut off valve is used as a
restrictor to provide this flow and should not be closed..

Our old pump had an adjustable bypass buit into it, and some
installations have one radiator, such as the bathroom, permantley
locked at a setting to provide this.

Harry