On Feb 4, 10:19*am, Harry Parkes wrote:
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.


I don't believe it is for the flow resistence; most CH pumnps are
designed to handle zero flow consitions. AIUI it's a tempertaure
issue: If TRVs all switch to off then (without this loop) there
would be no flow at all and the boiler "flow temperature" sensor would
not detect the rising temperature of the water in the heat exchanger
which could then lead to overheating.

Such a loop, with a tap in it set to 'trickle' is used so that there
is a small flow even if all the TRVs are off. Thus the flow temp
sensor detects the high flow temp and turns off the heat properly.


Robert