"David Hansen" wrote in message
...
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.

Correct. To prevent condensation inside the boiler by raising the return
temp. With having low temp UFH on and rads off, the return temp will be too
low for the boiler and condensation will occur inside the boiler rotting it.

It should have a blending valve on the boiler return keeping the return temp
just above dew-point.

As it stands it probably just needs the gate valve cracked open.