What is the purpose of the circled pipe, which connects the pipes that go to and from the boiler to the hot water cylinder in my vented domestic system? The diagrams I have seen of other systems do not seem to have it. Is it to prevent the pressure from becoming too high if the valve allowing the heated water into the cylinder is closed (by letting the water instead flow into the return pipe to the boiler)? I imagine that the white knob is the handle for a valve that controls the flow in this pipe. It appears to be a simple manual valve rather than one involving a thermostat. The installer of the system set this knob so that it takes about two rotations to close it. What is an appropriate amount for this valve to be open?enter image description here

1 Answer 1


Often seen with woodstoves so that the water circulating in the heating loop was not too cold.

If the heating coil in the stove is too cold, the acids condense and accelerate the corrosion of the water jacket.

The one we have is a motorized valve that is temperature controlled to modulate the coil return temperature. Once the tank is sufficiently hot then the valve is fully open.

  • In my case it is a gas boiler (about 10 years old), but perhaps the purpose is the same. In that case I imagine that having the valve a little open, as the installer left it, is appropriate so that some hot water returns to the boiler without diverting too much from the hot water cylinder. May 30, 2021 at 20:02

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