In May, I turn off the hot water that heats my condo in the winter. The photo shows the red-handled, quarter-turn valve in the OFF position (perpendicular to the piping) for the RETURN line. There is little hot water flow in the summer because the circulating-pump is not running, but with the valve OFF there is ZERO flow. This can improve condo cooling in the summer. Do NOT use the "HEAT" setting on your thermostat when the RETURN valve is closed.
If you turn off the wrong valve you'll get no hot water out of your spigots. All we need now is to remember to turn the valve back ON in September so that we can get heat.
The emergency-pressure-relief valve piping is also shown, properly positioned in the white, plastic funnel-drain.
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Your Friend in Real Estate, LLC
Arlington, Virginia, USA