by a heavy spring, opening when the pressure gets too high. Some valves come pre-set by the manufacturer, while others allow you to set the cut-off pressure yourself. All pressurized solar thermal systems require relief valves.

Pressure relief valves are always attached to a run-off drain pipe: if they open, hot, pressurized water sprays out and you don't want it hitting anyone. Most building codes require the run-off to terminate within 6 inches of the floor, preferably in an out-of-the-way location.

You can also use combined temperature and pressure relief valves, which open when either circumstance demands it to decrease heat and pressure. Most building codes require these valves on any tank connected to a heat source, but you should never use them on a solar fluid circuit – the constant high temperatures are likely to trigger the valve inappropriately.

Motorized or "zone" valves

These are sometimes used to control an automatic shunt loop to protect against overheating on closed-loop storage heating systems. They operate on AC current (120V or 24V) and work in a specified direction, marked on the housing. It's best to avoid using them on the hot side of the solar loop because of the electronics involved.