A check valve is a simple device that only allows liquids to flow through it in one direction. These valves have an arrow marked on the outside to show the allowed flow direction. Two different types are used in solar thermal installations: "spring" and "swing" check valves.

Spring check valves

These check valves control the flow direction with a spring, upon which sits a ball or door. The spring keeps the valve sealed unless the pressure of the flowing liquid compresses it and pushes it open.

Spring check valves can be used on any pipe orientation, whether vertical or horizontal, because they do not rely on gravity to operate. The price of this is more individual components to break and the fact that many valves are built for systems with powerful pumps and higher flow conditions than normal for a solar thermal installation.

While soft-spring versions are available for lower-flow systems, they are more expensive, so some experienced solar installers break open standard hard-spring commercial models and cut the spring to reduce resistance. If the valve is being used on a vertical pipe with upward flow, the spring can be removed completely, allowing the ball to work more like a swing check valve, relying on gravity.

Swing check valves

These check valves control the flow direction by using a simple swinging "door" mechanism. Like a house door, the valve's door only opens in one direction – when the liquid flows correctly, it pushes the door open, but if it tries to go back it pushes the door closed and stops the flow. They're generally a better choice in solar thermal systems because they work with lower-pressure flows.

Swing check valves are simple devices with no gaskets. They're normally made of very durable bronze, with the door closing onto a machined, flat surface to stop any liquid flowing. The contact is metal on metal.

Swing check valves can be used on any horizontal piping, but can only be used on vertical pipes where the correct flow is upwards, against gravity. Installing them on pipes with a downward flow – even a very slight downwards slope – will cause problems and valve failure. Up only!

Other valves

Some installer use motorized valves, but in almost all cases this is an unnecessary complicated and just adds additional parts which can fail. If a basic, mechanical valve will work, use one.

It's also important to note that many water pumps come with small, plastic check valves already installed. You need to remove these because they are not as reliable as a metal alternative and the solar loop should only have a single check valve in place (you risk isolating parts of the system if there are more).