One of the most complicated components to size in any solar thermal installation is the pump (or pumps if your chosen schematic requires more than one). All solar thermal pumps are centrifugal models, using vanes or paddles which spin within an enclosed space to create pressure. Your choice of pump will depend on the type of system and two measurements: flow and head.

• Flow is the volume of liquid moving through the piping circuit every minute.
• Head is the pressure produced by friction and liquid volume: static head is the pressure exerted by all the liquid in the system above the resting level (vertically) and friction head is the pressure produced by the fluid as it flows through the pipes.

Solar thermal systems use two main types of pump:

• High-head pumps can push a large vertical column of water a given distance. In solar thermal installations, they are mostly used to kick-start a solar loop which is not always full (in drainback systems). They cannot be powered by a PV panel as they need to provide a lot of instant pressure to overcome both static and friction head before the system is operational.
• Circulating pumps are low-head and are used in solar loops which are always full. They only have to overcome friction head, so they can start slowly and can be powered by a PV panel in many cases.

In either case, choosing the right pump size is very important. An undersized pump may not be able to start the system or may not pump solar fluid around quickly enough, possibly resulting in overheated fluid; an oversized pump may pump solar fluid too quickly and not allow enough time for it to heat, reducing the overall operating temperature of the system.

You will need to calculate the required flow through your system and the head the pump needs to overcome. Once you have done this, you can determine which type of pump you need and, using the flow and head figures, the precise model for your installation.

If you're installing a system that uses an antifreeze mix, you will also need to account for slipperiness. Glycol is not as easy to pump as simple water, especially at lower temperatures (though it's similar at high temperatures), so any solar thermal installation that uses an antifreeze mix needs a pump that can manage the extra requirement. This is especially true if you're using a high-percentage mix.

You also need to ensure that there is enough straight piping on either side of the pump, so that it operates properly. As a general rule, you need 4 times the pipe size on the suction side and 2.5 times on the discharge side. So if you're using one-inch diameter copper piping, you'll want 4 inches of straight line before the pump and 2.5 inches after. These measurements exclude any fittings, so remember to allow extra space for those!