Let's walk through some example calculations for choosing the correct solar thermal system pump. We'll assume it's for a pressurized antifreeze system, so there's no static head to worry about, and we've installed three collectors, about 100 feet of Type L copper piping, one heat exchanger and a variety of fittings and valves.

1. Collector array flow and head

We've used three flat-plate collectors, so we'll go with the manufacturer's recommendation of 3 GPM as the required flow (1 GPM per collector). If we had no other guidelines, we could use the SRCC's standard of 0.88 GPM per collector, resulting in 2.64 GPM.

Checking our manufacturer's documentation, we find that the collectors have a friction head of 0.25 feet per collector, which means a total of 0.75 feet of head.

2. Piping and fittings friction head

We've used 100 feet of ¾-inch Type L copper piping. In total, our fittings come out with another 33 feet of equivalent tubing, which makes a total of 133 feet. Referring to the graph that shows head per 100 feet of different sized Type L pipes, we can see that ours is rated at 3 feet per 100 feet of pipe (at 3 GPM).

(133 feet of pipe ÷ 100) x 3 feet = 4 feet of friction head

3. Heat exchanger friction head

Our chosen model of heat exchanger is rated at 1.15 feet of head. We'll add this to the total.

4. Calculate the total

Since we're installing a pressurized system, there's no static head. Our total head is as follows:

Collector head + Piping and fittings head + Heat exchanger head = Total friction head

0.75 + 4.00 + 1.15 = 5.90 feet of head

5. Look up the pump curve

All pumps have a pump curve – the amount of head they can push at a given flow rate. We need to check whether our chosen pump can manage, so we check the amount of head on the left and see where it crosses the curve. If it crosses at 3 GPM or better (but not too much), we're good to go.

You can find pump curves in the manufacturer's documentation, which is usually available online. Bear in mind that you want to aim for a pump which provides slightly more power than you need – it's easy to add head to a system (for example, by opening a ball valve on the loop and slowing the pump) but it's impossible to remove head without changing the installation layout.

You also need to remember to adjust the pumping curve if you're using a glycol mix at lower temperatures, where it is slippery and harder to pump around the system.