First, let me say that solar thermal is a reliable, low-maintenance technology, but just like anything that wears out or a homeowner will find a problem, the following list of common failures is based on my experience with over 100 solar thermal systems.
1 - The pump is running, but the cylinder is not getting hot, but the collector is hot.
I get this a lot; it's a flow problem, but not always a pump problem. The collector is doing its job and the controller is working. The thing to check is the pressure. If the pressure is low or zero, then it is likely that some heat transfer fluid is being lost. The system should be tested by a qualified person before a recommendation is made to replace the pump. Filling it with air first will reveal if there is a more fundamental problem (and it won't flood the attic!) . Once a leak is checked, the system can be refilled with fluid. Get the air out of the system and set the pressure to 1 bar and the job is done.
2 - Solar thermal doesn't seem to kick in even when the sun is shining, and the frost function on the controller often runs in cold weather.
This is probably the stupidest reason for a solar thermal failure, but it can be an expensive repair. Checking the controller will reveal that the temperature of the collector is the same as the outdoor temperature. This is because the sensor in the collector is being pulled out by the birds! That's why this is happening.
The expensive and correct repair is to hire a scaffold tower to safely access the collector (assuming it is mounted on the roof). If you are prepared to accept a lower efficiency (but still better than no solar heat), the sensor can usually be pulled back through the roof and secured to the collector's flow tube. It is important to place the sensor as close to the roof as possible and to use Jubilee clips with thermal conductive paste.
3 - Knocking sound and system not holding pressure.
Usually this is a problem with the expansion vessel, if the vessel is tapped gently and there is no rattle (when the system is cold), it has probably failed. Since the expanded solar fluid has nowhere to go, the system's safety device (pressure relief valve) will operate. When the pressure relief valve is mounted on the roof, you can even see steam coming out.
Eventually, the valve will fail and stick in the open position, and it is this that prevents the system from maintaining its pressure.
4 - The pump runs but there is no flow, the collector gets very hot and the cylinder stays cool.
If all the air has been removed from the system and it maintains pressure, then the pump must be suspect. Early installations used ordinary household heating pumps. In some cases, if the fluid in the system was very hot, the pump would fail. The aluminum impellers (rotors) of these pumps became soft at 100 degrees and began to slide on their main shafts. Eventually, the pump's motor spins, but the impeller remains stationary, so there is no flow in the system.
These four are the most common problems I encounter in standard home systems. The rest of the failures are usually related to the controller or sensors. Some are too obvious to mention here, like leaks and blown fuses!"