On top of the standard two-minute monthly maintenance checks , drainback systems need extra care. There are four main problems that could occur in a drainback system:

1. The system freezes and things go seriously wrong.
2. The controller fails.
3. There is a loss of solar fluid .
4. The main pump fails.

In all but the first situation, the result is thankfully quite simple: the system stops working until you repair the problem. A freeze is obviously a lot more complicated and problematic, since it can cause catastrophic damage to the solar thermal setup and all sorts of horrible peripheral messes from leaks and water damage.

If your system does freeze, it will be because of a controller failure or extreme climate conditions. A failed controller is the most common cause, as the failure may keep the solar fluid (which is often distilled water) circulating when it should be drained out of the pipes and collectors. Although freezes are thankfully unlikely, make sure you check the controller is working properly before switching the system back on.

The other three situations can all be detected by visual examination:

#2. Controller failures can be detected by the system pump(s) remaining inactive on a sunny day. The exception to this is if the high limit has been reached and the system has shut down to prevent overheating the storage, so check whether the storage tank is hot before assuming the controller is broken – if it is hot, the high limit has tripped.

#3. It is normal for solar fluid to evaporate over time, though an annual check and top-up is sufficient in normal circumstances. Your drainback tank should be fitted with a sight glass so you can see the fluid level inside – if it has dropped significantly in a short time, you will need to check for leaks in the solar thermal setup.

#4. Pump failures are very obvious because the system stops heating the storage tank. When pumps fail, they do it quickly – they either work or they break – and the results are simple to detect. A main circulation pump failure results in no heat in either the storage tank or the drainback tank after a sunny day. If you have a second heat exchanger pump in your system and that breaks down, then the drainback tank will be hot but the storage will be cold.