Pressurized solar hot water installations have a closed solar fluid loop that is always full of liquid. If the collectors get colder than the storage tank and are situated above the tank (as is usually the case), thermosiphoning can occur.

If this happens, the warmer fluid in the heat exchanger will rise up the pipes and enter the collector array, and colder fluid will fall to the exchanger. Basically, the system reverses direction and your domestic hot water supply starts to heat the collector array instead of the other way around. This is not good.

The simplest way to stop unwanted thermosiphoning is to install a check valve. This stops the solar fluid circulating in the wrong direction and also makes sure the fluid flows the right way when you're installing everything – two birds with one stone!

There's one additional consideration here: extended vacations and holiday homes. If you're setting up a solar hot water system on a building where the solar fluid can lie idle for a while (a couple of weeks or more), you increase the risk of the fluid overheating, degrading and corroding your equipment.

In this case, thermosiphoning can be used to your advantage: it doesn't matter to the hot water supply, since you're not living there, and it helps maintain a steady temperature throughout the system that prolongs overall equipment life. To set this up, you can install the check valve as normal and include a "vacation bypass" circuit of pipes that runs around it.

The diagram on the right shows how to set up the circuit. Use a full-port ball valve on the extra circuit so that you can keep it closed most of the time and, whenever you go away for an extended period, open the valve. Just remember to close it again when you get back!