Since a pressurized system has a solar loop which is completely filled with fluid (the industry standard being a mix of water and propylene glycol), it is very important to get all the air out of the system when you're setting up. This is doubly important because the solar fluid only needs to be checked every 10 or 15 years, so it's going to be in there a long time before you look at it again.

Air bubbles in the solar loop make it harder for the pump to push the solar fluid around and therefore reduces efficiency. In worst-case scenarios, the bubbles gather together and form a block which is extremely difficult to budge and could even prevent circulation. This is known as "vapor lock".

The best way to deal with air accumulating in the system is to install a release for it. You can use an automatic valve, a Schrader valve or a coin vent, but whichever you choose it must be installed at the highest point of the system. Trapped air is lighter than solar fluid, so it will always try to make its way to the highest point in the system. This is usually on the roof with the collector array.

Once you have located the highest point of your solar thermal system, install a short riser, separated from the solar loop by a ball valve. The system exit goes at the end of the riser, whatever type you choose. During maintenance, you can open the ball valve and let the trapped air into the riser, then close it and open the exit valve or vent to release the vapor.

If you decide to use an automatic air vent , make sure that it is a special "solar" valve with a cast body and a high-temperature float. Non-solar models are likely to be distressed by the high temperatures they will experience.

Special notes on automatic fill valves

Some solar installers are so used to setting up traditional hydronic heating systems that they put an automatic fill valve in the solar loop. A traditional heater needs an automatic filler to keep the system topped up with water and maintain pressure.

It is vital that you do NOT allow the installer to do this.

A solar loop is not part of a traditional system. Automatically injecting water into the loop dilutes the propylene glycol mix and decreases efficiency. It also increases the mix's freezing point and could result in catastrophic effects in freezing weather.

If you want to add a component to top up the solar loop, use an injection pump and supply it with a pre-mixed solution that matches your solar fluid mix. Be warned that the injection pump must be very carefully calibrated and regularly maintained to avoid building up dangerous excess pressure in the solar loop.

Heat exchangers

Pressurized systems can use external or in-tank heat exchangers. The simplest and most compact solution uses an exchanger inside the tank, though both types can work with a single pump.

If you opt for an in-tank exchanger, the pump