Pressure Concerns in Pressurized Solar Thermal Systems

You are here

As the name suggests, a pressurized solar thermal system operates under pressure. In particular, the solar fluid circuit is always under a certain amount of pressure, which can vary enormously depending on the local climate and a variety of other factors. It is vital to understand the effects of pressure and how the system is managed to avoid catastrophic events.

When the installation's solar fluid loop is first charged – usually with a mix of water and propylene glycol – it is put under pressure. This is because liquids change in size as they get colder or hotter and the pressure in the loop can vary enormously, depending on the local climate. It's not unusual to see the pressure go from 10 to 45lbs/in² seasonally, or even daily in some locations. Your solar hot water system needs to be able to handle those changes.

Liquids get smaller when they get colder and bigger when they get hotter. Let's look at what would happen if no measures were taken:

  • When the temperature drops, the pressure inside the solar loop falls and could even become negative. If this happens, physics demands that the pressure be equalized, so the system pulls air into the pipes by whatever means it can (through valves and other components). That air will inhibit or even block circulation through "vapor lock", which is very bad for your solar thermal installation.
  • When the temperature rises, the pressure inside the solar loop rises. If the temperature rises sufficiently high, the solar fluid expands so much that it could potentially burst the pipes. This is obviously catastrophic for your solar hot water system.

For colder climates, the pressure is typically a minimum of 35lbs/in² (with the fluid at 60°F); in hotter climates it may be less. You should also install a pressure gauge on the closed loop; make sure you install it upside-down. That may seem strange, but if you put the gauge above the pipe, you risk getting air trapped in it. You'll get used to reading it the wrong way up, don't worry!

Along with the gauge, you should also install a pressure release valve (NOT a combined pressure/temperature valve) set to open at 90-100lbs/in² or to the lowest-rated component in the system, so that it triggers before high-pressure air and liquid gets blown out through your gear!

Make sure there's a drainage pipe attached to the release valve to carry the hot fluid away safely, preferably to a termination point near the floor. It's best to have that pipe terminate indoors and put a pail under it – if you see liquid in the pail, there's a problem that needs handling.

You'll also need to install an expansion tank on the closed loop, to help handle the pressure variations. Like the pressure gauge, the expansion tank should be installed below the pipes – in this case to avoid as much heat as possible, to prolong the life of the membrane or bladder inside the tank.

The best place to install an expansion tank is just before the circulating pump (in