What are Active and Passive Solar Thermal Systems?

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When you're considering installing a solar thermal system, it's very important to consider all your options. Some of your choices will be governed by budget and climate; others are governed by what's practical for your home.

The choice of an open or closed loop depends on your budget and personal choice: a closed loop keeps the heating medium (air, antifreeze, water) completely separate from the water it's heating, whereas an open loop passes the water in your hot water tank around the pipes and heats it directly. Open loops are less expensive, slightly more efficient and require less maintenance; closed loops need their antifreeze changing periodically (much like a car).

Regardless of whether you opt for an open or closed system, you'll need to choose an active or passive pumping method for the water.

Active systems incorporate an electric pump which pushes the hot water into your home's plumbing and draws cold water into the heating zone. They work no matter what the weather does (including freezing as long as you use a gravity drainback setup) and where the panels are mounted but, of course, they use electricity. They're also a lot more complex, especially if you install a closed loop system, and therefore have higher maintenance demands.

Passive systems use the natural ability of hot water to rise to keep things circulating. Water is heated by the solar thermal panels and rises out, to be replaced by cold water coming in from the tank. They're cheap to install because they're extremely simple systems with no need for pumps, sensors or complex controls.

The downsides are that the water only circulates as long as the panels heat, the tank has to be above the collector (which pretty much rules out roof-mounted collector panels in most cases) and they can't be used in areas where freezing occurs.