Since evacuated tube collectors are, by necessity, a closed system inside their collector box, many of them use a neat technological idea called a heat pipe. Instead of simply passing the gathered heat out along a piece of metal to the manifold (and thence to the storage tank to heat your water), they use a two-stage approach.

Each evacuated tube in the collector contains a hollow tube which is partly filled with fluid – water, an alcohol/water mix, ammonia or some other proprietary mix are all used. A vacuum is created within that tube and it is sealed. This causes the liquid's vaporization point to be reduced (the laws of physics demand it), which means that the liquid will turn into a gas when it gets heated. Why is this important? Because when a liquid turns into a gas it absorbs an enormous amount of energy to fuel the transition – so when the transfer medium inside those heat pipes warms up under the sun, it'll vaporize and absorb a lot more of the sun's power for extra efficiency! Even better, when the gas turns back into a liquid, it releases that extra energy. So when the vaporized transfer medium rises to the manifold, hits the heat exchanger and cools down, it turns back into a liquid and releases a ton of extra heat to the solar fluid before dropping back into the evacuated tube. That heat then gets carried off to the storage tank and warms your water.

Some models that use heat pipes also include an automatic safety switch that changes shape when the pipes get too hot and cuts off any heat transfer to the solar fluid. This ensures that your solar thermal system doesn't get damaged.

If you're installing a system with heat pipes, remember to install the rack at an angle of at least 25 degrees, to ensure the fluid flows properly. It's important to note that heat pipes aren't used in all evacuated tube systems. Those that don't use them are called "direct flow" or "flow through" evacuated tubes and almost exclusively use two layers of glazing with a single, sealed end connecting them to the manifold. Their riser tubes come in at the sealed end, run down the absorber plate and back up the other side, exiting by the same end. It may seem odd but it means there's one less seal than if the riser passed straight through, which is one less thing to go wrong! Even though heat tubes are so efficient, direct flow systems are generally more efficient overall because the solar fluid gets right down inside the evacuated tubes, rather than depending on a tertiary heat exchange.