As soon as you start looking into a solar thermal installation, you will run into certain terms and principles that may be unfamiliar. It is essential that you understand these ideas, so that you can see what is needed and make intelligent decisions when offered choices by professional installers.

Water quality

Everyone's heard the terms "soft" and "hard" water. They refer to the amount of mineral content dissolved in the water supply in your area. Hard water has more minerals in it and, when the water is heated, those minerals can precipitate out and solidify into mineral buildup (or limescale, as it is also known). Water softeners remove those minerals and thus reduce the buildup.

Limescale causes all sorts of trouble in solar hot water systems. For starters, it reduces the efficiency of any heating elements and heat exchangers that are in direct contact with the water, just like in a standard kettle. On top of that, it reduces the efficiency of solar collectors by building up inside them and acting as insulation against the sun's heat.

Even worse, limescale buildup in systems with small waterways – especially those in the collectors – can block the flow of water or solar fluid. Not only does this seriously reduce efficiency but it also increases the pressure in the system, puts more load on the pump and adds to the risk of burst pipes.

Freezing

All liquids freeze, though some do it at much lower temperatures than others. Water is one of the worst culprits for freezing at relatively high temperatures: while glycol mixes will retain a liquid form well into negative temperatures, water freezes quickly, at 32°F (0°C).

Water is also an oddity. Most liquids get heavier and condense as they approach their freezing point (the opposite of when they're heated) but water doesn't. It gets densest and heaviest at 39°F (4°C), then expands by enormous amounts when it reaches freezing point. In fact, this oddity of water is one of the main reasons there is life on Earth: the water at the bottom of a lake is always above zero, which allows life to continue to thrive, even when the surface has frozen.

Unfortunately, solar thermal systems are negatively affected by this particular oddity of physics. If the water in your system freezes, it will expand enormously and is almost guaranteed to burst the pipes, catastrophically affecting the installation. There are no pipes that can resist this kind of pressure: freezing water can break solid rock, and rigid pipes are relatively easily fractured.

Heat transfer

The basis of all solar thermal systems is the transfer of heat – from the sun's radiation to a solar fluid , from the fluid to a heat exchanger and from the exchanger to the domestic supply. There's no need to understand the physics of actual heat transfer, but it is important to understand where heat goes and what happens when it does.

Heat does not rise. That may surprise you, but it's a common misconception. Heat moves from a warmer location towards a colder one, in