All the panel based systems we have discussed so far use the sun to directly heat the water that we are going to use in the house. But water freezing in a frost-prone area may damage your collector because water expands when it freezes.
If you live in a frost-prone area you will need to consider the frost tolerance of your panels. You have 3 options if you live in an area prone to frost:
1) Select a system that has been mechanically designed to be inherently frost tolerant.
2) Fit drain-valves that drain the panel if the temperature gets too cold.
3) Use antifreeze in the water that goes through the panel.
If the system has been designed to be naturally frost-tolerant due to smart mechanical engineering - then I would argue that that is the most elegant option. Ask your panel supplier if this is the case.
I'm not personally a fan of the drain-valve solution because if the valve fails you will be left with a bill for a new panel if the water freezes.
The final option is the anti-freeze solution. Obviously you don't want to put antifreeze in your tap water (unless you want it to taste like New Zealand Wine) so you have to separate the fluid that flows through your panel from the water that is being heated by using a heat exchanger. When the hot anti-freeze flows through the heat exchanger it transfers its heat to the water without touching it. That is called a closed loop system, because the antifreeze goes round in a closed loop.
Another reason that you may want a closed loop system is if you have very 'hard' water in your area. This can leave mineral deposits inside the collector's copper tubing which eventually clog the water flow completely. Using a secondary heat transfer liquid in a closed loop can get round this problem.
At last, we get to our final choice when configuring our Solar Hot Water system : How do we boost it? Gas or Electric?