The vacuum tubes are a thermos type consisting of one inner and one outer glass tube which are sealed together at the top and bottom thus, creating a enclosed vacuum layer. Inside the copper heat pipe is a liquid. Once the sun heats up the pipe, the liquid increases temperature and turns into a gas. Once the gas comes into contact with the anti freeze (which is located in the manifold/collector tube), it transfers into the condenser/heat exchanger. The gas decreases in temperature and condenses back into a liquid, returning to the vacuum tube. When the refrigerant contacts the gas, it boils and relocates into the central heating and hot water system and thus the process begins all over again.