Panel Type Solar Water Heater Heat escapes through the air SUN Tube Vacuum layer acts as a wall to prevent heat escape 2. The degree of air vacuum is demonstrated by various representative products in the illustration above. The closer this figure to 0, the more complete the vacuum condition. In the chart below, there is more vacuum as it approaches the bottom, exhibiting greater heat-blocking ability. Very close to complete vacuum.
Blocks heat well and keeps hot water hot.
SUN Tube is closer to near vacuum than even a thermos. Hot water is available morning, noon and night.