Evacuated-tube collectors can get very hot, exceeding the boiling point of water and can cause significant issues in an existing domestic solar water system. You need to use your hot water every day to ensure the temperature doesn't overheat in the tanks. Mixers are easily installed just after the last hot water tank and mix your regular (cool) water supply with the hot water, to ensure the temperature never exceeds a set limit.

Notice on the picture above, the black hose leading away from the pressure relief valve on the top of the tank. After only a single day without use, this tank could exceed the temperature and pressure levels, blowing hot water onto the lawn and killing the grass. The black pipe was added to safely take the hot water away to the drainage system. On a ground mounted panel, you can also cover the unit before leaving the house for an extended period. Be careful of the tubes when covering them, as they are made of annealed glass, which is more delicate than tempered glass. You need to consider overhangs (slide ice and snow loads), falling branches, etc., when selecting a location for your panels.

Evacuated-tube collectors don't heat up like flat-plate collectors. Thus in the winter, they won't melt large quantities of snow that fall on them at one time. It can be very difficult to clear the snow from the glass tubes without breakage, be careful. Of course, heat-pipe evacuated tubes will not impact the system if they break - you just have one less tube to heat the water. If a direct-flow tube breaks, you have a problem!