Evacuated tube solar collectors can get very hot, above the boiling point of water and can cause significant problems in an existing solar hot water. You need to use hot water every day to ensure that the temperature does not get hot in the tanks. Mixers are easy to install right after the hot water tank and mix their past regular (cool) water supply with hot water to ensure that the temperature never exceeds a set limit.
Note that in the previous image, the black hose which moves away from the pressure relief valve in the upper tank. After only one day without use, this deposit could exceed the temperature and pressure levels, blowing hot on the lawn and kill the grass. The black tube was added so you can use the hot water into the drainage system. In a panel mounted on the ground, you can cover the unit before leaving the house for an extended period. Take care when they covering tubes, since they are made of annealed glass, which is more delicate than the tempered glass. Be aware overhangs (ice slide and snow loads), falling limbs, etc., by selecting a location for the panels.
Evacuated tube collectors are not heated as flat plate collectors. So, in the winter, not melt large amounts of snow falling on them at the same time. It can be very difficult to clear snow from the glass tubes without breakage, be careful. Of course, heat pipe vacuum tube will have no impact on the system if they break - just a down tube to heat the water. If you break a direct flow tube, you have a problem!