Unlike flat plate collectors, vacuum collectors can reach very high temperatures if there is an imbalance between the collected energy and the load. A good design foresees the over-performing of vacuum tubes during high insolation* seasons (Spring, Summer, and Fall) and uses an artificial load to protect the water heating system from over-performing.
*) Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m²) or kilowatt-hours per square meter per day (kW·h/(m²·day)) (or hours/day))
An example of a heat rejection loop using a radiator: