I am analyzing the output data 'water water heater source temperature inlet' inlet in a simulation with a solar collector, when extracting the values ​​I noticed that at a certain time the temperature drops considerably and then increases again. What can cause this sudden drop in temperature?

I am analyzing the output data water heater source sorce temperature inlet in a simulation with a solar collector, when extracting the values ​​I noticed that at a certain time the temperature drops considerably and then increases again. What can cause this sudden drop in temperature?