Water tank heating - need help

[Beho]

Hi Everybody ...

I want to use LK3 to control the heating of 1000L hot water storage tank. 
If the water tank is cold, I need to start the heating until whole storage is hot. 
After that the heating will be off, until whole storage will be cold.

I need to start the output with one condition and stop it with another. 

Is it possible ?

There are two DS18B20 thermometers. One is on the top of the tank (T1) and one is in the bottom (T2).
If the temperature of T1 will be under 40'C the heating will start, until the T2 reach 40'C. 

I try to make an event, but i doesn't work, because if I use condition If (T1) < 40'C -> out 0 == 1;
the temperature of T1 increase to 40'C and out 0 goes 0, without waiting for second condition.

Can somebody help ?

Thx.