Finding source of coolant leak?

[c4tobe]

Hello everyone,

I recently noticed a coolant leak in my Citroen C4 (petrol, 1.4cc, 2004).
The leakage exacerbates when I use the cabin heater in the Hot position, in this case, the bottom part of radiator, and the surroundings gets really wet, and the coolant level at reservoir goes down significantly.

But, if I do not use the heater, the coolant level reduction in reservoir is not noticeable.

Now, the question is what are the common causes for this problem? and how to diagnose the problem and repair it?

Thanks in advance.