Helloo
I have an issue while driving my truck, i feel heat ( air heat) at the legs part. Someone told me that the cause for this might be solved if my install a valve at the heating hose and close it during summer season. Below is a picture of the heating valve . Please anyone could advise on this issue

 

You're truck already has a valve there from the factory. Make sure it's opening and closing with the lever on the dash.

If you're feeling air movement from the ducting, you most likely have a bad seal on the fresh air flapper door. That is located behind the glove box in the dash. The seal is just foam and it degrades and falls apart with age.