1999 4wd Blazer LT blows hot air all the time
 

My blazer blows constant heat even when all settings are turned off.
It does not have the auto climate control, I have to use a manual dial with the blue and red colors for temperature control.

Can some one help me to root cause the problem so I can fix/replace components and stop having this condition?

I red in the web, that this might be a federal or safety regulation which sounds very strange for me.

Please advice,
FT III

Could be a blend door issue. Check the vacuum lines under the hood & make sure they're all hooked up & not cracked. They get cracked & you lose the vacuum that is what makes the blend door move.
Try running the engine & spraying brake cleaner over the vacuum lines (don't be smoking when doing this lol). If the engine revs at any point, you've found a leak in the line.