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Mr Cool diy mini split
How do you know you have fully tightened the connection without tightening too much or too little when you are using 2 adjustable wrenches as advised
Just snug them up tight... they have O-ring seals so you don't really need to get them extremely tight.
Will you hear the connection being pierced for each of the 4 connections you make with the 25ft of lineset? Probably not on all 4 of them... not a problem.
Once you tighten the quick connections from the outdoor unit lineset to the 25ft of lineset, will you be able to tighten those lines any further without turning off the valves at the outdoor unit, if they require more tightening later, or do you need to turn off the valves with the allen wrench?
You can tighten those connections any time you need to with no regard to shutting off any system valves.
Lastly, how do you find out the pressure inside of the lineset lines as it seems it needs to be over 300psi for heating?
Measuring system pressures is only to be done by trained personnel using a gauge manifold and proper safety procedures when there is some reason to suspect a problem may exist... not necessary for end users.