Type M or Type L?

So I get the fact that everywhere is different, but I have a couple of plumbers saying two different things. The one says type L copper for everything except heating, and the other one says he really never uses type L. He says he uses M for everything unless it requires type K. My clients were asking me and think type L should have been used for the heat lines (I think they are thinking too deeply about pipes that have been there for 40+ years). My boiler has mostly type M, and all my supply lines are M. I almost always see this, but the first guys says no… What are your thoughts? I looked up all the info, I was curious more about opinion and practicality.

Need K for underground, main service line. As for M or L, comes down to $$. L is better for heat since thicker and less chance for any corrosive pin holes etc. but heating pipes dont need a high pressure rating, so M can be used and is seen a lot cause its cheaper. Get what you pay for. New heating and piping I personally would ask for L, at a higher cost of course, but average person had no clue about M or L and its probably whatever the plumber uses is what they get. Maybe some areas have local codes but overall both are ok.

Awesome, thanks

Great info. Thanks