WHat is differnce btwn Type M and Type L?

Is it interchangeable?

There is a difference. It's the thickness of the pipe's wall. Type L is thicker. I was advised, in no uncertain terms, that I wanted to use only Type L for when I replumbed my bathroom. I'm not sure if Type M would've worked or not.

Are they interchangeable? I suppose that depends on what you're gonna use them for. For residential plumbing, I hear Type M is more susceptible to pin hole leaks due to the weaker/thinner pipe walls. Course, that's not a ton of water preassure, but better to be safe than sorry.

Type "M" is made for DWV! (drain, waste, vent) Not for water pressure.. I know HD sells it in 1/2" plumbing size, and some unknowing folks buy it... Use type "L" for pressure............

PS. You guys comeing to the "reunion" next Aug.??
Lynne said it's OK if you just sit shyly in the corner!!

So it easier to remember to just always go wiht "L"...

asked the wife about it....says it sounds like fun...she's more of a people person than I.

Shes been wanting to go to NH for a while now...

They are interchangeable in that the O.D.'s are the same and, therefore, standard fittings will fit both types. I have a mix of M and L in my old house and have never had a problem with pressure failures on a well system with a pump switch set for a 40 PSI to 60 PSI range.

M is THIN walled, and L is MEDIUM walled, and K is THICK walled pipe.
See
www.acehardware.com/info/index.jsp?categoryId=1280921&infoPath=1280382

for Excellent info on the different types that I have found.

You can really tell the difference if you are working with "L" then pick up a piece of "M". I just did that.

I am installing baseboard heat. Not sure if I should mix typoes on this job. Have to leave it up to an expert.

You can use "M" for baseboard heating also!!
There are folks that use "M" for water pressure, but then again there are people that ride around on bald tires also!!

Yardmaster:

Type M copper pipe - is approved for residential construction (houses). It's the thinnest SUPPLY piping you can get. DWV (Drain, Waste and Vent) is thinner wall still, but it's only available in sizes 1 1/4 inch diameter and up, and it's used for drains and vents where there really is negligible pressure inside the pipe.
Here in Canada, Type M pipe will have red printing on it so that the inspector can tell what was used without cutting the pipe open. DWV piping will have yellow printing on it.

Type L - is thicker wall than M, and is approved for commercial use (office buildings and apartment blocks, etc.) Type L will have blue printing on it.

Type K - is the thickest supply pipe available, and it's used for underground applications. It'll have Green printing on it.

All of them have a 5/8 inch OD. It's the ID that changes. So, you can use the same fittings on Type K that you do on Type M or L. They don't make copper fittings in different grades like they do pipe.

It's actually hard to tell the difference between them by looking at the wall thickness in that they all look about the same. However, as mentioned by someone else, it's actually easy to tell by weight. Type L will weight more than you expect if you're used to Type M. Similarily, Type M will seem awfully light if you're used to Type L.

I try to avoid using Type M because it's been my experience (or bad luck) that Type M tends to go out of round much more easily when it's hot and you're fighting with a stubborn solder joint. When a copper pipe goes out of round when it's hot, it's a real problem because then you can't solder a fitting onto or off of it without a fight.