DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Plumbing (
-   -   Copper pipe size question (

greymous 01-27-2009 09:30 AM

Copper pipe size question
Sorry for the stupid question but I did not find an answer using the search feature.

I've got old steel pipes in the basement that I'm going to replace with copper. In looking around the internet I see that most of the diagrams use 3/4" pipes at first then switch to 1/2" pipes when they go upstairs.

Is there a reason to switch to smaller pipes (other than increased pressure)?


jporter5333 01-27-2009 02:12 PM

By upstairs, do you mean a second story upstairs or just through the floor from the crawlspace/ basement to a fixture?

PLUMBINGITALL 01-27-2009 06:39 PM

If you are talking about going from 3/4 to the fixture with 1/2; It is done to put a 1/2x3/8 stop on it. And then use a lav or toilet supply to the stop. Just for info the pressure does not increase by going to a smaller pipe. The pressure would be the same but the volume is decreased.

jporter5333 01-27-2009 07:25 PM

What PLUMBINGTALL said. I would recommend running a 3/4 trunk and then branching off with 1/2 to the fixtures.

Nestor_Kelebay 01-27-2009 11:20 PM


The most obvious reason for using 3/4 inch copper piping is that the inlet and outlet to the water heater are 3/4 inch in size. So, plumbers will typically connect to the water heater with 3/4 inch piping.

I agree with the previous posts about using 3/4 inch copper piping THROUGHOUT your house, and branching off to each plumbing fixture with a 3/4 X 3/4 X 1/2 inch tee. That way, the supply piping can provide full flow to two different plumbing fixtures simultaneously. That means your shower won't change temperature when someone flushes the toilet.

greymous 01-28-2009 07:38 AM

Thanks much! And another question.
Thanks everyone for the responses!

What got me curious was the pipe changes from 3/4 to 1/2 about 10 feet before it makes the turn up to the kitchen sink. Just seemed strange to change it there and not further along (or behind the sink). The other set of pipes going to the bathroom (on the 2nd floor) go up through the wall as 3/4.

New question: Looking at pipe types I see that there are "M", "L" and "K". I don't think I need to worry about "K" as from what I've read that is for outdoor use. For what I am doing (replacing old steel pipes in the basement) should I use "L" or "M"?

Are there elbows and connectors specific to "L" or "M" or are they generic.

Thanks again!

jporter5333 01-28-2009 08:44 AM

The fittings are generic as far as i know. In 13 years I've never seen a fitting marked with k,l, or m. The difference in each is the thickness of the wall of the pipe. I prefer to use type L but I am a commercial guy and it is generally required on any job I do. I use it on houses as well though. The main reason to use "m" is that it is a little cheaper than L so it gets used alot in residential to keep cost down and it will work fine.

greymous 01-28-2009 08:51 AM

Thanks jporter!

If the price difference is not too bad (I'll need about 110' of pipe) I'll probably go with "L" as I'd rather use better quality to begin with.

Maintenance 6 01-28-2009 11:27 AM

Type "M" is supposed to be used for low pressure spplications like heating systems. I'd never use it for domestic lines. The fittings are the same. As far as pipe size, a larger line will carry more water with less pressure drop. A 3/4 line will carry the same volume as 2-1/2 half inch lines will at the same pressure. That means exactly what Nestor said, flushing the toilet won't have as much impact on the pressure in the shower if they are fed from a 3/4 supply. Just keep in mind that for hot water lines, if you use a 3/4 supply, you'll need to run that much more water until you get hot water to the outlet. And when you are finished there will be that much more water laying in the pipe that you've paid to heat.

greymous 01-28-2009 12:32 PM

Thanks for the info Maintenance 6!

jporter5333 01-28-2009 07:29 PM

Not trying to start an argument but type m is fine for water distribution systems such as hot and cold domestic water. It is rated at over 300psi. But I do believe like we've all said that L is the better choice and it should be used unless you jusst can't afford it.

brokenknee 01-28-2009 09:00 PM

I really don't see the advantage of using L piping, IMHO it is just more costly. M has been used in residential plumbing for years. The weak points in any plumbing job is usually the solder joints, if you have good joints you should not have a problem. I have exposed plumbing in my laundry area that has been bumped and hit for years with no problems.

I am not sure were you are located but if freezing pipes is your concern then none of the three types are rated for freezing and they will all rupture. I know as I did a repair for a friend who thought if he used L piping to an outside faucet he would not have to worry about it freezing.

I would also consider pex piping, you will have some up front cost for some specialty tools. But once you have them your good to go. Working with pex is a breeze. You can buy it at the local home center in 500' lengths. The fittings are a little more costly but when you factor in you use fewer of them you are usually money ahead.

Pex pipe can freeze and not burst, however the fitting will.

I want to state as the previous poster said about starting an argument. I do not want one. The opinions I expressed here are solely my own. :)

Nestor_Kelebay 01-28-2009 10:40 PM

I agree that either Type M or L would work. Typically, Type M is used in homes and Type L in commercial applicaitons.

If it were my own house, I'd use Type L because it has a thicker wall. I've had several times when copper pipes have gone out of round on me, and every time it was a Type M pipe. The Type L is simply stronger because it has a 0.040 inch wall thickness as opposed to 0.028 inch for Type M (and 0.049 for Type K). That's more than a 33% increase in wall thickness over Type M, and that makes for a stronger pipe, even when hot at soldering temperatures.

If it were easy to change the type of copper pipe, it wouldn't matter. But since the lion's share of the cost of replacing the piping is going to be the cost of the labour, I'd opt to pay a few percent more on the total job cost and go with the Type L piping. It's going to change the cost of the job very little, but it'll make a big difference in the quality of the finished product.

brokenknee 01-28-2009 11:33 PM

Just curious, when you say gone off around you what do you mean? Are you saying the pipes ruptured? If it were a leak in the joint I do not think it would mater if the pipes were M or L, a bad solder joint is just that and will leak no matter how thick the pipe is.

I installed residential water softeners for about a year full time and a couple years part time after that. In all my installs that had copper pipe I do not recall one time any of the homes had type L copper in them. (I really enjoyed working on the old galvanized piping :laughing:)

I do not disagree that type L is a stronger thicker pipe, I just do not see the need for it in residential plumbing. The plumbing in my home is 23years old (I had the house built) The only problem I have had is one joint starting to leak. I cut it out and sweated in a new fitting, never have I had any other problems.

I know most, if not all of the builders in this part of the country have gone to pex for new home construction. I am aware they had some problems with one manufacture of the fittings when it first came out, but that has been resolved (not necessarily the pending litigation). With the price of copper, pex is now competitive cost wise and when you factor in the cost of the labor/time saved on installation in my mind it is the clear winner.

micromind 01-29-2009 07:15 PM

One disadvantage to using 3/4" everywhere and branching off with 1/2" is when you let the hot water run until it gets hot at the faucet, you'll waste a lot more hot water with 3/4".

When I built my house 12 years ago, I used type M everywhere except under the footing. It's 2 story, and on a well. I used 1" from the well to the water heater, then 3/4". I went down to 1/2" to feed the last 3 fixtures in the line.

This occurred in two places, both bathrooms. 1/2" feeds a shower, toilet, and sink. If I'm in the shower and another person flushes the toilet or washes their hands in the sink, I feel no water pressure or temperature change at all.

The key here is to keep the 1/2" runs short, and plan it out to minimize the number of 90's.


All times are GMT -5. The time now is 01:22 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1