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Bozeman, MT

Residential water pipes, 1/2" or 3/4" ?

I am looking toward sometime needing to replace the old galvanized water pipes in house. The pipes are currently 1/2" galvanized pipe. Originally installed in the 1930s or so with some few changes over the years. The basement is unfinished so getting to the pipes is not a big deal. I don't know if I would use copper pipes or go with PVC or PEX.

The water supply and water heater are directly under the kitchen. The bathroom is ~30 feet away.
The Cold water pipe runs directly up to the kitchen sink and then over to the bathroom sink, toilet and then shower.
The Hot water pipe runs to the bathroom sink first and tees to come back to the kitchen sink.
I don't know how come it was done that way... Water flow issues to try and avoid extra hot or frigid in the shower? It didn't work. Any time somebody is in the shower and somebody else runs water in the kitchen or bathroom sink or flushes the toilet, the water temperature in the shower changes. Run Hot water elsewhere, the shower gets cold. Run Cold water, the shower gets hot. Other Guesses? Why did they do it that way?

I don't really want to go with a special shower valve to try to deal with shower temperature changes.

If I use 1/2" pipe, then flushing the toilet or running water in the sink will still cause the temperature in the shower to change.
If I install 3/4" pipes, then I waste more water down the drain before the Hot water gets to the shower.

How do I figure out what size of pipe to install? Is it going to make a difference?
Should I do a home run with water pipes to the shower?

Fairfield, CA
Those 1/2" galvanized pipes have an inside diameter that is just a fraction due to corrosion after all those years, going with anything new makes a huge difference in comparison.
You may still have temp changes in the shower but not nearly as severe.

Carnegie, PA
3/4" piping will make no difference for scalding/freezing in the shower.

If I were doing your repipe, I would use a 3/4" drop of pex from the main to your water heater (picking up all colds along the way), then 1/2" pex for all other connections, hot and cold. Unless you have 3-4 bathrooms, and a big waterheater to accomodate simultaneous usage, 3/4" isnt necessary throughout.

If you can find a plumber that uses PEX-A product with the expander (NOT the crimper), I would highly recommend it. PEX-B (crimp stuff) isnt rated to expand like pex-a is, and could have less of a life and/or freeze protection due to its unexpanding nature.

For your shower issue, you do not need a new valve, you need one of these before your shower valve, easy to put in under the floor while you're repiping. I have one and love it.


good luck

if it aint broke, tweak it!!
currently on FiOS (kick aZZ!)

reply to ttiiggy
PEX tubing is so cheap and so easy to work with!

The cost of all of the fittings will be much more than the tubing.
Because of the low cost of the PEX I made a ring around the house for the cold in 1/2" PEX so I would get full water flow to all of my outside taps.
I installed 3/4 new water pressure reducing valve. (60 years should be enough to expect the old one to work)

The hot is in 1/2 inch PEX, no ring around the house, If I use the kitchen sink full hot water flow, which is before the bathroom, it does not change the shower temperature.

If I use the garden fitting, that has no restrictions to flow, yes the shower temp changes.
By the way, I now have hot and cold outside taps.

Did I say PEX is the best to work with?


reply to ttiiggy
actually, it's standard practice to use a 3/4" main line for hot and cold, and tap off that with 1/2" pipe for each fixture, that way, the pressure drop is minimized. think of it this way, if you made a T out of the same size pipe, and put a valve on each side of the T, and fed it from the bottom, if you open one valve, you'll have a certain flow, BUT if you open the other too, you'll only have 1/2 flow from either valve!. now if there's another T on top of that with valves too, you'll only get 1/4 of the flow at one valve with all of them open!. with 3/4" pipe used for the center of the T, and 1/2" pipe for the sides of the T, you'll get 1/2 the flow from one valve with them all open!. anyone that doubts this can make the T's up and prove it to themselves!.


Greenville, SC

2 edits
reply to ttiiggy
I just re-piped a house for a friend. He had old, blocked-up galvanized mixed with copper. Bad mix.

I went back with 3/4" for the "trunk" for both hot and cold. Then fed a manifold »www.google.com/products/catalog?···Q8wIwAw# that had 1/2" feeds off to each room. So there was a 1/2" feed to each room that fed both the tub and sink, not a separate 1/2" feed to each fixture.

No flow problems at all for him. Another benefit of the manifold is that you can turn off any fixture to work on it instead of the whole house.


Youngsville, NC
reply to ttiiggy
A manifold would be nice, but your house layout needs to be compact / good central point for it to really work well.

Since I have redone my house in PEX, I would offer the following suggestions -
#1 - 3/4 inch for main lines, or long runs. If you can find it - I'd even use 1 inch for your main trunks.
#2 - 1/2 inch with shutoff valves for all fixtures
#3 - DO NOT put compression fittings where you can't get to them !!

If you have bathroom out by itself, you can possibly get by with just 1/2 feeding all the fixtures for that room. I did this for a bathroom that doesn't get much use.

The worst problem is figuring out how to replace your water feeds from old to the new PEX. I ended up drilling down to the crawlspace for most of them, essentially abandoning the old fittings. You may not be able to do that, so you need to figure out how to join them.

All my PEX was done with crimps, not expanding. As long as all connections are done correctly, you shouldn't have a problem with either (and most of my PEX is in the unheated crawlspace). PEX will work with heat tape if necessary (my 40 gallon water heater sits outside the crawlspace, with insulated / heat taped connections. As long as the heat tape is working - I've never had an issue with it freezing , and it has seen single digits here.)


Decatur, GA
reply to PrntRhd
Those 1/2" galvanized pipes have an inside diameter that is just a fraction due to corrosion after all those..

Agreed! I couldn't see light through a 2 foot section of 1/2" galvanized pipe when I redid my house. The temperature drop in the shower is mainly a flow problem due to blockage. It will improve greatly with new pipe.

Usually 3/4" is run on the long main runs and it's reduced to 1/2" for the risers to sinks and faucets. Using newer faucets with flow restrictors thoughout the house may also help. The newer flow restricted faucets are better at providing a good steady stream of water at reduced flow, but you need to put them on all the fauctets and toilets to work best together. One unrestricted faucet will hog the flow away from the shower.