View Full Version : 1/2 inch to 3/4 main lines?

07-24-2012, 11:39 AM
the main coming into my house is 3/4 inch copper. As soon as it comes through the wall it is reduced to 1/2 inch and that is what is run throughout the house. My friend told me to replace the hot and cold mains throughout the house with 3/4 inch, but where it tees off to fixtures or applicances, leave it at 1/2 inch. He said it would increase pressure at each fixture and when someone flushes the toilet, it wont affect the shower if someone is taking one, or reduce the pressure at other fixtures. Is this true?


07-24-2012, 12:16 PM
At least 3/4" for a one bath home.
I use 1" on two and three bath homes.

Water pipe sizing (http://www.terrylove.com/watersize.htm)

A standard one-bath home with kitchen sink, dishwasher, water heater, clothes-washer, 1.6 tank toilet, lavatory, tub/shower combo and two hose bibs would be counted as 18 fixture units.

Most standard two bath homes consisting of kitchen sink, dishwasher, water heater, clothes-washer, two 1.6 tank toilet, two lavatories, one shower, one tub/shower combo, and two hose bibs would be counted as 23.5 fixture units.

Most standard three bath homes consisting of kitchen sink, dishwasher, water heater, clothes-washer, three 1.6 tank toilet, four lavatories, two showers, one tub/shower combo, one whirlpool bath and two hose bibs would be counted as 34 fixture units.

You can typically run two things like a lav and a toilet on 1/2"
Adding the third item can bump it to 3/4"

07-24-2012, 02:54 PM
will increase volume

07-24-2012, 05:23 PM
Pressure with everything off is the same, regardless of the pipe size. The pressure at the outlet effectively drops when you ask the pipe to deliver more than it can provide. Having a larger pipe running to the bathroom group will allow the volume to be greater, and thus maintain the pressure.

If you want another level of protection, change the shower valve to a pressure balanced valve as mandated for new construction to accommodate the anti-scald federal mandate.

07-24-2012, 09:30 PM
should i stop the 3/4 inch in the basement where it tees to each fixture, or run the 3/4 right up to the plastic supply hoses for each fixture?


07-24-2012, 10:49 PM
Only the main trunk lines and outdoor hose spigots are normally done in 3/4".

The downside of having the larger lines is that you will have to wait longer for the hot water to arrive from the water heater. There will be a greater volume of cold water to run down the drain before the water from the heater eventually arrives at the fixture. The further that the fixture is from the water heater, the more notable this will become.

07-25-2012, 07:11 AM
I'd consider running the 3/4" to those rooms that can use it, such as the bathroom and maybe the washing machine. Once in the area, run individual 1/2" lines the rest of the way to the fixtures - no need to run it directly to each. The possible exception is maybe the tub/shower where, if you have a big soaking tub or multiple showerheads, you may want to use a 3/4" valve and supply it with 3/4" lines. While running the lines, consider adding a hot water recycling line. If you can lay it out properly, you can get by without a pump. Make sure to insulate the lines while you have them exposed...this can help keep the hot hot, and the cold cold and helps to minimize losses if you decide you do want the recirculation system up and running. Gravity can do it, but it takes careful layout and good workmanship as any backwards slope can stop the circulation unless you add a pump.