I have a house built in 1940, which has a mix of copper, PVC and galvanized pipe for its plumbing at present. I am going to be gutting the upstairs bathroom in the near future, and have access to most of the rest of the pipes, giving me an opportunity to replace all of the corroded galvanized and to reconfigure my plumbing pretty completely.

I'd like to install a PEX manifold system in the basement, but what I don't have is any way to feed 3 cold and 2 hot lines up to the 2nd floor without opening walls I don't want to open, so I am thinking that I could install a small manifold on the 2nd floor just for that bath, and feed the rest of the house (kitchen sink, fridge, dishwasher, another full bath (2 hot 3 cold) and a half bath (2 cold 1 hot) from the basement manifold.

I have 3/4 copper coming into the house, if it matters.

What should my piping configuration be? Am I right in thinking 3/4 runs to the upstairs manifold and 1/2" runs from it to each fixture?

I was thinking about using using 2 3-port MINIBLOC manifolds (1 cold 1 hot 3 ports each) in the upstairs bath and 1 14-port MANABLOC manifold (6 hot, 8 cold) for the basement.

Total fixtures to be fed: - Upstairs Bath (separate manifold?) - 2 hot, 3 cold (sink, tub, toilet)

  • Line to upstairs bath (main manifold) - 3/4" - 1 hot, 1 cold
  • Downstairs bath (main manifold) - 2 hot, 3 cold (sink, tub, toilet)
  • Kitchen (main manifold) - 2 hot, 2 cold (sink, d/w, fridge)
  • Half Bath (main manifold) - 1 hot, 2 cold (sink, toilet)
  • Garden hose bib - 1 cold

So for the main manifold that's 5 hot, 8 cold. Is that enough or should I step up to the 24 port manifold (9 hot, 15 cold) even though that would have 4 unused hot and 7 unused cold?

Or - should I consider a completely different configuration altogether?

  • What size pipe do you currently have feeding the bathroom, and what material is it?
    – Tester101
    Sep 13, 2012 at 11:31
  • 2
    Just a comment from someone who has tried replacing plumbing and electrical in an older home without tearing down walls and ceilings. While you may not have to contend with insulation, many of them didn't brace pipes to wall studs and instead would toenail in a 2x4 block of wood between studs, and drill holes through this block to run the piping and electrical through. This made it nearly impossible to snake anything up the floors of my house from the basement. Sep 13, 2012 at 11:40
  • @Tester101 from basement to 2nd floor is galvanized, 1/2" I think - I'm never sure with that stuff. It's outer diameter is such that I can easily snake fit 3/4" pex in the existing holes. Once you get up to the bathroom they convert over to 1/2" cpvc (installed badly, I might add...) Sep 13, 2012 at 12:09
  • @maple_shaft A possibility, and if I need to I will open the necessary wall on the 1st floor but I think I can get by without having to do much damage there. Sep 13, 2012 at 12:10

2 Answers 2


First of all the information I'm providing is based on my local codes, your local codes my very.

Quick Answer

You will need to feed the lower floor manifolds with 3/4" PEX inlets and 1/2" outlets. No manifold would be required for the upper bathroom as only 1/2" PEX is needed for both the hot and cold, just tee off the 1/2" lines to feed the sink, tub and water closet. All individual fixture runs would be 1/2" as well.

Detailed Answer

Using manifolds and running to each fixture individually is called a homerun system. This can be less labour but may have a higher material cost. It all depends on how far the groups of fixtures are from the manifold. This method is typically used when the pipes are run through the slab.

Depending on the location of the fixtures, you may want to look at a truck and branch system. This is were you would run two mains and branch off to the fixtures as it passes them. The mains would get smaller as the fixtures are taken off.

Water lines are sized so that the velocity within the pipe is kept below a set point. This is to limit the wear and tear on the pipe material. Different materials can handle different velocities at different temperatures. The max velocity for copper and PVC is 5ft/sec for cold and 4ft/sec for hot. PEX pipe can handle a maximum velocity of 8ft/sec for both hot and cold.

Fixture units (FU) are given to typical fixtures. A FU is a design factor that is used to represent the typical requirement of a particular fixture (it is not a flow rate but).

Fixture Units for Private Use Fixtures (no flush valves):

| Fixture           |    Hot FU  |  Cold FU  |  Combined |
| Bathroom Group    |      4.50  |     4.50  |      6.00 |
| Bathtub/Shower    |      1.50  |     1.50  |      2.00 |
| Clothes Washer    |      2.25  |     2.25  |      3.00 |
| Dishwasher        |      3.00  |        -  |      3.00 |
| Hose Bib          |         -  |     7.00  |      7.00 |
| Kitchen Sink      |      2.00  |     1.50  |      1.50 |
| Water Closet      |         -  |     3.00  |      3.00 |
| Lavatory          |      0.75  |     0.75  |      1.00 |

The above table lists the common residential fixtures and their FU. You can see that a bathroom can be considered one group and has a lower FU then the sum of it's parts. This is because they are typically used by one person at a time so it is unlikely that all fixtures will be running at the same time.

Once we have the FUs, we can look up a sizing chart that will tell us the maximum FUs that a pipe of a set size and material can handle while staying below the maximum velocity. You can view these tables in the Domestic Water Sizing Tables (For Small Buildings) section below.

Based on this information, we can size your distribution system as follows for PEX:

| Fixture Group        |      Hot      |    Cold     |
|                      |    FU  Size   |    FU  Size |
| Upstairs Bathroom    |  4.50   1/2   |  4.50   1/2 |
| Downstairs Bathroom  |  4.50   1/2   |  4.50   1/2 |
| Kitchen              |  4.50   1/2   |  1.50   1/2 |
| Half Bathroom        |  0.75   1/2   |  3.75   1/2 |

So you can see that the pipes feeding each group of your fixtures will only need to be 1/2". Each individual fixture would also be 1/2". However, depending on how the groups of fixtures are located relative to each other, you may require 3/4" pipes to feed more then one group. The total FU of your house is 14.25 Hot FU (3/4"), 21.25 Cold FU (1") and 28.00 Combined FU (1"). The Combined FU is used to size the section of pipping from the cold line into the house to the hot water tank. Don't be alarmed that the cold and combined are sized at 1". This is due to the allowance of the hose bib, which our code now calls for 7 FU. This is too high and has been causing issues in large buildings with hose bibs on small decks, in that the lines are oversized and do not properly flow, allowing for growth in the water. Without the hose bib, your cold is only 3/4".

Domestic Water Sizing Tables (For Small Buildings)

  1. Minimum pressure available - 60 PSI at property line. (Greater acceptable).
  2. Pressure reducing valve set at 60 PSI minimum.
  3. Pressure loss for meter (3 PSI), maximum building height 25 ft. (10.82 PSI)
  4. Minimum 0.115 PSI for friction loss. If less than 0.115 PSI, system must bp fully engineered by detailed method or there will not be sufficient water to supply the fixture.

Pipe Flow Velocity Table For: Copper & PVC:

| Pipe Size | 5ft/sec (cold)  | 4ft/sec (hot) | 
|           |    GPM      FU  |    GPM    FU  | 
| 4"        | 186.65     850  | 149.32   600  |
| 3"        | 106.16     400  |  84.93   295  |
| 2-1/2"    |  74.37     245  |  59.50   170  |
| 2"        |  48.23     120  |  38.58    81  |
| 1-1/2"    |  27.72      46  |  22.18    34  |
| 1-1/4"    |  19.59      29  |  15.67    22  |
| 1"        |  12.86      18  |  10.29    14  |
| 3/4"      |   7.54       9  |   6.03   7.5  |
| 1/2"      |   3.64     3.5  |   2.91   2.5  |

Pipe Flow Velocity Table For: PEX, PE, PB, CPVC & Ductile Iron:

| Pipe Size | 8ft/sec (cold & hot)  |
|           |    GPM            FU  |
| 4"        |    300          1800  |
| 3"        |    170           750  |
| 2-1/2"    |    152           500  |
| 2"        |     78           265  |
| 1-1/2"    |     44           102  |
| 1-1/4"    |     30            54  |
| 1"        |     20            30  |
| 3/4"      |     12            17  |
| 1/2"      |    5.8             7  |
  • 3
    Wow, very detailed and informative. I will study this. However, same concern i voiced to Kendrick - we currently have 1 line to the upstairs bath with T branches off of it, and it causes wild variations if two do run at once (and yes sometimes we do end up sharing - plus right now our toilet takes forever to fill resulting in a long wait before using the shower, or a radical drop in temp about 2 mins after you hop in the shower after flushing the toilet)... thus the thinking about a mini manifold. Sep 13, 2012 at 9:09
  • 1
    I don't think that the issues you describe would be fixed by installing a homerun manifold, even if you increase the size of the supply pipe to the manifold. You mentioned that the house has a mix of different pipe material throughout, do you know what the pipe material supply the second floor bathroom is and how old it is? Has the issue increased overtime?
    – pdd
    Sep 13, 2012 at 15:26
  • Do you have the same issues in the ground floor bathroom? Depending on the mineral content of your water supply, the issue may be that the current pipes that feed the upstairs washroom have buildup within them restricting the flow. I have 1" pipes restricted to 1/4" due to this. The issue with the slow toilet fill may be the caused by the same issue. The pressure drop and temp change in the shower caused by flushing the toilet would be fixed by installing a pressure independent shower valve. Do you know what the static pressure of your water supply?
    – pdd
    Sep 13, 2012 at 15:27
  • The material to both full baths is galvanized. The main floor bath isn't used for showering, but flushing does noticeably affect sink water temperature. No doubt the galvanized pipe corrosion is a big factor in this. Sep 13, 2012 at 17:00
  • I would bet that the galvanized lines are the major culprit of the water supply issues. Do you notice any difference in flow from the sink when switching from full open cold to full open hot?
    – pdd
    Sep 14, 2012 at 21:45

Usually the in the wall is 1/2 and from city or well is 3/4 - 1+ it all depends on what your supply capacity and needs are. Some have 1/2 in from residential city water but often have capacity problems.

The run from your mains to the bathroom doesn't really need a manifold, if it's cheaper great but not needed. T connectors will work just fine and a single run from one end to the other branching off at each fixture. I have a mobile with original pex. I had to rerun the hot water to the shower as occasionally the water lines freeze in the winter. That's the only reason I ran a separate line, it runs along the ceiling so that it won't freeze. There is about 16' from the water tank to the toilet on the same wall.

The biggest decider would be how easy it is to run the lines. You will probably want a separate line for each room as I suspect they won't be in the same area. Also flow rate through 1/2 is plenty for 1 maybe 2 rooms but if they aren't separate you may get a cold shower when some one turns on the dishwasher etc...

  • The point of the manifold with larger volume going to it is to maintain pressure distribution so that flushing the toilet while taking a shower doesn't cause the shower to become scalding, which is the problem we have now with our pipes configured as you suggest. Sep 13, 2012 at 9:04
  • Most modern shower temperature control units have flow adjusters built in to prevent the toilet scald issue. I notice a pressure drop in my shower, but not change in temperature. Sep 12, 2013 at 17:44

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