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Thread: Second Floor Pressure/Flow Problem or Maybe I'm over thinking this a bit

  1. #1
    DIY Junior Member dylane's Avatar
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    Default Second Floor Pressure/Flow Problem or Maybe I'm over thinking this a bit

    I have been helping a friend through some pressure issues in his new home. The shower on his second floor barely works. And I mean barely works. The toilet also seems to fill slowly, but the bath sink seems to work just fine (however it is the old dual faucet style).
    The house only has one wet wall which runs from the mechanical room in the basement up to the first floor in the kitchen and then up to the second floor in the bathroom. The basement plumbing has been changed from galvanized pipe to copper (3/4 inch from the city, 3/4 inch throughout the basement). The pipe from the basement to the first and second floor is all galvanized, and by the looks of it, it is about 80 years old.
    My first impression was to remove the galvanized pipe, run 3/4 inch copper to the first floor, branch 1/2 inch to the kitchen sink and then run 3/4" cold to the second floor and 1/2" hot to the second floor. But a plumber told me to first buy a pressure gauge and make sure the pressure was good. So I purchased a pressure guage and at the hose connection on the outside of the house (3/4 copper) we got 70PSI and 15 GPM when we did a flow test. I proceeded to test the pressure at the kitchen sink and at the tub on the second floor. Both tested at 65-70PSI, but when we did a flow test in the tub (through the faucet with hot and cold running) we only got about 3.3 GPM, which seemed low even though we were going through the faucet. We did not do a flow test in the kitchen sink, it does seem to run slow to me, but that could be the style of sink as it is one of those sink spout/sprayer combined faucets.
    So, here is my question, am I over thinking this too much? Is the problem most likely the galvanized pipe and I shouldn't worry about the pressure and flow at the faucets? Or should I look at maybe replacing the bath faucet first? We took apart the shower head and made sure it wasn't clogged and the faucet handles are in good working order. I am a little concerned about the bath faucet because it is a claw foot tub and the feed from the top of the faucet to shower head is restricted to a 3/8" fitting. But that could be normal for older claw foot tub faucets.

    Any input would be greatly appreciated. I would hate to rip apart the walls and find that we did a bunch of unnecessary work.

    I have posted some pictures of the project.
    https://picasaweb.google.com/dylan.e...umbingProblem#

  2. #2
    Nuclear Engineer nukeman's Avatar
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    The galvanized is probably all clogged up assuming there isn't a shutoff valve somewhere that is only partially open. See, for any sized pipe (even if it was pinhole sized) will show the same pressure when everything is off. However, the flowrate capability will be quite different for different sized pipes. Measuring pressure doesn't tell you much at all. A bit better measurement (in regards to pressure) might be to measure pressure at one fixture and then turn on another fixture nearby. If that other fxture has a low flow (such as a lav faucet) and the pressure drops like a rock, that means that the fixture is exceeding the plumbing capacity by quite a bit. This means there is a restriction which could be either clogged up galvanized pipes (most likely) or a partially closed shutoff valve in the line(s) that feed the upper floors.

    The pressure you measured at the upper floors was lower due to gravity head. For every foot you go up, expect to lose about 0.5 psi. If the hose connection was at the 1st floor and the other measurements were at the 2nd floor (roughly 10' or so in elevation), you would expect the pressure to be lower by about 5 psi.

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    DIY Junior Member dylane's Avatar
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    Thanks for the reply nukeman. There isn't a shutoff valve in the way. In fact, the only shutoff valves in the whole house are on the kitchen sink and then the main shutoff. If I understand you correctly, if I attach the pressure guage to one of the shower supply lines and then turn on the lav. sink, the pressure should drop dramatically if the lines are restricted, but they should only drop a little if they are free and clear. Do you have any idea what kind of drop is normal in this type of test?
    Thanks

  4. #4
    Nuclear Engineer nukeman's Avatar
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    I don't have a good number, but ideally, with a small flow from say a lav, the water pressure will remain up in the "good" pressure range. This may be 30-50 psi, but might be higher depending on what you start with.

    If the lav flows say 1 gpm and turning it on drops the pressure from 70 psi to say 30 psi then there is a serious problem. While you do the test, you might open up other fixtures at that level and see how the pressure drops.

    We can get an idea of the resistance by looking at your flow rates.

    - at the hose, you are getting 15 gpm
    - at the tub, you are getting 3.3 gpm (and this is with H and C, correct?)

    For a given resistance, 4x the pressure difference gives 2x the flow. In this case, the pressure difference (pressure at the street to atmospheric pressure) is the same. What you are finding is that (2) 1/2" (assuming that is what your galvanized pipes are) are able to flow 3.3 gpm at a 70 psi dP.

    According to this, for a single 1/2" sch 40 steel pipe, at 3.2 gpm, you should get a loss of 4.9 psi/100'. If both pips are flowing, then you are around 1.6 gpm per pipe, or about 1.4 psi/100'.

    http://www.engineeringtoolbox.com/pr...pes-d_307.html

    I know that your effective pipe length is probably well under 100', but you are dropping about 70 psi under these conditions. We can estimate what you have for area. Let's assume that you have 100' effective length (including elbows, etc.). Velocity is volumetric flow (say gpm) divided by area. Area is related to diameter squared. Then we have hydraulic diameter, that is also based on diameter. So overall, the dP goes as 1/D^5 (assuming the other values remain the same).

    Δp = λ (l / dh) (ρ v2 / 2)

    Say we should be 1.4 psi/100' and now we are 70 psi/100' . This is 50x the loss. Very roughly speaking, your diameter is now about 1/2 what is should be (like a 1/4" pipe).

    The actual numbers depend on many factors. For instance, the pipe roughness wouldn't be the same as a new, clean pipe. However, the numbers tell us the diameter of these pipes are probably about 1/2 of what they were when new. If the pipes are 3/4", then they are in even worse shape in terms of % of plugging.

    If there aren't any other shutoff valves that could be causing the high loss, then I would say that those galvanized pipes are probably in pretty bad shape. Running new lines in copper or PEX would help a bunch.
    Last edited by nukeman; 04-06-2011 at 12:01 PM.

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    DIY Junior Member dylane's Avatar
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    Nukeman-
    thanks for helping me get my head around this. while I have done several plumbing projects, they all have revolved around adding to an existing and functioning copper system. I've never had to deal with galvanized before, or worry to much about the science behind plumbing conventions.
    Thanks alot.

  6. #6
    Nuclear Engineer nukeman's Avatar
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    In a perfect world, the supply pipes would be large enough so that running maximum flow through the expected number of fixtures operating at the same time won't cause a noticable drop in pressure. However, cost of materials, limitation on the meter size, and additional labor/access issues with larger pipe keeps people from going crazy with the size. For instance, if your whole house was done in 2" copper (including meter and out to the street), you would never notice a drop in water pressure even with everything going full bore. This isn't practical, though. In addition, large pipe can be a negative on the hot water side. It means that it takes longer for the line to flush out after it has cooled down and you'll be waiting forever to get hot water if the pipe is too large.

    Typically, the line is sized based on the number of flush units for the house. Most of this takes into a probability of how many fixtures will be used at one time. For instance, it isn't likely that every toilet will be flushed, with all showers going, DW running, clothes washer filling, all bath lavs running, etc. at the same time. Thus, the line is sized based on what the house might typically use at any given time instead of the maximum.

    With the galvanized pipe, it was probably fine when it was new (probably had good flow). When galvanized ages, it tends to close up and restricts the flow.

    I'll be interested in hearing how the tests go.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Replace the galvanized from the first to the second floor and any branches you can get to and the problem likely will go away. While it can last a long time, galvanized that old is probably the equivalent to about 1/4" pipe and just can't flow much volume. People often have problems getting their head around volume and pressure.

    Most fixtures in a home are restricted, so you won't get the same flow out of them as say a hose bib. A tub spout is often unrestricted and can flow as much as the pipes going to it and the openings in the valve can provide.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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