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Pipe Sizing Advice

cwcjr09 | Posted in Construction Techniques on December 11, 2011 08:07am

I’m having some recent issues with water pressure in my building.

Existing main is 3/4″ copper with 80 PSI. The first floor of the building is a salon with (1) toilet, (3) shampoo bowls, (1) lavatory, (1) washing machine, (1) 50 gal hot water heater and (1) kitchen sink. Second and third floor contain an apartment with (1) kitchen sink, (2) toilets, (2) showers, (1) tub, (1) washing machine, (1) dishwasher, (1) 40 gal hot water heater and (3) vanity sinks.

3/4″ main comes into the house and branches to upstairs apartment and salon hot water heater. All trunk lines are 3/4 before stepping down to 1/2″ as few feet before the fixture. Max elevation change in the building is 30ft.

Pressure was never an issue for the first year. But now there are times where in the upstairs apartment there is absolutely no water due to the fact that it is all going to the salon fixtures and there isn’t enough supply to go around. So I am assuming that the main needs to be increased – any ideas on what size to go with?

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  1. DanH | Dec 11, 2011 09:11pm | #1

    What you want is two mains, one for the salon and one for everything else.

    The salon main should probably be larger than 3/4", though.

    (But before you do anything, double-check that the main shutoff valves are all the way open.)

    1. cwcjr09 | Dec 12, 2011 06:02am | #2

      2 mains directly from the meter?

      Or am I just branching off two lines as soon as the existing 3/4 main comes in the house?

      1. Piffin | Dec 12, 2011 06:49am | #3

        It will depend on what is permisable from your water supplier. You might have to pay a tap fee for a second line.

        My experience is primarily with wells for water, and I would not dream of doing your building with less than a 1" main coming in.

        A creative solution might be t install a pressure rgulator to reduce what the Salon gets. Supply a separate 3/4" to them with 35# and keep the 80# going to the rest of the building

      2. DanH | Dec 12, 2011 07:09am | #4

        Two mains directly from the city main -- a new service.

  2. rdesigns | Dec 12, 2011 09:55am | #5

    If a second main is too pricey, you could consider installing a 40-gallon pressure tank to serve the upstairs unit.

    Be sure to install a check valve in the line that feeds the new pressure tank, so that its reserve pressure and volume don't backfeed to the lower unit.

  3. oops | Dec 13, 2011 09:35pm | #6

    pipe sizing

    You said it worked fine for the first year.  Has anything changed in the way that it is being used since the first year? Like were both the solon and the apartment both occupied the first year? Does the apartment have pressure, say at night, when the solon is not open?

    If it did indeed work fine for the first year  with a single 3/4' service, it should work fine now.  However, as has been stated, the service is not adequate for the number of fixtures listed in any case.

    1. JohnWalker | Dec 17, 2011 02:00am | #7

      old pressure reducing valve?

      Sometimes an old pressure reducing valve will partially pack it in and although they provide 80psi pressure the VOLUME is greatly reduced.

      A new water service will cost you at least $5k, probably a lot more if the main is on the far side of the street, and your have to replace curb and sidewalks etc. Also depends on your frost depth and whether the city requires unshrinkable backfill (lean concrete) for trenches across main roads...

      Here's my suggested list of things to look into before jumping to a new water service:

      1) Replace your main shut off valve (just inside the house) with a ball valve. These provide an unrestricted water passage that fully matches the pipe size (3/4" in your case). Regular "globe" valves are actually restrictive. Rent a valve key to shut off the curb stop near the property line so you can make this repair. Some municipalities will do it for you if you tell them you are "repairing" the main shut off valve inside.

      2) Replace the pressure reducing valve. As mentioned above old units can partially fail or simply be low volume designs.

      3) Consider back flushing your building's plumbing by connecting a hose from the main valve to some upstream point on your plumbing to blow any debris backwards towards the location where the main shut off is. You'll need to improvise some hose bibs to feed and drain this setup. To be clear I am not saying back flush the water service, just the buildings internal plumbing. Keep the "gap" between the main valve and the building plumbing. Sometimes solder or other debris will collect in an elbow and reduce the flow. This is a bit of a long shot but if you are looking for every cheap fix before springing for a new water service this might be worth doing.

      4) Assess the situation for changes in the last year. Are you sure someone isn't leaving a sink running in the salon? A leaking toilet? Is the salon now very busy? These things will rob pressure but not as much as you describe.

      Anyway I'd start with 1 and 2.

      John (former municipal inspector of water main and water service replacements)

  4. JohnWalker | Dec 17, 2011 10:34pm | #8

    No more trenches - in the right soils but if you're in glacial till or there are boulders god only knows where your torpedo will end up!

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