Thread: Pressure drop in two fixtures

The great state of Texas requires home inspectors to "report as deficient the water supply... as determined by viewing functional flow in two fixtures operated simultaneously."

Granted, the wording is vague; however, previous to this particular SoP, the former stated that we would operate two fixtures and if there was a noticeable drop in pressure we were to report it.

Today's brand new house had just that problem. The pressure as tested at the hose bib was 70 psi. When I operated any combination of two bathroom fixtures there was a definite drop in pressure.

There is a backflow device just past the supply meter.

The house has Pex and copper (no manifold).

Any thoughts what would cause this when I've got 70 psi?

Bruce

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Insufficient pipe sizing.

Originally Posted by Matt Vozzella

Insufficient pipe sizing.

I thought about that as well as other things. This house is new and the plumbing "should" be the right size. But one never really knows....

I wondered about friction loss for Pex. Is it higher than copper or other materials?

Originally Posted by JB Thompson

I thought about that as well as other things. This house is new and the plumbing "should" be the right size. But one never really knows....

I wondered about friction loss for Pex. Is it higher than copper or other materials?

I have seen the pex in attics going to partially hidden manifolds. I have had the same occurance in a couple homes. To small of a line to be split into multiple lines and you have a volume decrease.

Originally Posted by JB Thompson

I thought about that as well as other things. This house is new and the plumbing "should" be the right size. But one never really knows....

I wondered about friction loss for Pex. Is it higher than copper or other materials?

Not all pex is created equal. Top quality pex will almost match copper in flow rate from what I've seen, the benifit to pex if it is used in a maniblock setting is less fittings from source to the fixture. Using pex in a "standard" distribution set up is counterproductive I think.

What is the supply size pipe?

What is the distribution supply line size?

Did you check the screens on the individual fixtures to see if they were clogged with crud? Very common in new construction.

Did the house have a manifold?

Was this a single level home?

If the volume of water dropped, this is a sign of some type of restriction. It could be pipe size, clogged screens at the fixtures, improper bends in the PEX pipe (common), or a few other things.

Originally Posted by Scott Patterson

What is the supply size pipe? 3/4" meter with 1" line

What is the distribution supply line size? Of the ones I could see, they were the standard Pex red and blue (sorry, I don't what size those are)

Did you check the screens on the individual fixtures to see if they were clogged with crud? Very common in new construction. Not clogged

Did the house have a manifold? No

Was this a single level home? yes

If the volume of water dropped, this is a sign of some type of restriction. It could be pipe size, clogged screens at the fixtures, improper bends in the PEX pipe (common), or a few other things.

Answers above

It could be something as simple as the valve at the meter or supply shut-off that is partially open (or partially closed depending on your outlook on life.)
No matter, it is deficient and needs to be corrected.

Originally Posted by Jim Luttrall

It could be something as simple as the valve at the meter or supply shut-off that is partially open (or partially closed depending on your outlook on life.)
No matter, it is deficient and needs to be corrected.

Doh! I feel like an idiot. I didn't look at the valve to see if it was fully open.

Originally Posted by JB Thompson

Doh! I feel like an idiot. I didn't look at the valve to see if it was fully open.

That could be it.

All you can do is to report that you discovered a loss in water volume when you operated two or more fixtures. Let the builder discover the reason.

Originally Posted by Scott Patterson

That could be it.

All you can do is to report that you discovered a loss in water volume when you operated two or more fixtures. Let the builder discover the reason.

Water pressure was found to be noticeably lower when two fixtures were operated simultaneously. The main water pressure was adequate; however, this particular condition should be investigated. I recommend contacting the builder or plumber.

You are talking about two different things here. Pressure is one thing and volume is another. Functional flow (what your SOP requires you to test) is related to volume.

So what exactly does the current SOP require? I bet it is flow, which is related to volume. You can have moderate or even low pressure and still have adequate volume.

You might be getting your requirements crossed up and scrutinizing the wrong thing.

Originally Posted by John Dirks Jr

You are talking about two different things here. Pressure is one thing and volume is another. Functional flow (what your SOP requires you to test) is related to volume.

So what exactly does the current SOP require? I bet it is flow, which is related to volume. You can have moderate or even low pressure and still have adequate volume.

You might be getting your requirements crossed up and scrutinizing the wrong thing.

I'm also required to make a note of the pressure...which was 70 psi.

The 2 fixture req't is for flow....which was noticeably lower.

Originally Posted by JB Thompson

I'm also required to make a note of the pressure...which was 70 psi.

The 2 fixture req't is for flow....which was noticeably lower.

Gotcha...I didn't realize you were required to do both. Even if you weren't, what stops you from going beyond requirements, right?

I was just a bit crossed up because two different elements are being discussed as though they were one in the same. That's the way it seemed anyway.

Originally Posted by John Dirks Jr

You are talking about two different things here. Pressure is one thing and volume is another. Functional flow (what your SOP requires you to test) is related to volume.

"Functional flow ... is related to volume" AND "pressure".

You can have a high static pressure and think you're ok but it means almost nothing. What you are interested in is what the fire service refers to as 'residual pressure' or others call 'flow pressure'. You can have a pipe clogged to all but a pin hole and still record a high static pressure which drops out upon flow. It is not uncommon for a fire hydrant of a 6" main to show a static pressure of 60 pounds but a residual pressure of barely 5psi when flowing 500 gpm or even go negative. BTW, 5 psi is the min. below that you can literally suck pipes out of the ground if you are using hard suction pipes on the hydrant.

In the home, partially closed valves or clogged pipes rate high. If you increase pipe diameter, you can increase flow while decreasing pressure. With the same pump pressure, you can increase the height of a fire stream with smaller tips but at reduced flow.

I was recently working at a house where the new owners were finding all sorts of sabotage by contractors hired by the previous owner, an attorney who was, shall be say, not loved by all who met him. When there was no flow in the shower, we found a lead fishing sinker placed upstream of the valve. You never know....

Fluid dynamics apply to gas or liquids. When you increase demand on a system without increasing pressure, you get a drop in flow and pressure. This accounts for pilot outages and improper combustion. If you are taking a shower and suddenly lose pressure/ flow, most probably someone started a washing machine or other high flow. If there is a backflow preventer or PRV, it could be clogged.
Bob