SOLVED: Yet another posi-temp flow issue (but probably not what you think)

[Basilisk]

Howdy folks,

Obligatory "long time lurker, first time poster" - have been reading this forum for a while and it helped me a lot, so I was hoping to tap into the collective wisdom.

One of the showers (posi-temp) is having low flow issues - 1.1 gpm, as measured with the shower head removed, and 1.1 gpm, as y'all can imagine, doesn't exactly translate into great shower experience.
Replacing the cartridge with a brand new one did not help at all. 4 years ago replacing the cartridge did help to get from ~1.1gpm to ~1.5gpm, but not this time.
Flushing the valve using a specially designed flush plug (Moen PN 101975) also resulted in exactly the same 1.1 gpm flow, which leads me to the conclusion that the flow is getting restricted somewhere in the valve body as it travels from the capped diverter outlet to the shower arm. This is a shower only, so no tub spout, unfortunately.

So the questions are, 1) is my diagnosis correct, and 2) anything can be done here besides replacing the valve? I was thinking about pouring some weak acid solution (vinegar, phosphoric acid or some commercially available descaler that is approved for potable water) into the shower arm and let it sit for a few minutes to work its magic. Any issues with this approach?
The fact that it's been getting worse over time gives me hope that it's mineral deposits and not something like a glob of solder. Our city water is not hard (< 150 ppm), but I've seen a lot of build-up in supply hoses and angle stops of the most frequently used faucets.

Looking for other ideas as well. Many thanks in advance!

-B

 

[Helper Dave]

Late reply ... still having this problem?

What kind of pipes do you have supplying the shower?

Another little test could help you out. Take the cartridge out, and have someone slowly turn your main valve on as you watch the water come straight outta the valve body. Put a towel around it so you don't soak the wall, and don't get full blast on your valve unless you notice bad pressure coming out the valve, too. That'd tell you if your problem exists before the valve. Then it might mean repiping a bit.

 

[Sponsor]

 

[Basilisk]

Hey @Helper Dave , thank you for your response!

Yup, still having the problem... The pipes are all copper and are not that old (15 years), the valve is soldered in.
Going to do the test with the cartridge removed again over the weekend. Last time I did that, a lot of water was coming out of the valve body, but it's quite difficult to tell if one of the sides is more restricted than the other. I wish the valve had separate stops so the test would be 100% conclusive, but, of course, it doesn't, as it's all builder grade stuff.
My thinking was that if I can get 1.1 gpm at any temperature setting, it must mean that the valve is still capable of getting at least 1.1 gpm from the cold inlet and 1.1 gpm from the hot inlet. So when the cartridge is removed and the flush plug is in place, I should've been getting more than 1.1 gpm through the shower riser - except that it didn't happen
So far it looks like trying to de-scale the valve with something like sulfamic acid is still my best bet.

-B

 

[Basilisk]

Just in case anyone else runs into a similar issue (flow slowly getting worse over time, replacing cartridge doesn't help):
Sulfamic acid trick worked!
I removed the cartridge and the shower arm, and used a large syringe with a clear tube to pour sulfamic acid solution down the shower riser, adding some fresh solution every few minutes. After about 15 minutes of doing that, put everything together and the flow immediately increased to 2.2gpm!
This turned out to be an incredibly cheap (a small tub of dry sulfamic acid is about $8 and will last you a lifetime) but a very efficient solution.
Of course, don't forget to flush for a few minutes to remove any leftover acid solution; and use a rag around the valve so the acid solution leaving the valve body doesn't go down the wall.

-B

 

[Breplum]

Hard to imagine but you seem to have proven your case about the riser having impaired flow.
In our not-hard-water S.F. Bay Area, I've demo'd hundreds of bathrooms and other copper piping and never seen more than minor scale deposits inside copper tube (with the exception of recirculating hot water systems that had been running for many years).
Especially because a shower head riser gets very little time "on" so, it doesn't make sense.
But, good to hear your story, so thanks!

 

[Basilisk]

@breplum My theory is that these deposits were bits and pieces of deteriorating anode rods from the past, that clump together around any edges they can attach to.
Last year, when I went around proactively replacing all faucet supply lines, the hot side would look something like this:

Now, this is by far the worst one I've seen, but gives you an idea of what I've been dealing with.
Our water is actually pretty soft, and when I pulled angle stops, both cold and hot pipes were squeaky clean inside with no signs of any deposits, so at least the "blast radius" of this problem is limited to supply lines, angle stops, and, apparently, posi-temp valve bodies. Always learning something new!

-B

 

[Reach4]

https://www.austinutilities.com/assetmanager/downloads/documents/pdf/WaterQualityReport.pdf says
"The average water hardness for Austin is at 16 grains per gallon (gpg)."

When you get a softener, you usually size it and set it up for maximum hardness.

Anyway, you would benefit a lot from a water softener. You said you water was pretty soft, and that would seem to imply that you have a softener. But if you have one, you probably would have said that.

When you first go from no softener to softener, you will typically think you can't rinse off the soap. When I switch to hard water, I think I have a problem getting the dirt off. With a softener, use a lot less soap than you are used to .

 

[Basilisk]

heh, I also ran into that site more than once by mistake That's Austin, MN.
Our water is currently at 100mg/L, which is less than 6 gpg, unless my math is wrong:

https://www.austintexas.gov/sites/default/files/files/Water/WaterQualityReports/WQS%20Q1%202022.pdf

 

[Reach4]

heh, I also ran into that site more than once by mistake That's Austin, MN.
Our water is currently at 100mg/L, which is less than 6 gpg, unless my math is wrong:

https://www.austintexas.gov/sites/default/files/files/Water/WaterQualityReports/WQS%20Q1%202022.pdf

Yes, not so hard.

Do you collect solids in your aerators on your faucets? Consider an incoming whole-house cartridge filter. How else would I imagine how the stuff you have accumulated accumulated.