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I have very low hot water flow (0.7 GPM) to my master shower. Other fixtures served by the same hot water line get about 2.5 GPM hot water or more.

I suspect that there is a kink in the final PEX run to the shower, but want to eliminate the possibility that the thermostatic valve in the shower is malfunctioning and restricting the flow before I start cutting into drywall to find the restriction.

Here are the clues I have gathered so far:

  • Shower hot water flow is the same 0.7 GPM before and after the water in the supply line has heated up.
  • Shower cold water flows fine.
  • I am turning the temperature control all the way up past the safety detent when measuring hot water flow.
  • In the summer, our showers are great. I attribute this to the fact that the cold water supply comes in at 85F or hotter (I live in Texas), so the trickle of hot water is enough to give a comfy shower.
  • In the winter, the shower is unusably cold. I presume that is because the cold water is coming in at closer to 40F, there just isn't enough hot water volume to balance it out.
  • This shower system was installed in early 2022, and we don't have particularly hard water where we live.
  • Our previous shower system had a thermostatic valve that was broken and couldn't adjust the temperature. With that old broken valve, we had good showers in the summer, cold showers in the winter.
  • The PEX line was run by the previous homeowner, who was not living in the house at the time, did a DIY remodel of the whole upper floor (where the problematic master shower lives) in a hurry to sell, and was (according to neighbors) a certified weirdo. I can very, very easily believe that he just didn't feel like going down the street to Lowes to get the correct coupler and bent the PEX more than he should have.

I'd like to avoid the cost of testing out a new cartridge (or thermostatic valve or whatever) if these signs point clearly to a restriction in the supply. I'm not even sure I could find a replacement cartridge for my shower system, honestly.

Is this enough information to start ripping into drywall to examine the supply line to the shower? If not, are there any other experiments I could do to narrow it down to restricted supply (or alternately, confirm a faulty valve)?

(note: I posted a related, but rather less-informed question about this problematic shower a couple months ago - now that it's getting colder, I can confidently say that the capacity of my tankless water heater is not the problem based on the amount of hot water I get at other fixtures)

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  • And this is why, when I put in a tile shower, I bought a couple of used cabinet doors at Re-Use-It, and made access panels. Watch youtube vids on fishing cables through walls. Will reduce the amount of drywall patching you have. Commented Nov 26, 2023 at 8:13
  • Do you have any fixtures on the same floor as the master shower and preferably the same or a larger distance from the water heater? Do those fixtures have sufficient hot water flow? Commented Nov 26, 2023 at 16:37
  • yes, the master shower is actually the closest fixture to the tankless heater. All other fixtures (including some supplied off of smaller lines) have plenty of hot water Commented Nov 26, 2023 at 19:22

2 Answers 2

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You can reduce the amount of drywall you destroy by getting a fiber optic snooper. Should be able to use a 1/4" hole every few feet.

Is there access to the valve?

I think these work with a bi-metalic diaphragm. the diaphragm is backed by a spring. Spring tension is adjusted by faucet handle. As temperature increase, the valve either increases the amount of cold, or restricts the amount of hot. If diaphragm is cracked or the spring is broken.

Get your ducks in a row, and order the valve from amazon. Open the wall. Disconnect the hot line from the valve, and run it into a bucket. If you still get only 0.7 gpm, then the valve is not the problem. Send the valve back to amazon for a refund, and start taking your wall apart.

Some of thse valves can be taken apart from the front for maintenance.

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  • I really like the fiber optic probe idea! I'll see if I can pull the valve to inspect it without destroying too much first. Commented Nov 26, 2023 at 14:09
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If you sort out how to remove the cartridge from the valve body (with water shut off upstream) you should be able to check the flow (messily) from the open valve body directly by turning the hot back on upstream. If the shower valve body has integral shutoffs (typically slotted screw adjustments in the valve body) you'd also have access to make sure that they are fully open.

Typically that involves removing the handle and trim and a clip or two on the valve, but they do vary and a new question with pictures of the relevant parts might help to identify what it is and how to take it apart (usually easy as following the instructions once you know what it is, which is often insanely hard to track down rather than being plainly marked.)

That would also give you the valve cartridge in hand to go shop for a replacement if the flow is fine with it out.

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  • Ahh, I didn't realize those were integral shutoffs. Unfortunately they seem to be fairly fouled w/ grout on both sides and I'm not able fully close them (I can back them out until they start leaking, that doesn't seem ideal). Might just need a beefier screwdriver though. I don't even know the brand of the shower system, we bought it many years ago, long before we actually had it installed. Currently spelunking through email receipts to see if I can dig it up. Commented Nov 26, 2023 at 14:12
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    Well, SOME flathead adjustments (generally just two) are integral shutoffs, and integral shutoffs are nearly always as flathead adjustments. So if it only has two, that's probably what they are. Post a picture of it in a new question about what it is if you can't find documents and somone may recognize it. A tiny chisel or awl might help with getting grout in the slots out.
    – Ecnerwal
    Commented Nov 26, 2023 at 14:18

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