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I am running into a repeating issue with my thermostatic shower valve. It's a Delta T27T899 / TempAssure® 17T. My model has 3 handles, one at the top for the diverter, and 2 on the same axis at the bottom, with the back one for controlling the amount of water and the front one the temperature.

The way the maximum temperature is adjusted on this valve is by simply placing the temp control handle on the splines in a specific location so that it can't be spun past a certain point due to stops built into the handle. So there is now way this can adjust itself over time in any way. Picture A3 / page 8 on this manual outlines the step for my specific valve.

I also just have it set to be max out at the the full temperature my water heater delivers, since my water heater is set to 125F and my wife like really hot showers. About 6-8 months ago (which was about 1 year after the shower went into service), we noticed the shower did not seem to get as hot. I confirmed even at the hottest setting with the temp knob removed, it would only get to about 105F, where as my tub spout (which is down line from the shower) would hit 125F. This rules out an issue with the water supply temperature. At the time, I contacted Delta, and they did not offer any additional troubleshooting or repair steps. Instead, they sent a replacement cartridge.

Now, about 6 months after installing the replacement cartridge, the same issue is occurring again. My question: Is this just an issue with the cartridge design, or could there be something with my water (or something else) causing this? We are on well water and it is fairly hard and has some iron, but we do have a newer softener and iron remover. Is there anything that can be done to repair these cartridges instead of replacing them? Delta should replace them for free, but it's kind of a pain to replace it every 6 months.

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  • Type of water heater? For tanks with temperature reducing valves included in the plumbing lines, it is recommended to have the tank heat above 130F Commented yesterday
  • It's an electric tank heater. I don't believe it has a temperature reducing valve. Would it be integral to the water heater? If it is external to the water heater, than I am sure we do not have one. Commented yesterday
  • You have one at the shower and maybe sinks. In the old times they were not available and people got burnt skin with hot water. Recommended tank temp was lowered to about 120F but nasty stuff started to grow in the tanks, so anti-scald (thermostatic) valves were added and tanks could go back to 130F or above. Commented yesterday

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Thermostatic valves become calcified with excessively hard water. I have found that valve life can be prolonged by occasionally rotating the temperature control from minimum to maximum and back again through a few cycles. If done on a regular basis itseems to help break up calcium deposits in the valve and extend the time between valve replacements.
For safety, this should be done while standing outside of the shower and being careful to avoid contact with scalding water.

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A bit of googling shows that a thermostatic valve has moving parts in it. This Youtube video claims there's a piece of wax that melts and solidifies inside the valve based on the temperature of the hot water, moving a sliding piece which controls the output temperature.

A moving part can be jammed by mineral deposits from hard water. A bit more Googling says "white vinegar, lemon juice, or commercial cleaners" might remove hard water mineral deposits. You might be able to soak the valve in one of these substances to dissolve deposits and get the valve working again. Softening the well water may help prevent further trouble.

If the wax thing is true, it could also be possible that the manufacturer's wax source has provided wax that over time is not following its specified expansion/contraction curve. If so, it's a manufacturing defect and probably not much can be done about it except to replace with a different company's valve.

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