Because the problem is the shower valve, not the water heater. A bad heater would starve the whole house. When only the shower runs cold, the cause is the pressure-balance cartridge or its rotational limit stop, often set too cool or clogged with Phoenix hard-water scale.
Why it is the valve and not the water heater
A water heater feeds the whole home from one tank or one heat exchanger. When it fails, the failure shows up everywhere at once: the sink, the tub, the laundry, the shower. So the single most useful test you can run is the one you already did by accident. If hot water reaches your other taps and only the shower comes up short, the heater is doing its job and the trouble sits at the shower itself.
Modern showers use a pressure-balance valve, also called an anti-scald valve. It blends hot and cold behind the wall and holds your set temperature steady even when someone flushes a toilet or starts the dishwasher. The heart of that valve is a cartridge, and most cartridges carry a built-in temperature limiter called a rotational limit stop. Two things on that valve cause a shower that runs cold while the rest of the house runs hot: the limit stop is set too cool, or the cartridge is scaled up or worn out.
This matters in Phoenix more than in most places. City water here runs hard, roughly 10 to 17 grains per gallon, which the U.S. Geological Survey rates at the top of "hard" and into "very hard." That mineral load builds scale inside valve cartridges and showerheads faster than it does in soft-water regions, so a cartridge that should last years can clog or stiffen sooner.
The rotational limit stop, the usual cause
The rotational limit stop is a plastic ring or gear that sits under the shower handle and caps how far you can turn toward hot. Plumbers set it during installation so a child or an unsteady adult cannot crank the handle to full heat and get burned. If it was set conservatively, or if it shifted when someone last serviced the valve, your "all the way hot" position may only be lukewarm. The water heater is fine. The handle simply will not let you reach the hot you expect.
On a Moen PosiTemp valve, the limit stop adjusts by hand. Moen's instructions are plain: "To make the water hotter, remove the stop and reposition it to the left. To make the water cooler, reposition it to the right." You pop off the handle, pull the limit-stop ring, rotate it a notch or two toward hot, and reseat it. Delta, Kohler, and other brands use a similar splined ring or a screw, so check the model's own guide before you turn anything.
After any adjustment, set the cap with a thermometer, not by feel. Run the shower at its hottest setting, hold a kitchen thermometer in the stream, and reposition the stop so the maximum lands at or below 120 degrees Fahrenheit. The U.S. Consumer Product Safety Commission ties that number to scald risk: water at 140 degrees can produce a third-degree burn in about five seconds, while 120 degrees takes far longer and gives a person time to react. Set it hot enough for a good shower, never hot enough to scald.
When the cartridge is scaled or worn
If the limit stop is already set toward hot and the shower still runs cold, the cartridge itself is the likely culprit. Hard-water scale collects on the moving parts and in the hot-side ports, slowly choking off the hot feed while cold still flows freely. A worn cartridge can also let the internal balancing spool stick, so the valve favors cold no matter where you point the handle. Signs include a handle that turns stiffly, temperature that wanders mid-shower, or hot that has faded over months rather than failing all at once.
You have two paths. Light scale sometimes clears with a soak: shut off the water, pull the cartridge, and soak it in a 50/50 mix of white vinegar and warm water for 15 to 30 minutes to dissolve the buildup, then lubricate the grommets with silicone (never petroleum) grease and reinstall. A cartridge that is cracked, gouged, or heavily scaled should be replaced outright. Common replacements are the Moen 1222 (PosiTemp), Delta RP46074 (MultiChoice), and Kohler Rite-Temp. Our cross-reference page on how to replace a shower cartridge walks through pulling the retaining clip and seating a new one step by step.
Before you blame the cartridge for cold water, rule out a simpler twin problem. Delta notes that debris and scale can also block the valve and starve flow, and its fix for a clogged valve is to remove the cartridge and flush the lines by running the supplies for about a minute to clear loose grit. If your real complaint is weak pressure rather than wrong temperature, see our page on why your water is not getting hot enough, which covers heater-side limits like a low thermostat setting.
How to confirm it is the valve in ten minutes
You can settle this without opening a wall. Work through these checks in order.
- Test other fixtures. Run hot at the kitchen sink and the bathroom sink. If both deliver hot and only the shower fails, the heater is cleared and the valve is your target.
- Check the heater setting. Confirm the water heater thermostat is near the U.S. Department of Energy's recommended 120 degrees. A heater set far too low would weaken hot everywhere, not just the shower.
- Adjust the limit stop first. It is the cheapest, fastest fix and needs no parts. Reposition it toward hot, then verify the max with a thermometer at or below 120 degrees.
- Watch how the hot faded. A slow decline over months points to scale or a tiring cartridge. A sudden change right after plumbing work points to a bumped limit stop or a cartridge installed backward.
- Feel the handle. Stiff, gritty handle action is a strong sign of hard-water scale inside the cartridge.
Keep the temperature honest while you test. The CDC advises storing water hot enough to limit Legionella growth, which thrives in the warm 77 to 113 degree range, while still delivering no more than about 120 degrees at the tap to prevent scalds. The shower handle should never be your tool for chasing dangerous heat.
When to call a plumber
Adjusting a limit stop and swapping a cartridge are reasonable do-it-yourself jobs if you can shut off the water and you are comfortable with hand tools. Call a licensed plumber when the cartridge is seized and will not budge, when you cannot find the model or the right replacement, or when you spot signs of a deeper fault: water dripping inside the wall, a valve body that leaks when the handle is off, or hot that is weak at several fixtures rather than the shower alone. A behind-the-wall valve-body or soldered-joint leak risks real water damage and belongs with a pro.
HQ Plumbing & Air services shower valves across metro Phoenix and carries the cartridges that hard water wears out, so a stuck or scaled valve does not have to mean cold showers for long. Reach our team at (602) 675-1555, 24 hours a day. We will confirm whether a limit-stop adjustment, a cartridge swap, or a fuller valve repair gets your shower hot again, set safely under the 120-degree scald line.