That split — hot at the sink, tepid in the shower — rules out most heater failures in one test. The heater can clearly make hot water and carry it to a nearby tap. What sits between the wall and the showerhead is a short list of parts, and most of them are cheap.
Measure, don't guess. Run the problem shower on full hot for two minutes and catch the water in a cup. Do the same at the nearest sink. Write both numbers down. A sink at 118–120°F and a shower at 100–105°F confirms a fixture problem. A sink that also tops out near 105°F points back at the heater instead.
Check the heater setting once. If the tap test shows low heat everywhere, confirm the dial sits near 120°F. Our guide to what temperature a water heater should be set at shows how to verify it at the tap. If the sink is hot and only the shower is not, leave the dial alone — turning it up masks the fault and adds scald risk elsewhere.
Most modern single-handle valves hide a rotational limit stop under the handle. It caps how far the handle can turn toward hot. Builders often set it conservatively, and it can slip over time. The result is a shower that feels capped, no matter how long you run it.
Remove the handle. Most handles come off with a hex key on the underside or a screw under a small cap. Set the screws somewhere they cannot roll into the drain.
Find the limit stop. On many valves it is a plastic ring or disc with notches, marked with a plus direction or a "hotter" arrow. On others it is a small tab you lift and rotate.
Move it one or two notches toward hot. Small moves only. Refit the handle loosely, run the shower, and measure again. Aim for a shower maximum around 115–120°F. Repeat until the temperature is right, then refit everything snugly.
Inside the valve, a cartridge blends hot and cold. Mineral scale and worn seals change that blend over time. Signs include a shower that starts hot and drifts cool, a stiff or gritty handle, and temperature that jumps when another tap opens. In hard-water areas, cartridges can misbehave in as little as 5–7 years.
A replacement cartridge usually runs $25–$75 for common brands. The swap is valve-specific: water off, handle and trim off, retaining clip out, old cartridge pulled straight out, new one seated in the same orientation. If the old one will not budge, use the maker's puller tool rather than prying — a cracked valve body turns a $50 fix into a plumber's bill. A scaled head can also change how hot the spray feels, so cleaning a shower head is a cheap side check.
A failed cartridge — in the shower itself or in another single-handle faucet — can let cold water push across into the hot line. The clue is timing: the shower runs warm for a minute, then fades. Try an isolation test: shut the cold supply valve under the nearest sink, then run the shower. If it suddenly runs properly hot, cold water was crossing over at that faucet. If nothing changes, repeat at other single-handle faucets, one at a time.
A dip tube sends incoming cold water to the bottom of the tank, keeping hot water at the top ready to leave. If it cracks or shortens, cold water dumps in at the top and mixes with the outgoing supply. That fault affects the whole house, not one shower: the first shower is fine, then every tap runs lukewarm until the tank rests. If your test showed one hot sink and one cool shower at the same moment, the dip tube is an unlikely culprit. If every fixture fades after a few minutes, raise it with a plumber — tank work involves gas or high-voltage connections on many models, so diagnosis is the DIY limit there. Sediment can produce a similar fade, and draining a water heater yearly is the standard way to slow it down.
| Pattern | Most likely cause | First move |
|---|---|---|
| One shower capped at the same lukewarm temperature every time | Anti-scald limit set too low | Raise the limit one notch and re-measure |
| Shower drifts cool, handle stiff, temperature jumps when a tap opens | Worn or scaled cartridge | Replace the cartridge for that valve model |
| Warm for a minute, then fades; improves when a sink's cold valve is shut | Cold-water crossover | Isolate each single-handle faucet in turn |
| Every tap fades after a few minutes of use | Dip tube, sediment, or a failing element | Check heater setting, then call a plumber |
Call in help if the valve has no visible limit stop and no cartridge you can identify, if water leaks behind the trim, or if the whole house fades and the tank is gas-fired or hardwired. A plumber's cartridge swap typically lands around $150–$350. Tell the plumber your two thermometer readings and what changed during the crossover test — that shortens the visit.
Because the fault sits at the shower valve, not the heater. A low anti-scald limit, a scaled mixing cartridge, or cold water crossing over at a single-handle valve can each cap one shower while sinks on the same hot line deliver full heat.
Remove the handle, find the notched limit ring or tab underneath, and move it one or two notches toward the hotter side. Refit the handle, run the water, and check with a thermometer. Repeat in small steps until the shower tops out around 115–120°F.
Yes. A cartridge that fails open between its hot and cold sides can let cold water cross into the hot line and cool other fixtures. The isolation test — shutting a faucet's cold valve and retesting the shower — spots it quickly.
The part alone usually runs $25–$75 for common brands. A plumber's swap, parts and labor together, typically lands around $150–$350. Discontinued valves cost more, mainly because the cartridge takes longer to source.
No, not when the sinks are already hot. Raising the tank temperature hides the valve fault and raises scald risk at every tap. Fix the limit stop or the cartridge instead, and keep the tank near 120°F.