Hot water in the sink but not the shower: why it happens

Why it's probably not the water heater

If one faucet gets properly hot, the water heater is making hot water. Whatever is cooling your shower is between the pipe and the shower head, and that's the shower valve.

There's one exception. A sink only uses a little hot water, while a shower draws much more for much longer. If the shower starts hot and turns cold after a few minutes, the heater may be running short, and that's a water heater problem. If the shower never gets hot at all, even for a moment, read on.

1. The anti-scald limit stop is set too low

Most single-handle shower valves have a limit stop that controls how far the handle can turn toward hot. It's there to prevent scalding. If it's set too far toward cold, the shower never gets hot no matter how far you turn the handle.

Delta, for example, calls this the rotational limit stop. Its support page says that each tooth the stop is rotated clockwise lowers the maximum water temperature by about 2°F, so moving it the other way raises it. Adjusting it usually means taking the handle off. Follow your valve maker's instructions, and don't remove the stop altogether.

2. A clogged or worn cartridge or balancing spool

Pressure-balancing valves contain a part that shifts to keep the temperature steady when someone flushes a toilet. Moen's troubleshooting guide says the typical cause of no hot water from a tub or shower valve is debris or an obstruction keeping that balancing spool from moving freely. Depending on the model, the fix is cleaning or replacing the spool, or replacing the cartridge and flushing the valve body.

3. Shutoff stops partly closed

Some shower valves have small built-in shutoffs, called stops or check stops, on the hot and cold inlets. Moen's guide says to make sure they're fully open. They're sometimes left partly closed after a repair.

4. Hot and cold lines reversed

If the problem started right after a new valve was installed, the hot and cold supply lines may be connected backward. Moen suggests feeling the supply pipes, where they're accessible, to check which is which. This one needs a plumber to re-pipe.

What you can safely check

• Run the sink and shower in the same bathroom. If both are lukewarm, it's the water heater, not the valve.
• Turn the shower handle all the way to hot and wait a full minute.
• Find your valve's brand and model. The maker's support site usually has the adjustment steps.

If you're not comfortable pulling the handle and cartridge, a plumber can usually sort out a shower valve in one visit.