Quick Answer:
If the sink produces steady hot water but the bathtub stays lukewarm or cold, the problem is usually at the tub’s mixing valve, cartridge, or in-wall stop valves — not the water heater. Start by confirming the sink’s hot supply, then run the tub on full hot and check the tub faucet’s anti-scald limiter and cartridge for a hot-side blockage.
Why This Happens
Most homes use a mixing valve or cartridge in each faucet to blend hot and cold water. If the tub’s valve or cartridge is blocked on the hot side, or the anti-scald limit inside the valve is set too low, the tub will not draw enough hot water even though the heater and sink are fine. Partially closed supply stops behind the tub or a faulty diverter can cause the same symptom.
Step-by-Step What to Do
1. Confirm the sink hot water is steady
- Run the sink’s hot tap for a couple of minutes. Watch temperature — it should become consistently hot and stay that way.
- If the sink is not steady or goes cold, consider troubleshooting the heater or plumbing supply first and consult the guide on no hot water after turning it back on.
2. Run the tub on full hot
- Turn the tub faucet to the hottest setting and let it run for 2–3 minutes. Note if the flow increases in temperature or stays cool.
- Use a thermometer or a careful hand test at first. Compare the tub’s hot temperature to the sink’s.
3. Check the anti-scald limit
- Many single-handle tub valves have an internal temperature limiter (anti-scald device). Remove the handle per the manufacturer instructions to inspect the limiter.
- If the limiter is set too low, it restricts hot water. Adjust it incrementally and retest the tub.
4. Inspect the cartridge for a hot-side blockage
- Shut off the water to the tub before removing the cartridge (see your shutoff procedure). Pull the cartridge and look for mineral buildup or broken parts on the hot-side inlet.
- Clean or replace the cartridge if you find debris or corrosion. If you aren’t comfortable with removal, skip to calling a pro.
5. Verify valve stops are open if accessible
- Some tubs have in-wall or access-panel shutoffs. Make sure both hot and cold stops are fully open. If you must open them, do it slowly to avoid pressure spikes — remember to turn the water back on slowly.
- If the stops are hard to reach, corroded, or leak, a plumber should handle them.
6. Recheck and narrow the cause
- After each change, run the tub on full hot again to see if temperature improves.
- If the tub remains cool while the sink is hot, the tub valve or in-wall plumbing to the tub is the likely cause and may need replacement or professional attention.
What Not to Do
- Do not remove safety stops without a plan.
- Do not keep testing until scalding.
- Do not assume the water heater is the issue.
When to Call a Professional
- You cannot access the cartridge or in-wall stops, or the valve is corroded and won’t come apart.
- The tub leaks after you work on it, or you find damaged shower/tub piping.
- Temperature problems persist after checking limiter, cartridge, and stops — a plumber can diagnose in-wall mixing valves, blocked piping, or replacement options.
Safety Notes
- Always shut off the water supply to the fixture before disassembling valves. If you don’t know where to shut it off, stop and get help.
- Avoid repeated hot testing that risks scalding. Test gently with a hand or a thermometer and keep children away.
- Wear gloves and eye protection when working on valves. If a repair looks beyond your tools or experience, hire a licensed plumber.
Common Homeowner Questions
- Why does the sink get hot but the tub does not? Most often the tub’s mixing valve, cartridge, or anti-scald limiter is restricting hot water to the tub.
- Can I fix this myself? Maybe — if it’s a limiter adjustment or an accessible cartridge with minor buildup. Stop and call a pro for corroded parts or in-wall work.
- Is the water heater likely the cause? Unlikely if the sink is hot; the problem is usually local to the tub’s valve or supply stops.