Quick Answer:
A tub spout that drips only when you run hot water is usually a sign of worn hot-side seals, thermal expansion, or a valve/cartridge issue on the hot side. Start by confirming the leak happens only in hot-only mode, then check seals and compare hot pressure to cold. If simple checks don’t find or stop the leak, call a plumber.
Why This Happens
Hot water affects materials differently than cold. Rubber O-rings and seals can swell, shrink, or become brittle with repeated heating and cooling. A worn or heat-damaged seal on the hot side will let water escape when the hot supply is used but stay closed when only cold is on. Thermal expansion from hot water can also force a weak connection to leak, especially if your water heater or expansion tank isn’t working properly.
Other triggers include sediment buildup, a failing cartridge or valve seat, or recent service events that stressed the system. If the timing suggests a supply interruption or freeze-thaw event, check these related notes: Leak started after city outage and Leak after winterization.
Step-by-Step What to Do
1. Compare hot-only vs cold-only shower mode
- Run the tub in hot-only mode and watch the spout for drips or steady leaks.
- Switch to cold-only and check again. Note whether the leak appears in hot-only, cold-only, or both.
- Record exactly when the leak happens (during flow, after shutting off, or continuously).
2. Verify hot pressure isn’t unusually high
- Turn on another hot faucet (kitchen or bathroom) and compare flow and pressure to the cold side. A big difference suggests pressure or supply issues on the hot line.
- If you have pressure symptoms (hammering, surges, or constant trickle when other taps are off), pressure or thermal expansion could be forcing the spout to leak.
- If you’re unsure about pressure, a plumber can measure it and check the expansion tank and heater settings.
3. Inspect the spout and adjacent fittings for heat-related seal issues
- Visually inspect the spout base and the overflow area for mineral deposits or wetness that points to a seal failure.
- Feel the spout and valve trim for looseness. Do not force or remove parts unless you’re comfortable shutting off the water first.
- If you decide to remove a spout, shut off the water supply before doing any disassembly and follow the manufacturer’s instructions or call a pro.
4. Check the valve/cartridge operation
- Operate the handle slowly through hot and cold positions and watch for leaks at the spout. A cartridge that seals poorly on the hot side will leak only when hot is selected.
- If you see wetness around the valve trim or behind the handle, the problem is likely inside the valve body rather than the spout itself.
5. Try simple, safe fixes
- Tighten any visible mounting screws or set-screws on the spout (do this after turning off water if disassembly is needed).
- If you’re comfortable and the spout type allows it, replace a worn O-ring or slip-on spout gasket. Only attempt this if you know how and have shut off the supply.
- After repairs, test hot-only and cold-only again to confirm the leak is gone.
What Not to Do
- Don’t raise heater temperature trying to “fix” hot-only leaking—this can increase expansion and make the leak worse.
- Don’t ignore a steady leak; small leaks waste water and can damage finishes over time.
- If hot-only persists after basic checks and safe DIY steps, a plumber is appropriate rather than attempting major valve replacement without experience.
When to Call a Professional
- The leak continues after you compare hot vs cold, check seals, and try simple fixes.
- You find signs of unusually high hot-side pressure, hammering, or water heater problems that could need system work.
- The valve cartridge or internal valve body needs replacement and you don’t have the tools or confidence to shut off and drain the system safely.
- There’s active water damage, mold, or you can’t fully stop the flow with the shutoff valves.
Safety Notes
- Always shut off the water supply before disassembling a spout or valve. If you don’t know where the shutoffs are, stop and call a plumber.
- Be careful of scalding. Test water temperature with your hand before prolonged exposure.
- Wear eye protection and gloves if you remove parts. If you smell gas or notice other plumbing systems behaving oddly, stop and seek professional help.
Common Homeowner Questions
- Why does it only leak with hot water?
Worn or heat-damaged seals and thermal expansion on the hot side commonly cause this pattern. - Can I fix it myself?
Simple checks (compare hot vs cold, tighten spout, replace an O-ring) are doable, but internal valve work is best left to a pro. - Will raising the water heater temperature help?
No. Increasing temperature can worsen expansion-related leaks and is not a fix.
For more related articles, see the Tub Spout Leaks When Shower Is On hub.