Quick Answer:
If the shower temperature swings only on the hot side, the issue is usually in the hot supply or the shower valve’s hot-side components. Start by comparing the hot side flow to another fixture and check the valve stop on the hot line. Small blockages, a failing cartridge or a restricted stop can cause the hot supply to vary while the cold side stays steady.
Why This Happens
Many shower valves mix hot and cold with a cartridge or pressure-balancing mechanism. If the hot side has lower or intermittent pressure, or the hot inlet is partially blocked, the valve cannot hold a steady temperature. Mineral buildup, a partially closed stop, or a failing cartridge are common culprits. If you recently worked on the shower valve, problems can also appear shortly after and are discussed in Temperature unstable after replacing valve.
Step-by-Step What to Do
1. Check hot water at another fixture
Open a nearby hot-only tap (kitchen or bathroom sink) and note the flow and temperature while someone uses the shower. Compare hot pressure at the shower to hot pressure at a sink. If the sink shows steady hot flow while the shower does not, the issue is likely at the shower valve or its hot inlet, not the water heater or main supply.
2. Inspect the hot-side stop for restriction
Locate the shower shutoff stops (usually behind an access panel or under a nearby sink). Partially closed or clogged stops can throttle the hot feed. To inspect:
- Turn off the hot-side shutoff fully, then open it again to flush any debris.
- With water off, remove the stop cap (if accessible) and check for mineral buildup or a broken washer.
- If you see a clear restriction, clean or replace the stop cartridge or the stop assembly.
3. Check the cartridge or mixing valve
Many hot-side fluctuations come from a worn cartridge, a damaged seat, or debris lodged in the hot inlet. Steps:
- Turn off both hot and cold supply at the stops and relieve pressure by running the shower.
- Remove the handle and trim, then pull the cartridge per the manufacturer’s instructions.
- Inspect for scoring, worn seals, or sediment. Clean parts gently with vinegar solution if mineral deposits are present, or replace the cartridge if worn.
4. Test again and observe
After cleaning or replacing parts and reopening the hot stop, run the shower for a few minutes. Watch for fades or returns in heat—this pattern can match the issue described in Hot water fades then returns. If fluctuations continue, note when they happen (e.g., when another fixture runs) to help diagnose cross-connections or pressure issues.
What Not to Do
- Do not force the handle hotter to overcome hot-side fluctuation.
- Don’t disassemble the valve without shutting off both hot and cold stops first.
- Avoid using excessive force on trim screws or the cartridge—broken parts make repairs harder.
When to Call a Professional
Call a licensed plumber if:
- You cannot access the stops or valve without major demolition.
- The hot supply to the valve is steady at other fixtures but fluctuates only at the shower after basic checks.
- There are signs of leaks behind the wall, persistent scalding risk, or unclear pressure-related issues that may involve the home’s main plumbing or pressure regulator.
Safety Notes
- Always shut off both hot and cold supplies before removing the cartridge or internal valve parts.
- Beware of scalding when testing temperature—use short tests and keep hands clear of the spray when first restoring flow.
- Wear gloves and eye protection when working with cleaning agents or removing mineral deposits.
Common Homeowner Questions
-
Why does the hot side fluctuate but cold stays steady?
A restricted hot inlet, a partially closed stop, or a failing cartridge typically affects only the hot side. -
Can mineral buildup cause this?
Yes—scale in the stop, inlet screens, or cartridge can reduce hot flow and cause temperature swings. -
Is this dangerous?
It can be if sudden hot surges occur; take care when testing and call a plumber if you cannot stabilize it safely.