Quick Answer:
Short pressure or water-main work during a city outage often sends sediment and sudden pressure changes into home lines. Start by flushing at a tub spout or a sink to clear debris, then test the shower. If the shower still swings between hot and cold, the pressure-balance spool or cartridge is likely clogged or sticking and may need cleaning or repair by a professional.
Why This Happens
When a city outage ends, water flow resumes with pressure spikes and disturbed pipe sediment. That material can travel through your service line and lodge in faucets and valves. Shower valves that use a pressure-balance spool or cartridge are sensitive: debris can cause the spool or cartridge to stick briefly, leading to unpredictable temperature shifts. If the issue shows up only on one side, check the hot feed specifically; for that situation see Temp fluctuates only on hot side.
Step-by-Step What to Do
1. Flush at a tub spout or sink first
- Open a nearby tub spout or sink faucet on both hot and cold for 1–3 minutes. Use full flow if practical.
- Flushing from a tub spout or sink first directs debris out of the branch lines before it reaches the shower valve.
2. Test the shower
- After flushing, run the shower and move the handle between hot and cold several times.
- Note whether the temperature is stable, or if it still swings when other fixtures are used (toilets flushing, washing machine filling, etc.).
3. Repeat flush and isolate
- If instability continues, repeat the flush for a few more minutes. Try flushing other fixtures on the same hot or cold branch to see if the problem follows one line.
- Shut off and isolate appliances briefly (dishwasher, washing machine) while testing to eliminate their effect.
4. Suspect the valve spool or cartridge if instability persists
- Persistent swinging after proper flushing usually means debris is in the pressure-balance spool or cartridge. That part can stick or not seat correctly until cleaned or replaced.
- Cleaning or replacing the cartridge is a common fix, but it requires the right replacement part and basic plumbing skill. If you recently had a valve service and now see shifts, consider the related issue described in Temperature unstable after replacing valve.
What Not to Do
- Don’t ignore post-outage instability—debris events are common and can get worse with continued use.
- Don’t force parts or use excessive tools on the valve trim; that can damage cartridge seals or trim components.
- Don’t assume the problem will fix itself without flushing and testing; if flushing doesn’t fix it, a plumber is appropriate.
When to Call a Professional
- Call a plumber if flushing and basic checks don’t stop the temperature swings.
- Call if you are uncomfortable removing trim or cartridge parts, or if the valve is hidden behind finished walls.
- Call if you see leaks, persistent extreme scalding risk, or if multiple fixtures behave oddly after the outage.
Safety Notes
- Be careful when testing with hot water — sudden scalding can occur. Keep water at a safe level while diagnosing.
- Turn off the main water only if you understand how to restore it; otherwise leave isolation and repairs to a pro.
- Avoid electrical contact when working near fixtures (wet hands, power tools). If in doubt, hire a licensed plumber.
Common Homeowner Questions
- Why did this start after the outage? — Pressure surges and sediment released during the outage often reach valves and cause sticking.
- Can I clean the cartridge myself? — You can if you know the model and have basic tools, but many homeowners prefer a pro to avoid damage.
- How long until it’s safe to use again? — After flushing and a successful test run, it’s usually safe; if instability continues, stop using the fixture and call a plumber.