Quick Answer:
If your shower temperature started jumping after a new water heater was installed, first check the heater temperature setting and any mixing or tempering valve installed at the heater. Many causes are simple to test and fix—pressure changes, a mis-set mixing valve, or a cross-connection at a faucet are common—but don’t raise the heater thermostat to “fix” it. Start with the quick checks below and call a pro if you can’t identify the cause safely.
Why This Happens
- New heater settings: The installer may have set the thermostat higher or lower than your old unit, or a built-in tempering valve may be active.
- Mixing/tempering valve behavior: These valves blend hot and cold to reduce scalding, and they may need adjustment after installation.
- Pressure changes: Variations in hot or cold water pressure will shift the mix at the shower valve. This often shows up as sudden spikes or drops in shower temperature—see Temperature swings after pressure change for more on pressure-related causes.
- Cross-connections: A faulty single-lever faucet, a worn cartridge, or a clothes washer filling can let cold water flow into the hot line and cause swings.
- Poor installation: Incorrect piping, a missing check valve, or a reversed connection at the heater can produce unstable output.
Step-by-Step What to Do
1. Verify the heater temperature setting
- Locate the thermostat control on the water heater (tank or tankless). Note the current setting.
- Set the thermostat to a safe, typical temperature (many homes use around 120°F). Avoid setting it higher to “stop” swings—see What Not to Do.
- After changing the setting, wait for the tank to reach temperature and test the shower again.
2. Check for a mixing or tempering valve at the heater
- Look for a valve assembly on the hot outlet pipe near the heater. It may have adjustment screws or a labeled temperature cap.
- Confirm the valve is set to the expected output temperature and that it is functioning (no obvious leaks, corrosion, or loose parts).
- Gently adjust per the manufacturer markings, then retest the shower temperature.
3. Test the shower valve and other fixtures
- Run the shower at a steady flow and watch temperature behavior for a minute or two.
- Open other hot fixtures one at a time (kitchen sink, tub) to see if their use causes shower swings. This helps locate cross-connection or pressure-balance issues.
- Check for a worn cartridge or a pressure-balancing valve in the shower that may need replacement if it no longer holds temperature.
4. Look for pressure and supply issues
- Check that hot and cold supply valves to the heater and to the house are fully open.
- Note whether water use elsewhere or a change in municipal pressure correlates with the swings—pressure changes can alter the hot/cold mix.
5. Consider appliance interactions
- Watch the shower while the washer fills. If the shower shifts when the washer starts, the washer is affecting the supply balance. For more on this pattern, see Shower temp changes with washer use.
- Try running the washer and shower separately to confirm the interaction.
6. Inspect piping and connections
- Look for obvious cross-connection points (mixing valves, bypass lines) and ensure hot and cold lines are correctly routed on the heater outlet.
- Check for a missing or faulty check valve where recirculation or solar systems are connected.
7. Flush and check the heater equipment
- If safe and you are comfortable, flush sediment from a tank unit as heavy sediment can change heat delivery.
- For tankless units, ensure inlet filters are clean and flow sensors are operating. If you’re unsure, stop and call a pro.
What Not to Do
- Do not raise heater temperature to compensate; this increases scald risk.
- Do not remove electrical panels, burner assemblies, or gas controls yourself unless you are qualified. Turn off power/gas first and call a licensed technician for those tasks.
- Do not bypass or disable a tempering or pressure‑balance device as a “quick fix.”
When to Call a Professional
- Temperature swings continue after checking thermostat and mixing valve adjustments.
- There are strong pressure fluctuations in the house or suspected cross-connections you cannot locate.
- There is a leak, gas smell, tripped electrical breakers at the heater, or any sign of unsafe installation.
- You are uncomfortable working near gas, electrical, or pressurized plumbing; call a licensed plumber or HVAC technician.
Safety Notes
- Always turn off power to an electric heater or set a gas heater to pilot/off before doing any work on the unit.
- Use a thermometer to check actual water temperature at the shower—don’t rely on feel alone.
- Be cautious of scald risk when testing. Run tests at lower flow or have someone assist while you monitor temperature with the thermometer at the outlet.
- If you smell gas or see corrosion, stop and call a licensed professional immediately.
Common Homeowner Questions
- Why did the problem start only after the new heater?
Install settings, added tempering valves, piping changes, or a different hot output profile can cause new instability. - Can I adjust the mixing valve myself?
Minor adjustments are often possible, but if you’re unsure how the valve works, call a plumber to avoid creating scald risk or code violations. - Is a fluctuating shower dangerous?
Yes—sudden hot spikes can scald. Test carefully and avoid increasing the heater thermostat to “fix” swings.