Quick Answer:
Most often this is not a heater failure. First confirm the heater outlet temperature and any mixing/tempering valve settings are correct. Then fully purge the hot-water lines to remove air or debris and retest the shower. If the heater is producing hot water but the shower stays cold, the issue is usually in the local plumbing or a mixing valve, not the new unit itself.
Why This Happens
- Air or debris introduced during installation can block a shower cartridge or restrict flow until lines are flushed.
- Thermostatic mixing/tempering valves (at the heater or a shower valve) can be set too cool or may have been disturbed during the install.
- If the heater wasn’t allowed to fully heat after refill, outlet water can be lukewarm until the tank reaches set temperature.
- Isolated shutoffs, zone valves, or faulty cartridges can leave only one fixture cold even when other taps are hot.
Step-by-Step What to Do
1. Confirm the heater is producing hot water
- With the heater on and set to its normal temperature, open a hot faucet near the heater (a sink or laundry sink) and let it run for a minute. Measure the water with a simple kitchen thermometer or feel the flow carefully. The outlet should be noticeably hot—if it’s not, allow the heater 30–60 minutes to fully heat the tank and recheck.
2. Check mixing/tempering valve settings
- Locate any tempering or thermostatic mixing valve near the heater or under a utility cabinet. These valves blend cold with hot to limit outlet temperature. If the valve was moved during install it could be set low. Adjust only small amounts and recheck outlet temperature at a nearby tap.
3. Purge air and debris from hot lines
- Start at the fixture closest to the water heater and open all hot-water taps in the house (sinks, tubs, showers) one at a time. Run them until the flow is steady and any sputtering stops—this clears trapped air. A strong, steady flow typically means the line is purged.
- For a stubborn shower, remove the showerhead and run the hot valve to flush the line directly for several minutes; small particles that clog a cartridge will often be flushed out this way.
- After flushing, reinstall the showerhead and test the shower again.
4. Test the shower and isolate the problem
- Check other hot fixtures. If sinks or other showers are hot but this one remains cold, the issue is local to that shower — a cartridge, valve, or isolated shutoff.
- If no fixtures are hot, the problem is at the heater or a main control valve and needs broader troubleshooting.
- For help with a single affected bathroom, see Hot water missing only in one bathroom. If the upstairs shower is the only problem, see No hot water only upstairs shower.
What Not to Do
- Don’t crank the heater temperature to fix a shower-only issue — scald risk increases and raising the tank temperature is unsafe if the problem is a local valve or cartridge.
- If mixing valves or thermostatic devices are involved, don’t attempt major adjustments or replacements unless you are confident in how they work — a licensed plumber is appropriate for those tasks.
When to Call a Professional
- If the heater outlet stays cool after it had time to heat, the installer or a plumber should inspect the unit and piping.
- If you suspect a faulty mixing/tempering valve, or if flushing doesn’t clear a clogged shower cartridge, call a plumber to avoid damage or incorrect adjustments.
- Call a plumber immediately if you find leaks, gas smell, or electrical problems around the heater.
Safety Notes
- Avoid long exposure to very hot water—test with a thermometer or brief hand test before full use to reduce scald risk.
- Turn off power or gas to the heater only if you are sure how to do so safely; if unsure, contact the installer.
- When removing fixtures (showerheads, cartridges), close any local shutoffs first to avoid water damage.
Common Homeowner Questions
- Q: Why is my shower cold right after installing a new water heater?
A: Air in the lines, a cool mixing valve setting, or the tank not being fully heated are the most common causes. - Q: How long should I wait after filling the tank to test hot water?
A: Wait 30–60 minutes for most residential tanks; larger tanks may take longer. - Q: Can I fix a cold shower by adjusting the heater thermostat?
A: No—don’t raise the heater temperature to cure a shower-only problem because that increases scald risk; troubleshoot the local plumbing first.
For more related articles, see the No Hot Water at Shower but Everywhere Else hub.