A tankless water heater should give steady hot water.
When the temperature jumps, something is not right.
This homeowner has an A. O. Smith tankless unit bought in 2020.
The heater is set to 120 degrees.
Yet the display sometimes shows 100 degrees.
At other times, the outlet water jumps to 138 degrees.
That is a real safety concern.
Water at 138 degrees can scald skin fast.
Question
Is the tankless water heater installed correctly?
Can a white PVC vent pipe be moved up four feet near a door?
Answer
Do not assume the installation is correct from photos alone.
A licensed plumber or gas technician should inspect it on site.
The temperature swings may come from gas supply, flow rate, venting, or a bad mixing valve.
A bad gas mix valve can cause uneven firing.
Low gas pressure can also cause poor burner control.
The wrong pipe size may starve the heater.
That can make the unit heat unevenly.
Flow also matters with tankless heaters.
If the water flow is too low, the temperature can swing.
Dirty inlet filters can lower flow.
Scale buildup can also reduce flow.
A yearly flush may help some units.
Venting is a bigger safety issue.
Do not move the PVC vent pipe without checking the manual.
Tankless vents have strict rules.
They include pipe size, slope, length, and termination clearances.
Vents near doors and windows need special clearance.
Exhaust can enter the home if placed incorrectly.
White PVC may be allowed on some units.
But only if the manual lists it as approved.
Some heaters require special vent material.
The pipe rating must match the exhaust temperature.
Check the exact model number first.
Then compare the installation to the A. O. Smith manual.
You can start at the A. O. Smith support.
Also, review safe water temperatures from the CPSC scald safety guidance.
Do not keep using 138-degree water without caution.
Use a thermometer at a faucet.
Write down the results.
Then call a licensed pro.
Ask the pro to check the gas pressure.
Ask them to inspect venting clearances.
Also, ask them to test the mixing valve.
This problem needs a safety-first repair.
I’m Chris Mayer, writing for Plumbing 101.
