Quick answer
Error code 11 on a Rheem tankless water heater means the flame rod does not detect a flame. Rheem's service manual says the cause is commonly gas supply, venting or both. The unit locks out. Confirm the gas valve is open and the vent termination outside is unobstructed, then reset once. If code 11 returns, stop resetting and call a licensed gas technician.
Error code 11 on a Rheem tankless water heater means the flame rod is not detecting flame. Either no flame appeared when the heater tried to fire, or one did and the control could not confirm it, so it shut the gas and locked out. Rheem's tankless troubleshooting manual adds that the cause is commonly gas supply, venting or both, and Ruud tankless units use the same chart. Check the code list in your own manual too: charts change between model generations, and the paperwork for your unit is the authority.
Is it safe to reset a Rheem tankless showing code 11?
One reset is reasonable; a second or third without a cause found is not advisable. The lockout is protective, since the heater will not keep feeding gas to a burner it cannot prove is lit. Repeated resets ask it to try again under the same conditions, with the same result.
If there is any smell of gas near the heater or the meter, do not reset at all. Go outside and call Enbridge Gas; the steps are in what to do if you smell gas.
Before you call: the gas valve, the vent and one reset
- Gas valve. Follow the gas pipe to the heater and confirm the shut-off handle is parallel to the pipe. If no gas appliance in the house will light, the supply itself is off and that is a call to the utility.
- Vent termination. Go outside and look at where the heater exhausts. Remove snow, ice, leaves or a nest from the opening, and from the air intake if the heater has one beside it.
- Reset once. Close the hot taps, switch the heater off and on at its remote or power button, then run one hot tap.
- Note what happens. Hot water that arrives and then cuts out on code 11 is different information from a heater that never fires, and the technician will ask.
Gas, venting, igniter, flame rod: the likely causes in order
A flame rod proves flame by passing a very small electrical current through it. Anything that weakens the flame at the rod, or insulates the rod itself, reads as no flame. Rheem's manual puts gas supply and venting ahead of the parts:
- Gas supply. Low inlet pressure, an undersized line, or pressure that drops when the furnace and the water heater fire together on a winter morning. The burner lights weakly or not at all.
- Venting. A blocked termination, an over-long run, or exhaust drawn back into the intake leaves the burner short of clean air, and the flame becomes unstable.
- The igniter rod. A cracked or fouled igniter gives no spark, so there is never a flame to detect.
- The flame rod or rods. Oxide build-up or a loose lead blocks the signal from a flame that is burning normally.
- The gas control valve, when it does not open or regulate the way the control commands.
- An altitude DIP switch set wrong, so the control meters fuel and air for an elevation the heater is not at.
Why Rheem's manual sends technicians back to gas and venting
Rheem's service manual is blunt on one point: if every component tests OK, the gas supply and venting must be thoroughly inspected. That instruction points outside the heater's cabinet. A line that fed a 40,000 BTU tank without complaint may not carry a tankless burner at full fire, and the shortfall only appears when demand is high.
The technician measures inlet gas pressure at rest and then under load with the other gas appliances running, checks the vent run against Rheem's length limits, tests the igniter and reads the flame signal. If the gas line turns out to be the cause, our answer on whether a tankless water heater needs a bigger gas line explains the fix. Older houses that have added a range, a fireplace and a barbecue line to the original piping over the years are where an undersized run tends to show up.
What a Rheem code 11 repair costs
- Diagnostic visit: $90–$150, applied to the repair when you go ahead.
- Water heater repairs: $150–$800, depending on the part.
- Gas line upsizing: adds $400–$1,200 where the existing run is too small.
Igniter, flame rod and gas valve prices depend on the model, so that number comes in a written quote after testing and before any part is ordered. If the heater is also overdue for a descale, the flush can be done on the same visit.
Call when code 11 comes back after a single reset. Gas pressure testing and anything behind the heater's cover is licensed gas work under Ontario's TSSA rules. ZK Mechanical's technicians hold TSSA gas licences and take tankless water heater repair calls at (647) 801-1252, or you can book a visit through the contact page. For the general version of this problem across brands, see why a tankless water heater is not igniting.