On the covered Rinnai tankless water heater, 05 means a fault in the by-pass flow control device. First, turn the water heater's power off and back on once and run a hot tap. Then: Rinnai directs measuring the resistance of the by-pass flow control and replacing the device if it is out of range. If 05 reappears after the reset with hot water running, Rinnai's diagnostic manual says to replace the bypass valve.
Differential
Likely causes of “Rinnai tankless water heater code 05”, and how to tell them apart
- 1
A failed bypass flow control
What separates it Code 05 returns while hot water runs
How to confirm Rinnai says to replace the bypass valve
- 2
A wiring or connector fault
What separates it The measured resistance is wrong
How to confirm Technician measures the resistance
- 3
Scale in the valve
What separates it Hard water
How to confirm Flush and treat the water
- 4
A stuck valve
What separates it Temperature swings
How to confirm Technician inspection
Before you touch a setting
Record this first — changing settings destroys the evidence
- Whether hot water was running
- Your water hardness
Try in this order
0 of 6 completedWhat to do first
ReversibleTurn the water heater's power off and back on once and run a hot tap.
Correct only the documented fault domain
ReversibleRinnai directs measuring the resistance of the by-pass flow control and replacing the device if it is out of range. If 05 reappears after the reset with hot water running, Rinnai's diagnostic manual says to replace the bypass valve.
Turn the power off and on once
ReversibleThen run a hot tap.
Watch for the code
ReversibleWith water running.
Check your water hardness
ReversibleAnd any softener.
Note the code history
ReversibleFor the technician.
Model-specific
What is true of this model and not of its siblings
- The bypass valve
- A tankless heater can overshoot, so the bypass valve mixes in cold water to hold the set temperature. If it fails, the temperature swings or the control reports code 05.
- Rinnai's diagnosis
- Rinnai directs measuring the resistance of the bypass flow control and replacing it if the reading is out of range. If 05 returns after a reset with hot water running, its manual says to replace the bypass valve.
- Preventing a repeat
- Scale makes valves stick. A yearly flush and treating hard water extend the life of the valve.
- Why the bypass exists
- A tankless heater heats water as it passes and can overshoot when the flow changes suddenly, for example when someone turns down a tap. The bypass flow control mixes in a little cold water so that the outlet temperature stays steady. When the valve cannot move, the heater cannot hold the temperature, and it reports 05.
- What you would notice
- Hot water that swings between too hot and too cold, particularly at low flow, is the typical sign. Running a tap on hot at a trickle makes the swing obvious. A steady supply at higher flow may seem normal, so the fault can hide until a shower is turned down.
- What the technician measures
- Rinnai's diagnosis is a resistance measurement of the valve's motor, compared with the range in the service manual. A reading outside the range means replacement; a good reading sends the technician on to the wiring and the board.
Stop boundary
When this stops being a self-service repair
- !Rinnai says to replace the bypass valve if 05 returns with hot water running
- !This is a technician repair
- !Treat hard water to prevent a repeat
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 05 mean on a Rinnai tankless water heater?+
A fault in the by-pass flow control device.
What does the valve do?+
It mixes some cold water to set the temperature.
Is it a DIY job?+
No.
What causes it?+
A failed valve, wiring or scale.