- Heat pumps
- Air-to-water heat pump
L6
Mitsubishi Electric L6 on an Ecodan heat pump: circulation water freeze protection
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On Ecodan systems, L6 means the circuit water has cooled to the freeze limit. Keep the heat pump powered so it can protect the circuit, do not switch it off and call the installer today.
Safety first
Stop and check this first
- L6 is the circuit water freeze protection. With L6 in winter, do not cut power to the heat pump and do not use flames to thaw pipes. Call the installer the same day.
- Mitsubishi Electric states that the cylinder and outdoor unit must not be disassembled, altered or repaired by the user, and that all electrical work must be done by a qualified technician. Do not open the cylinder or work on the water circuit inside it.
Safe checks you can perform
- Heat pump power supply
- Visible leaks or frozen pipes
- Flow, valves and THW1 and THW2 sensors
Leave these tasks to a qualified professional
- Do not attempt this professional task: Flow and valves before closing the job
- Do not attempt this professional task: Measure the sensors against Mitsubishi's table
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Flow and valves before closing the job
What they doIf the sensors are fine, the technician checks pump, purging, strainer and the 2-way and 3-way valves.
They must show youthe flow measured after the check and the valve tested from the service menu.
In writingthe cause found and the operating test without L6.
Measure the sensors against Mitsubishi's table
What they doThe technician disconnects THW1 and THW2, measures their resistance and compares it with the manufacturer's table.
They must show youthe kΩ measured against 15 kΩ at 0 °C or 6.3 kΩ at 20 °C and the sensor firmly in its holder.
In writingwhich sensor failed or that both were ruled out.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Heat pump power supply
- Who:
- you.
- How:
- check at the consumer unit that the heat pump is powered and do not switch it off while it is cold outside.
- Result that matters:
- L6 is the circuit freeze protection; without power, the circulation pump cannot move the water.
- If it was unpowered:
- restore power once and call the installer; the freeze risk was real.
- If it was powered:
- the controller saw 1 to 3 °C water with the unit running; the installer must check the flow.
- Evidence to keep:
- a note of the outdoor temperature and whether there was a power cut.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Visible leaks or frozen pipes
- Who:
- you, without opening the cylinder.
- How:
- look at outdoor pipework and the floor under the cylinder for ice, water or damaged insulation.
- Result that matters:
- Mitsubishi includes primary circuit leaks among the causes.
- If you see water or ice:
- do not thaw with a flame; call the installer urgently.
- If you see nothing:
- a visible leak is ruled out, not low flow or the sensor.
- Evidence to keep:
- photos of the pipework.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Flow, valves and THW1 and THW2 sensors
- Who:
- the installer or a Mitsubishi technician.
- How:
- they purge, check the strainer and pump, test the 2-way and 3-way valves and measure THW1 and THW2 resistance with the sensor disconnected.
- Result that matters:
- Mitsubishi gives 15 kΩ at 0 °C, 9.6 kΩ at 10 °C and 6.3 kΩ at 20 °C for these sensors.
- If resistance does not match the real temperature:
- a faulty or misplaced sensor is confirmed.
- If the sensors are fine:
- the cause is flow or a valve.
- Evidence to keep:
- a job sheet with the kΩ measured and the water temperature at the time.
Requires opening the cylinder and measuring with a multimeter. Qualified technician only.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- Mitsubishi Electric
- Product type
- Air-to-water heat pump
Known code variants
- Ecodan L6
- Ideal boiler L6
- Ideal boiler L6 false flame
- Ideal L06
- Ideal L6
- Ideal Logic L6
- Mitsubishi Ecodan L6
- Mitsubishi L6 heat pump
What it means
On Ecodan FTC5 cylinders, L6 trips when flow sensor THW1 reads 1 °C or less for 10 consecutive seconds, or return sensor THW2 reads 3 °C or less for 10 seconds. It is not shown with the freeze function disabled or for 10 minutes after pump 1 starts. Mitsubishi links it to low flow, 2-way or 3-way valves, a THW1 sensor detached from its holder, faulty sensors or the FTC board. The code does not say whether the water actually froze.
Warnings and stop conditions
- Higheditorial warning
L6 is the circuit water freeze protection. With L6 in winter, do not cut power to the heat pump and do not use flames to thaw pipes. Call the installer the same day.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- Highsource verified
Mitsubishi Electric states that the cylinder and outdoor unit must not be disassembled, altered or repaired by the user, and that all electrical work must be done by a qualified technician. Do not open the cylinder or work on the water circuit inside it.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Symptoms and causes
Probable causes · in order
- Reduced primary flowCommon
A faulty pump, insufficient purging, a blocked strainer or a leak: water stands still and cools. Mitsubishi gives the same steps as for L9.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- 2-way or 3-way valvePossible
An actuator that does not switch leaves part of the circuit without circulation. Mitsubishi asks to test it electrically or operate it manually from the service menu.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- THW1 detached or faulty sensorsPossible
If THW1 came out of its holder or the sensors read wrong, the controller sees a temperature the water does not have. Measuring sensor resistance separates it.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- FTC boardLess common
If sensors, flow and valves are correct, Mitsubishi says to replace the board.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Virtual technician
Does L6 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Mitsubishi Electric · 2016 · manual
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