- Heat pumps
- Air-to-water heat pump
L9
Mitsubishi Electric L9 on an Ecodan heat pump: low primary circuit flow
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Ecodan systems, L9 means the flow sensor measures low water flow in the primary circuit. Mitsubishi points at water circulation, air in the circuit or a valve or pump not energising. Note the code and call your installer.
Safety first
Stop and check this first
- 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
- Code and when it appears
- Visible leaks and valves
- Purge, strainer, pump and flow sensor
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure flow against the model range
- Do not attempt this professional task: Electrical test before replacing sensor or board
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.
Measure flow against the model range
What they doThe technician reads the actual flow and compares it with the recommended range Mitsubishi gives for your outdoor unit.
They must show youthe flow reading in litres per minute against your model's range.
In writingwhich cause they found, air, strainer, valve, pump or sensor, and what they corrected.
Electrical test before replacing sensor or board
What they doIf the circuit is fine, the technician tests the flow sensor and checks CN1A, IN2 and the SW2-2 setting before replacing the FTC board.
They must show youthe sensor test result and a photo of the connector.
In writingthe part replaced, its part number and the operating test without L9.
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.
Code and when it appears
- Who:
- you.
- How:
- note the code on the main remote controller, the date and whether the house was heating or making hot water.
- Result that matters:
- Mitsubishi only shows L9 after 10 seconds of low flow and never in the first minute after the pump starts.
- If it appears with the pump already running:
- the low flow is real for the controller; it is not a start-up event.
- If it appears with another code:
- note them all; each has different causes.
- Evidence to keep:
- a photo of the controller with the code and the time.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
Visible leaks and valves
- Who:
- you, without opening the cylinder.
- How:
- look under the cylinder and along visible pipework for water, and check whether anyone closed a valve on the heating circuit.
- Result that matters:
- Mitsubishi includes primary circuit leaks among the causes of L9.
- If you see water or a closed valve:
- tell the installer; it is a specific cause.
- If you see nothing:
- air, strainer and sensor are not ruled out; the installer measures those.
- Evidence to keep:
- photos of any drip or closed valve.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
Purge, strainer, pump and flow sensor
- Who:
- the installer or a Mitsubishi technician.
- How:
- they open the purge valve, check the strainer, look for leaks, check that flow is within the recommended range for your outdoor unit and electrically test the flow sensor and the 2-way valve.
- Result that matters:
- the measured flow against Mitsubishi's recommended range for the outdoor unit model.
- If flow rises after purging or cleaning the strainer:
- a hydraulic cause is confirmed.
- If flow is correct and L9 persists:
- the sensor, the CN1A connector, the SW2-2 setting or the board is the suspect.
- Evidence to keep:
- a job sheet with the measured flow and what was purged or cleaned.
Opening the cylinder and testing electrical parts is qualified technician work. The user must not disassemble the unit.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
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 L9
- Mitsubishi Ecodan L9
- Mitsubishi L9 heat pump
What it means
On Ecodan FTC5 cylinders, L9 appears when the flow sensor detects low flow for 10 seconds; it is not shown for 1 minute after circulation pump 1 starts. Mitsubishi lists insufficient pump head, a faulty pump, insufficient air purge, a blocked strainer, a leak, valves installed incorrectly, the 2-way valve actuator, the CN1A connector or IN2 terminal, the sensor itself, the SW2-2 setting or the FTC board. The code does not say which.
Warnings and stop conditions
- 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
- Air or blocked strainerCommon
Insufficient purging and a blocked strainer are the hydraulic checks Mitsubishi lists first. They fit after refilling or after work on the circuit.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
- Pump head too low or faulty pumpPossible
If the pump cannot overcome the system pressure loss, flow stays short even without air. Mitsubishi asks to compare the pump with its requirements table.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
- Valves, leak or 2-way valvePossible
A leak, valves fitted wrongly or a 2-way valve actuator that does not open reduce flow. Mitsubishi's homeowner page also mentions a zone valve or pump not energising.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
- Flow sensor, wiring or settingPossible
With a correct circuit, the fault can be the sensor, the CN1A connector, the IN2 terminal, the SW2-2 setting or the FTC board. Only an electrical test separates them.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C · Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
Virtual technician
Does L9 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
- 2
Mitsubishi Electric Living Environmental Systems UK, support for Ecodan owners
Mitsubishi Electric · official_doc
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