- Heat pumps
- Air-to-water heat pump
L3
Mitsubishi L3 on an Ecodan heat pump: circulation water overheat
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On an Ecodan, L3 means the flow or return water reached a protection temperature and the system stopped. Note the code, look at the temperatures on the controller and the circuit pressure, and tell your installer if it keeps coming back.
Safety first
Stop and check this first
- The system water may be at 80 ºC or more when L3 trips: do not open air vents or touch bare pipes while the system is hot.
- Mitsubishi states the user must not dismantle or repair the cylinder or the outdoor unit; electrical and refrigerant work belongs to an authorised installer.
Safe checks you can perform
- Read the code and temperatures on the main controller
- Look at the heating circuit pressure gauge
- Check that the visible circuit valves are open
- Look for drips or water marks
Leave these tasks to a qualified professional
- Do not attempt this professional task: Primary flow measurement and pump check
- Do not attempt this professional task: Test of the valves and booster heater relays
- Do not attempt this professional task: Check of thermistors THW1, THW2 and THW5
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.
Primary flow measurement and pump check
What they doThe installer purges air, checks the strainer and the circulation pump and confirms the flow is inside the range Mitsubishi recommends for your outdoor unit.
They must show youthe measured flow against the manual's range for your model and the state of the strainer.
In writingthe flow obtained, whether the strainer was cleaned or air purged and any change to the pump.
Test of the valves and booster heater relays
What they doThe technician tests the 2-way and 3-way valves electrically, can drive the 3-way valve manually from the service menu and checks relays BHC1, BHC2 and BHCP.
They must show youthe 3-way valve moving in manual mode and the relay test.
In writingwhich valve or relay failed, the part replaced and its reference.
Check of thermistors THW1, THW2 and THW5
What they doThe professional checks the thermistors sit in their holders, measures their resistance against the manual's table and compares the FTC board reading with a hand-held thermometer.
They must show youeach thermistor's measured resistance against the table and the thermometer reading.
In writingthe values measured, the supply voltage and any parts replaced with the reason.
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.
Read the code and temperatures on the main controller
- Who:
- you, from the main controller, without opening anything.
- How:
- photograph the error screen showing L3 and, if your controller displays flow, return and tank temperatures, write them down with the time and the mode it was in (heating, hot water or legionella prevention).
- Result that matters:
- a record of when L3 trips and in which mode, which helps the installer target the visit.
- If L3 always appears in the same mode, for example during the legionella cycle:
- tell the installer exactly that.
- If it trips in different modes with no clear pattern:
- report that too, as it points more to flow or a thermistor.
- Evidence to keep:
- photos of the screen with the code and of the temperatures shown.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Look at the heating circuit pressure gauge
- Who:
- you, just looking.
- How:
- find the primary circuit pressure gauge on the cylinder or in the pipework and note the reading with the system stopped. Compare it with the figure your installer gave you or the mark on your system.
- Result that matters:
- knowing whether the circuit has lost pressure, which often goes with a leak or air in the pipes.
- If the pressure is clearly lower than usual:
- do not top up yourself unless you were shown how, and call the installer.
- If the pressure is normal:
- write the figure down anyway for whoever comes to check it.
- Evidence to keep:
- photo of the gauge with the needle visible.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Check that the visible circuit valves are open
- Who:
- you, no tools.
- How:
- look at the isolating valves you can see on the flow and return pipes next to the cylinder and on the underfloor or radiator manifolds. Do not touch motorised valves or open any covers.
- Result that matters:
- ruling out a closed valve stopping the water from circulating.
- If a valve was shut or half shut:
- tell the installer which one before running the system again.
- If they are all open:
- any low flow has another cause that a professional must measure.
- Evidence to keep:
- photo of the valves and their handle positions.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Look for drips or water marks
- Who:
- you.
- How:
- check under the cylinder, at the pipe joints you can see and around the outdoor unit for drops, limescale or puddles. A leak is one of the low flow causes Mitsubishi names.
- Result that matters:
- spotting a visible leak before the technician arrives.
- If you see water or scale at a joint:
- take a photo and report it without tightening or dismantling anything.
- If everything is dry:
- the technician will check the pump, strainer and valves.
- Evidence to keep:
- photos of the spot where water appears.
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
- aerotermia Mitsubishi L3 sobrecalentamiento
- Ecodan error L3
- Ecodan L3
- Ecodan L3 agua demasiado caliente
- Ecodan L3 water too hot
- error L3 Ecodan
- FTC5 L3
- L3
- Mitsubishi Ecodan L3
- Mitsubishi Electric L3 aerotermia
- Mitsubishi Electric L3 heat pump
- Mitsubishi heat pump L3 overheat
What it means
The Ecodan FTC5 cylinder service manual (OCH570C) shows L3 when the flow thermistor THW1 or the return thermistor THW2 reads 80 ºC or more for 10 seconds in a row. Mitsubishi links it to low primary flow (pump, trapped air, a blocked strainer or a leak), 2-way or 3-way valves not operating, a booster heater relay, a supply voltage rise or a thermistor that is loose or faulty. Other models may use a different threshold. Seeing L3 does not on its own prove that any of those parts has failed.
Warnings and stop conditions
- Mediumeditorial warning
The system water may be at 80 ºC or more when L3 trips: do not open air vents or touch bare pipes while the system is hot.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- Highsource verified
Mitsubishi states the user must not dismantle or repair the cylinder or the outdoor unit; electrical and refrigerant work belongs to an authorised installer.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Symptoms and causes
Probable causes · in order
- Not enough flow in the primary circuitCommon
Mitsubishi lists a pump without enough head, poorly purged air, a blocked strainer or a leak: with little flow the water overheats.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- 2-way or 3-way valve not switchingPossible
If the 3-way valve actuator or a site-fitted 2-way valve fails, the hot water does not go where it should and the temperature climbs.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- Booster heater not switching offPossible
The manual includes a fault in the BHC1, BHC2 or BHCP booster heater relays, which would leave the heater on when it should be off.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- THW1 or THW5 out of its holder, or a faulty thermistorPossible
A displaced thermistor or one giving a wrong value makes the FTC board read temperatures that are not real and trip the protection.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
- Rise in supply voltageLess common
Mitsubishi also points to an increase in mains voltage, which the technician checks by measuring the unit's power supply.
Evidence: Mitsubishi Electric Ecodan FTC5 cylinder unit service manual OCH570 revised edition C
Virtual technician
Does L3 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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