LP on a Mitsubishi Electric heat pump: Out of water flow rate range for outdoor heat pump unit
Short answer
On a Mitsubishi Electric heat pump, LP means: Out of water flow rate range for outdoor heat pump unit. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Ecodan
The manual says: Out of water flow rate range for outdoor heat pump unit.
Part involved: Primary circuit flow rate / flow sensor.
When it shows: Measured primary flow rate outside the permitted range for the connected outdoor unit.
Likely causes:
- Primary flow rate outside the range in the installation table; incorrect 'Heat pump flow rate range' setting; reduced flow (see L3).
On the Ecodan Co2 Quhz
The manual says: DHW supply temperature overheat protection (THW4 >= 70C for 5 consecutive seconds).
Part involved: DHW supply thermistor THW4 / thermal store temperature.
When it shows: THW4 >= 70C for 5 s (not while no hot water is drawn, first 30 s of draw, or if DHW supply max temp set > 61C / within 24 h of lowering it).
Likely causes:
- 1 Thermal store tank water temperature high (THW5A >= 70C); 2 THW4 fault; 3 FTC board failure.
Is it safe?
On the Ecodan
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated as auto-resetting; code remains until reset.
Manual reset: Installation manuals: press F4 (RESET) then F3 (Yes) on the main remote controller (older controllers: hold button E for 3 s); 'To cancel error codes please switch system off'. Fix the cause first.
On the Ecodan Co2 Quhz
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated as auto-recovering.
Manual reset: Press F4 (Reset) then F3 (Yes) on the main remote controller, or switch system off (installation manual).
What you can check
On the Ecodan
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing. Check the primary circuit pressure gauge (typically about 1-2 bar when cold on sealed systems) and look for visible leaks; do not top up or bleed unless your installer has shown you how and the filling loop is then closed.
On the Ecodan Co2 Quhz
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing.
What the engineer will check
On the Ecodan
- Check the flow-rate table in the installation manual (FTC6 Table 4.3.1 / FTC7 Table 3.4.1); check remote controller Service > Heat pump settings > Heat pump flow rate range; see action for L3.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
On the Ecodan Co2 Quhz
- 1 Refer to L3/L4; 2 Check THW4 resistance; 3 Replace board.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit).
What you might notice
What owners and engineers report
37 owner and engineer reports mention this.
- Short cycling on the Ecodan: 12 reports
- No heating on the Ecodan: 9 reports
- No hot water on the Ecodan: 9 reports
- Keeps stopping / cutting out on the Ecodan: 6 reports
- Hot water too hot / scalding (incl. legionella cycle) on the Ecodan: 1 report
Sources
- Ecodan FTC7 PAC-IF08(1-3)B-E Installation Manual (DG79H200H01) ↗ · DG79H200H01 · page 32 (PDF p.33, 6.2 Self-check) · Reviewed 23 September 2026
- Ecodan EHPT20Q VM2EA Service Manual (GCH15010C) ↗ · GCH15010C · page 39 (10-4 Self diagnosis and action) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026