UA on a Mitsubishi Electric Ecodan Hydrodan: Low ambient loop flow rate (flow switch operated below 5.5 L/min during compressor operation)
Short answer
On a Mitsubishi Electric heat pump, UA means: Low ambient loop flow rate (flow switch operated below 5.5 L/min during compressor operation). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Low ambient loop flow rate (flow switch operated below 5.5 L/min during compressor operation).
Part involved: Ambient loop flow switch / ambient loop (water or brine).
When it shows: Flow switch operated (under 5.5 L/min) during compressor operation.
Likely causes:
- 1 Valve or PICV of ambient loop closed; 2 SW6-4 on FTC not active when TBI.4 7-8 connected; 3 Disconnection/loose connector (63L) on C.B. or 63L; 4 Defective C.B.; 5 Leakage or shortage of water/brine in ambient loop; 6 Misconnection of water/brine pipe.
Is it safe?
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
What you can check
- 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.
What the engineer will check
- 1 Check valve/PICV; 2 Activate SW6-4 on FTC; 3-4 Power off/on and check if F3 is displayed; 5 Correct water/brine amount; 6 Check water/brine pipe connection.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable) (if refrigerant circuit work is needed).
What you might notice
Sources
- Ecodan Hydrodan EHWT17D-MHEDW Installation Manual RK79D203K04 ↗ · RK79D203K04 · page 51 (PDF p.52) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026