Mixer Unit / Recirculation Fans / Filters — Engineering Details

Zone Controller / Trim Air showed how the zone controller heats each zone. But before the heated air reaches the seats there is one last loop — recirculation: the recirculation fans draw used cabin air to the underfloor, through filters, into the mixing unit where it blends with the packs' fresh cold air, and back to the cabin. This deep-dive opens up the mixing unit, the two recirculation fans, the two recirculation valves, the eight filters, the clogging indicators, the check valves, the cockpit's independent fresh-air supply, the freighter MORV, the main-deck-smoke cascade, the CAB FANS and OVHT COND FANS RESET pbs, and the effect of recirculation on cabin humidity.

Scope: cargo isolation is in Cargo Environmental Control (this article covers only the freighter main-deck-smoke cascade onto the fans/packs); avionics ventilation using the upstream recirculation air is in Avionics Ventilation; the trim-air heating downstream is ata-21-09.

1. Component locations

Freighter configuration adds: Mixer Overpressure Relief Valve (MORV) 260HG (mixing-unit upper outlet port) and main-deck shut-off valves 253–256HG (one per main-deck area).

2. The full recirculation loop

                        cabin
                          │ used air (sinks / through grilles)
                          ▼
          ┌────────[underfloor area]────────┐
          │  ┌──[recirculation filters]──┐   │
          │  │  5250–5253HG (FWD)         │   │
          │  │  5254–5257HG (AFT)         │   │
          │  │  8 filters + 2 housings    │   │
          │  └────────┬───────────────────┘   │
          │           │ filtered (no particles)│
          │  ┌──[clogging indicators]──┐       │
          │  │  214/215HG (ΔP switch)   │       │
          │  └─────────────────────────┘       │
          │           ▼                        │
          │  ┌──[recirculation fans]──┐         │
          │  │  210/211HG              │         │
          │  │  3-phase induction      │         │
          │  │  180 °C thermal switch  │         │
          │  │  self-containing housing │        │
          │  └────────┬─────────────────┘        │
          │  ┌──[recirculation valves]─┐          │
          │  │  212/213HG (electric)    │          │
          │  │  open / closed (= restrictor)│      │
          │  └────────┬─────────────────┘          │
          │  ┌──[recirc check valves]─┐             │
          │  │  5214/5215HG (one-way)  │             │
          │  └────────┬────────────────┘             │
          └───────────┼────────────────────────────┘
                      │ recirculated air (clean, ~cabin temp)
                      ▼
           ┌────[mixing unit]────┐ ◄── pack 1 fresh air (cold)
           │  belly fairing,     │ ◄── pack 2 fresh air (cold)
           │  near FR 40         │ ◄── (freighter) MORV 260HG → relieve
           │  mix ≈ 50/50         │ ◄── (dual-pack fail) emergency ram air
           └─────────┬───────────┘ ◄── low-pressure ground inlet
                     ▼
           [cabin distribution] (trim-air valves heat here) → outlets
                     │
                     ├─► cabin zone outlets (ceiling / floor)
                     ├─► lavatory / galley (vented overboard)
                     └─► (freighter) main-deck cargo via 253–256HG

           (cockpit independent: pack → cockpit, not via the recirculation mix)

Sources: AMM 21-21-00 §3/§6 + FCOM DSC-21-10-20.

3. The cockpit's independent fresh-air supply

Only fresh air is used for the cockpit (not recirculated air). — AMM 21-21-00 §5.A

This unit mixes cold fresh air from the packs with the cabin air being recirculated through recirculation fans. The mixer unit is also connected to the emergency ram air inlet and the low pressure ground inlets. Note: In case both packs are inoperative, the recirculation valves are partially closed. — FCOM DSC-21-10-20

[!warning]- Counter-intuitive: the cockpit's independent fresh air is not "the cockpit being fancier"

The real reason is anti-smoke cross-contamination: if the cabin produces smoke (electrical overheat, a passenger item), the recirculation fans would draw it back through the filters into the mixing unit and around the whole aircraft. If the cockpit also used recirculated air, the crew would breathe that smoke and could be incapacitated. The cockpit's independent fresh-air supply isolates it from the cabin — the cockpit always gets all-fresh air. The cost: lower cockpit humidity (no recirculation to retain moisture) and smaller flow volume. Operationally, this is the physical basis for the crew being able to keep working during a cabin smoke event, and it underlies the AVNCS SMOKE logic (see Smoke / Fumes / Avionics Smoke).

4. Recirculation fans (210/211HG)

The recirculation fans 211HG (210HG) have a three-phase induction motor and a fan wheel in a housing. ... Thermal switches are installed to stop the electrical power supply to the induction motor if the temperature increases to 180 DEG.C (356.00 DEG.F). ... If the fan wheel breaks, the housing is sufficiently strong to contain the broken pieces. — AMM 21-21-00 §6.B

[!note]- The fans have no speed control — only ON/OFF

Unlike the stepper-driven temperature/trim valves, the recirculation fans use three-phase induction motors at a fixed speed — no variable-frequency drive. State = ON/OFF: CAB FANS pb ON → fans run; OFF (or overheat) → stop. Flow is modulated by the recirculation valves (closed = restrict). So a single-fan failure halves the recirculation flow (the other runs normally); a dual-fan failure drops recirculation to ~zero → cabin relies on pack fresh air → drier, lower flow, slightly negative pressure.

[!warning]- The housing is designed to contain a broken fan wheel

The AMM states the housing is strong enough to contain a fan wheel that breaks (metal fatigue, foreign-object strike, overspeed) — a fail-safe so fragments cannot enter the cabin distribution. Unusual vibration/noise from the recirculation area is a write-up item, not a "fragments into the cabin" risk.

5. Recirculation valves (212/213HG)

The recirculation valves 213HG (212HG) have an electric motor, a manual lever, an actuator and a butterfly valve. ... The recirculation valves have two positions, open and closed. In the closed position they operate as restrictors and in the open position there is not a large decrease in pressure. — AMM 21-21-00 §6.C

[!warning]- Counter-intuitive: "closed" is not full shut-off — it is a restrictor

First reaction: "closed = shut = no flow". The AMM is precise: in the closed position they work as restrictors. Physically: open = flap parallel to the flow → near-zero pressure drop → free recirculation; "closed" = flap near-perpendicular (but restricted) → large pressure drop → recirculation reduced, not zero. Why not zero: to guarantee a minimum airflow to the avionics bay (avionics ventilation) — in a dual-pack failure the recirculated air is the avionics bay's only cooling source (see Avionics Ventilation). Normal flight: valves open → maximum recirculation. Dual-pack failure: FCOM "recirculation valves partially closed" → restrictor mode to preserve avionics ventilation. On the SD COND page: green = open; crossed = closed (restrict).

6. Filters + housings

The recirculation filters 5251HG (5250HG), 5253HG (5252HG), 5255HG (5254HG) and 5257HG (5256HG) filter the used air that flows from the underfloor area into the mixer unit. ... The recirculation filters 5251HG (5250HG) and 5253HG (5252HG) are installed in the FWD recirculation-filter housings 5211HG (5210HG). The recirculation filters 5255HG (5254HG) and 5257HG (5256HG) are installed in the AFT recirculation-filter housings 5213HG (5212HG). The FWD filter housings each have a pressure port for the clogging indicators 215HG (214HG). — AMM 21-21-00 §6.K / §6.H

        Left (pack 1 / cabin left)        Right (pack 2 / cabin right)
        FWD housing 5210HG                FWD housing 5211HG
        ├─ filter 5250HG                  ├─ filter 5251HG
        └─ filter 5252HG                  └─ filter 5253HG
                │                                 │
        clogging indicator 214HG          clogging indicator 215HG
                │                                 │
        AFT housing 5212HG                AFT housing 5213HG
        ├─ filter 5254HG                  ├─ filter 5255HG
        └─ filter 5256HG                  └─ filter 5257HG
                │                                 │
        recirc fan 210HG                  recirc fan 211HG
        recirc valve 212HG                recirc valve 213HG
        check valve 5214HG                check valve 5215HG
                └──────────┬──────────────────────┘
                           ▼
                       mixing unit

[!note]- Eight filters but only two clogging indicators — only the FWD housings are monitored

The clogging indicators connect only to the FWD housings' pressure ports, monitoring the overall blockage of the four forward filters. The four aft filters have no clogging indicator — the assumption is that both sets are used and age in step. Maintenance rule: a clogging-indicator alert → replace all eight filters (not just the front four).

7. Clogging indicators (214/215HG)

The clogging indicators 215HG (214HG) are differential pressure switches ... When blockage occurs in the recirculation filters, the air pressure there increases. When the air pressure is at a specified value, the pressure switch operates and sends a signal to the ventilation controller 280HN. — AMM 21-21-00 §6.E

A blocked filter raises the flow resistance → the upstream pressure rises relative to downstream; the ΔP switch senses the difference and, at a specified value, signals the ventilation controller (which drives the ECAM indication + a CMS fault code).

8. Recirculation check valves (5214/5215HG)

The recirculation check valves 5215HG (5214HG) have two semicircular flaps installed on a hinge bar. Springs hold the flaps in the closed position. The airflow from the recirculation fans 211HG (210HG) opens the flaps ... Springs close the flaps and stops the airflow in the opposite direction. — AMM 21-21-00 §6.J

They prevent back-flow into a stopped recirculation fan, and prevent pack 2's fresh air back-flowing into the left recirculation line when pack 1 fails — the same philosophy as the pack's downstream check valves (5533/5534HB, ata-21-07 §11).

9. The mixing unit (no individual FIN)

This unit mixes cold fresh air from the packs with the cabin and Main Deck cargo air being recirculated through the recirculation fans. The mixer unit is also connected to the emergency ram air inlet and the low pressure ground inlets. The mixer unit is equipped with a Mixer Overpressure Relief Valve (MORV). ... — FCOM DSC-21-10-20 (freighter)

The mixing unit's interfaces: pack 1 + pack 2 outlets (fresh cold air); left + right recirculation lines (used cabin air); the emergency ram-air inlet (dual-pack failure — see ata-21-22); the low-pressure ground inlet (ground source — ata-21-04); the outlet to the cabin distribution; and, on the freighter, the MORV outlet + four main-deck shut-off valve outputs.

[!note]- The mixing unit has no individual FIN — it is a ducting junction, not an electro-mechanical part

There is no "mixing unit FIN" because it is a ducting structure (a welded-aluminium plenum), not a part with an actuator/sensor/motor. AMM 21-21 treats it as a system name with no individual §6 component description. The pack-outlet downstream check valves (5533/5534HB) sit at its inlets (ata-21-07 §11); the freighter MORV (260HG) sits on its upper outlet port; the cabin distribution ducts run aft from its outlet.

10. MORV — Mixer Overpressure Relief Valve (260HG, freighter)

The mixer overpressure relief valve 260HG prevents overpressure in the mixing unit. The mixer overpressure relief valve is a circular mechanical valve with three springs which hold the valve flap in position ... When the pressure in the mixing unit is above specified limits, the valve opens and releases the overpressure. ... two limit switches to show ... open or close ... installed on an outlet port at the upper mixing unit section. — AMM 21-21-00 §6.D (freighter)

[!warning]- The MORV is freighter-only — the passenger configuration has none

The MORV is fitted only on the freighter configuration; the passenger configuration has no MORV. Why the freighter needs it: the freighter cabin volume is large and the main deck connects to the mixing unit via the shut-off valves (253–256HG); when any shut-off valve closes, the mixing-unit outlet path changes and the internal pressure can spike. The MORV is a mechanical fail-safe — springs + spring rods, no electrical power — that releases the overpressure to prevent a duct burst. The passenger configuration does not need it because its mixing-unit downstream is always the cabin distribution → outlets (a fixed path) with no shut-off-valve path switching.

11. Main-deck-smoke 5-action cascade (freighter)

Main deck smoke alarm. If there is a smoke alarm, the main deck cargo compartment shut-off valves close to stop the air supply into the fire area. To prevent an overpressure in the mixer unit, the two recirculation fans stop, one pack stops and the other air conditioning pack decreases the flow of air to 85%. — AMM 21-21-00 §7.B (2) (freighter)

   main-deck smoke detector (ATA 26)
            ▼  ventilation controller 280HN
   ① close main-deck shut-off valves 253–256HG  → cut air into the fire area
   ② stop both recirculation fans                → stop drawing more into the mixer
   ③ stop one pack                               → halve the fresh air into the mixer
   ④ other pack flow → 85%                        → further limit the mixer inflow
   ⑤ mixing-unit pressure kept safe; cabin still ventilated on the single 85% pack

[!warning]- Why stop the fans + reduce the packs after the fire area is already isolated?

First reaction: the fire area is isolated (shut-off valves closed), so why stop the fans and reduce the packs? Physics: once the shut-off valves close, the path that used to carry pack air to the main deck is cut, but the mixing-unit inflow is unchanged (both packs still running) — so the mixing-unit pressure rises; the recirculation fans still drawing from the cabin into the mixer raise it further → a duct burst / the MORV held open. The fix: stop both fans (less inflow), stop one pack (halve the fresh air), and drop the other pack to 85 % (limit inflow further) — bringing the mixing-unit pressure into the range the distribution + cabin can take. From the pilot's view, a main-deck smoke alarm puts the whole air-conditioning system into an "emergency ventilation mode" — the cabin still breathes on ~85 % × single pack (≈ 40–50 % NORM flow), enough to live on but markedly reduced.

12. Dual-pack failure → recirculation valves partially closed

In case both packs are inoperative, the recirculation valves are partially closed. — FCOM DSC-21-10-20

[!note]- Counter-intuitive: why partially CLOSE the recirculation valves on a dual-pack failure?

Intuition: a dual-pack failure needs more recirculation to keep cabin air moving → the valves should be fully open. The FCOM states they are partially closed. The real reason: with both packs out, the emergency ram air feeds the mixing unit (large flow, no pressurisation); partially closing the recirculation valves routes more recirculated air to the avionics-ventilation path (§5). The design priority is avionics cooling over cabin comfort — an overheated avionics bay would take down the flight-control computers, FMGEC, pack controllers, and zone controller, disabling the aircraft. Protecting the avionics is the top priority.

13. CAB FANS pb (3HG)

You can push the CAB FANS pushbutton-switch 3HG (the OFF light comes on) to stop the recirculation fans. — AMM 21-21-00 §7.A

ON (pressed in) = fans run per the ventilation controller; OFF (out, OFF light) = fans stopped (crew action). Uses: cabin smoke suspected from the recirculation system → OFF to isolate it; recurring fan fault / abnormal noise → OFF; maintenance troubleshooting. Consequences of OFF: recirculation ~zero → the cabin relies entirely on pack fresh air → reduced flow + much lower humidity + possibly slightly negative cabin pressure. Not recommended for long-haul unless smoke isolation requires it.

14. OVHT COND FANS RESET pb (3HN)

If the ventilation controller 280HN receives a fan overheat signal, it stops the electrical power supply to the recirculation fans 211HG (210HG) and on the overhead panel, the FAULT legend of the OVHT COND FANS RESET pushbutton 3HN comes on. When the temperature of the fan decreases below a specified value, and you push the OVHT COND FANS RESET pushbutton, the ventilation controller supplies electrical power to the recirculation fans 211HG (210HG) again. The FAULT legend goes off. — AMM 21-21-00 §7.B (1)

   recirculation fan temperature rises
        ▼ thermal switch (~180 °C, §4) cuts the fan's own power (fan stops)
   ventilation controller 280HN detects the overheat signal
        ▼
   ECAM: L+R CAB FAN FAULT (if both) / single-fan fault
   overhead: OVHT COND FANS RESET pb FAULT light on
        ▼ fan cools (housing dissipation / time)
   pilot presses OVHT COND FANS RESET pb
        ▼ ventilation controller re-powers the fans; FAULT light off; fans restart

[!warning]- OVHT COND FANS RESET is a pilot pushbutton, not a maintenance-only procedure

Unlike the zone-controller reset (3HK, a maintenance-level procedure the crew rarely uses, ata-21-09 §18), the OVHT COND FANS RESET (3HN) is a routine pilot pushbutton: on an in-flight fan overheat, the pilot presses RESET to recover the fan; if RESET still shows FAULT, the temperature has not yet fallen enough or the fan is genuinely failed → wait a few minutes and retry, or accept the fan loss. No maintenance needed. Single-fan FAULT → try RESET → if not, accept single-fan operation (recirculation halved); dual-fan FAULT → try RESET → if not, accept dual-fan loss (recirculation ≈ zero, all on the packs).

15. ECAM L + R CAB FAN FAULT

L & R Cab Fan Fault. If there is a failure in both recirculation fans 211HG (210HG): a single chime is heard, the amber MASTER CAUT lights come on, on the EWD ... COND L + R CAB FAN FAULT and the necessary steps are shown, on the SD ... the COND page ... and the FAN symbol is shown amber. — AMM 21-21-00 §7.C (1)

16. Recirculation ratio + humidity

Typical cruise split (integrative reasoning from the architecture): ~50 % pack fresh air + ~50 % recirculated air.

Humidity physics: ambient air at altitude is extremely dry (dew point ~−50 °C); pack fresh air, after compression + condensation + the water extractor, is near-zero humidity; recirculated air (used cabin air, warmed and moistened by occupants) carries ~15–25 % RH. The mixed output is ~7–15 % RH (vs 30–60 % on the ground).

Recirculation is the key to saving bleed (fuel) and retaining humidity — less recirculation → more bleed used → higher fuel burn + drier cabin. On long-haul, healthy recirculation fans are the hardware basis of cabin comfort; avoid long-term CAB FANS OFF unless smoke isolation requires it.

Self-test

[!note]- Q1. Does the mixing unit have an individual FIN, and what are its interfaces (incl. freighter)?

No individual FIN — it is a ducting junction, not an electro-mechanical part. Interfaces: pack 1 outlet, pack 2 outlet, left recirculation line, right recirculation line, emergency ram-air inlet, low-pressure ground inlet; the outlet → cabin distribution. Freighter adds the MORV outlet (260HG) + four main-deck shut-off valve outputs (253–256HG). The passenger configuration has no MORV.

[!note]- Q2. Is a recirculation valve "closed" a full shut-off? Why this design?

No — the AMM says the closed position works as a restrictor: the flap is near-perpendicular but restricted → a large pressure drop but recirculation reduced, not zero. The reason: guarantee a minimum airflow to the avionics bay (the recirculated air is the avionics' only cooling source on a dual-pack failure) and keep some cabin convection in abnormal conditions. The "open" position has the flap parallel to the flow → no large pressure drop.

[!note]- Q3. How are the eight filters distributed, and why only two clogging indicators?

FWD four (FWD housings 5210/5211HG, two filters each: 5250+5252 / 5251+5253) + AFT four (AFT housings 5212/5213HG, two each: 5254+5256 / 5255+5257). The two clogging indicators (214/215HG) connect only to the FWD housings, monitoring the forward four; the aft four have no indicator (both sets age in step). A clogging-indicator alert → replace all eight.

[!note]- Q4. Why are the recirculation valves "partially closed" on a dual-pack failure + RAM AIR?

Intuition says a dual-pack failure needs more recirculation; the FCOM states they are partially closed. The reason: with the emergency ram air feeding the mixing unit (large flow, no pressurisation), partially closing the recirculation valves routes more recirculated air to the avionics-ventilation path — the design priority is avionics cooling over cabin comfort, because an overheated avionics bay would take down the flight-control computers / FMGEC / pack and zone controllers.

[!note]- Q5. What are the five actions of the freighter main-deck-smoke cascade, and why so many?

Per AMM 21-21-00 §7.B(2): ① close the four main-deck shut-off valves (cut air into the fire area); ② stop both recirculation fans; ③ stop one pack; ④ drop the other pack to 85 % flow; ⑤ keep the mixing-unit pressure safe. Why: once the shut-off valves close, the pack air that used to flow to the main deck has no outlet → the mixing-unit pressure rises; stopping the fans + reducing the packs cuts the inflow to keep the mixer pressure within the ducting/cabin limit. The cabin still ventilates on ~85 % × single pack (≈ 40–50 % NORM).

Key takeaways

Common misconceptions

Scope — what this deep-dive covers and defers

References

A330 specifics per FCOM DSC-21-10-20 (mixing unit, the freighter MORV, the dual-pack-failure partial-close of the recirculation valves) and AMM 21-21-00 §3/§5/§6/§7 (the recirculation loop, the cockpit independent fresh-air supply, the three-phase 180 °C recirculation fans with self-containing housings, the recirculation valves working as restrictors when closed, the eight filters in four housings with two clogging indicators, the recirculation check valves, the mixer overpressure relief valve and main-deck shut-off valves on the freighter, the main-deck-smoke five-action cascade, the CAB FANS and OVHT COND FANS RESET pbs, and the L+R CAB FAN FAULT indications) — the English AMM being the fact source where the Chinese FCOM carries no AMM content. The 50/50 split and humidity figures, the filter topology pairing, and the main-deck-smoke overpressure rationale are integrative syntheses. All engineering detail is from the A330 knowledge base; no cross-type comparison is made.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.