AN A320 CREW RETURNED TO THE GATE WITH A 'VENT AVIONIC SYS FAULT' ECAM ALERT PRIOR TO TKOF. AFTER TKOF THE CREW RETURNED TO LAND WHEN THE 'VENT SKIN VALVE FAULT' ECAM ALERTED.

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DURING TAXI WE HAD AN ECAM 'VENT AVIONIC SYS FAULT.' THE CAPT BROUGHT THE AIRPLANE TO A STOP AND SET THE PARKING BRAKE. WE COMPLETED THE ECAM AND THEN CALLED MAINT. MAINT HAD US PULL AND RESET CIRCUIT BREAKERS TO RESET THE VENT SYS COMPUTER. THE ECAM SCREEN RETURNED TO NORMAL AND WE RECEIVED A NEW RELEASE FROM MAINT. WE CONTINUED TO THE RWY AND JUST BEFORE TKOF THE SAME ECAM VENT AVIONIC SYS FAULT OCCURRED. WE RETURNED TO GATE AND HAD MAINT FIX THE PROB. AFTER INVESTIGATING THE FAULTS; MAINT DEFERRED THE FAULT AS A VENT SKIN VALVE FAULT. FLT TAXIED OUT AGAIN WITH NORMAL INDICATIONS AND THE DEFERRED VENT SKIN VALVE. THE FO WAS THE PF AND DURING THE TKOF PHASE OF FLT WE GOT ANOTHER ECAM. THE ECAM DISPLAYED A VENT SKIN VALVE FAULT. THE FO FLEW THE AIRPLANE AND HANDLED THE RADIO. THE CAPT MANAGED THE ECAM. THE ECAM CHKLIST CALLED FOR PLACING THE BLOWER AND EXTRACT FANS TO OVRD. THE VENT INLET VALVE WAS CLOSED; BUT THE EXTRACT VALVE POS INDICATOR WAS CYCLING BTWN CLOSED TO OPEN TO AN AMBER COLOR FAILED (XX'S). THE EXTRACT VALVE POS INDICATOR EVENTUALLY DISPLAYED FAILED (AMBER COLOR XX'S). THE ECAM CHKLIST RESTR THE AIRPLANE'S ALT TO 10000 FT; CAB PR MODE SEL SWITCH WAS POSITIONED TO MAN; AND THE MAN VERT SPD CTL SWITCH WAS USED TO CTL THE CABIN ALT AND PRESSURIZATION. THE CAPT MADE THE DECISION TO RETURN TO ZZZ THAT WAS LESS THAN 5 MI AWAY. THE FO HAD ATC TO VECTOR FLT BACK FOR A LNDG WHILE THE CAPT CONTACTED ZZZ STATION AND DISPATCH THAT FLT IS RETURNING TO THE FIELD DUE TO A CABIN PRESSURIZATION PROB. WE COMPLETED THE LNDG BACK TO ZZZ. UPON CLRING THE RWY THE FO AND CAPT BOTH CONFIRMED THAT THE ACFT WAS DEPRESSURIZED. HOWEVER; WE BOTH WERE CONFUSED BY THE ECAM PRESSURIZATION PAGE INDICATIONS. THE CABIN DIFFERENTIAL PRESSURE WAS 0; THE OUTFLOW VALVE WAS FULLY OPEN; BUT THE CABIN ALT INDICATED 1450 FT. THE FIELD ELEVATION AT ZZZ IS ABOUT 2000 FT. THE CABIN ALT INDICATION OF 1450 FT INDICATED THAT THE AIRPLANE SHOULD BE PRESSURIZED; BUT CABIN DIFFERENTIAL PRESSURE WAS 0 INDICATING THAT THE ACFT IS UNPRESSURIZED. TO CONFIRM THAT THE ACFT WAS INDEED UNPRESSURIZED THE CAPT HAD THE FO TO OPEN HIS WINDOW. BOTH PLTS FOUND THAT WHEN THE CABIN PRESSURIZATION MODE SEL SWITCH IS POSITIONED TO MAN; THEN THE CABIN ALT INDICATOR IS ERRONEOUS BY -150 TO -950 FT. WHEN THE CABIN PRESSURIZATION MODE SEL SWITCH IS POSITIONED TO AUTO; THEN THE CABIN ALT INDICATOR IS ACCURATE. THIS ERRONEOUS CABIN ALT INDICATION CAN AND DOES CAUSE CONFUSION. IT COULD EVEN CAUSE A SERIOUS SAFETY HAZARD TO PAX AND/OR DAMAGE TO THE ACFT. IN MY OPINION; THIS ERRONEOUS CABIN ALT INDICATION IS A SAFETY HAZARD THAT NEEDS TO BE ADDRESSED BY ACR'S; THE FAA; AND AIRBUS.