Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 65; Flight Crew Type 4000
Communication Breakdown; Distraction; Human-Machine Interface; Other / Unknown
Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport
Environment - Non Weather Related; Human Factors
As we were climbing through about 5;000 feet MSL; I told the Captain that I was turning the packs on HIGH to cool the cabin; as it was very hot. I looked up at the pack switches and unintentionally moved them in the wrong direction to the OFF position instead of HIGH. Contrary to what I normally do; I did not verify my action. Climbing through 10;000 feet I called for the Climb Checklist; the Fuel Caution illuminated (due to the center tank being empty) and ATC gave us 'direct ZZZZZ' intersection and 'climb to FL 210' which added to the workload and distracted me. As the Captain read 'Pressurization' from the checklist; I did look up at the Cabin Altitude Differential Pressure; and rate gauges and the position of the needles looked normal; so I replied: 'checked.' In hindsight; I realize that I missed the first point of my usual Pressurization Check Flow; which includes checking the two needles on the Duct Pressure gauge; and the packs switches position. Shortly above 20;000 feet the Cabin Altitude Warning light illuminated and the horn sounded. In short order we donned the oxygen masks and established communication; leveled off and silenced the horn. The Captain ran the QRC; QRH; and requested a lower altitude to ATC.We agreed that we were going back to ZZZ. Sometimes in this time frame; I realized that the packs were OFF due to my error in positioning the pack switches. I told the Captain and reached up to move them back to HIGH. I did not realize that the Captain had not heard me and did not see me. With the noise of the masks and the various tasks of running the QRC; QRH; the flight attendant calling us; and the return to ZZZ; I should have communicated much more effectively and assertively with the Captain to make sure he had heard me and that we were on the same page. When we leveled off at 10;000 feet and took the oxygen masks off is when I told him again that I inadvertently turned OFF the pack switches and that there was no issue with the aircraft.  We agreed that the safest course of action was to run the appropriate checklists and return to ZZZ.Cause - I am usually pretty consistent and methodical in double and triple checking what I do; but I did not realize that this morning the combination of a number of risk factors could lead me to let my guard down and become more distracted. The heat made me rush and I reverted to muscle memory by moving the pack switches up (like when I start the engines or transfer to APU bleed). The only other couple of times in six years that I needed to move the packs to HIGH I remember thinking how easy it could be to make the error of moving the switch in the wrong direction as this is what we normally do with each engine start. AM trip against my preferred PM circadian rhythm. Busy four-day with the ETOPS check out and with earlier show times than scheduled due to reroutes for ZZZ1 cancellations.General uncertainty of COVID-19 and preoccupation of being furloughed (for both myself and my husband who is junior to me).
Air carrier First Officer reported inadvertently moving the pack switches to off instead of high flow causing a cabin depressurization during climbout.