Off the top of my head. Please add others.
Heated patios with plastic or plexiglass walls
By The Way Café
Bar Raval (very busy at night, nicer to visit weekdays during the day)
Heated back patios
The Three Speed (this patio is not always open, depending on staff and demand, it seems. Call first or have a back-up plan)
Front patios, not sure about heaters (I couldn’t see any today)
Pera Café in Kensington has a plastic tent that’s heated. @rstuart and I had lunch on the open patio (4 ⁰C today)
Arch Café has closed its back patio.
By The Way Café (patio open for weekend Brunch, not at night)
Emmer’s patio at 2:30 pm on a Sunday. They close at 4 pm. Half empty.
Union has sealed up their patio.
I’m a little disappointed, to be honest. It was as the only heated patio I had found that had good food and good ventilation. I will update the CO2 over brunch
Pleasantly surprised that the back patio at Union had good ventilation (CO2 under 800) throughout our hour long brunch.
Swiss Train Ride (a Croque ) My friend had the Rib Benny.