A couple notes about this place in order to help you make the right decision. Please note that Alberto's place is on the first floor, but in Spanish terms meaning that he is one (narrow) flight of stairs above the ground floor, no elevator that we saw. The other point I would be cognizant of is that Barrio Carmen is a young, vibrant neighborhood, meaning that it never really gets quiet.
We LOVED staying here. The flat is perfectly located in order to eat well, drink well, and especially to see the entire old city of Valencia. We were able to see everything we wanted to within the old city on foot, however we rented bikes in order to see the city of arts and sciences (we rode through the entire park that runs around the old city, starting at the museum of modern art and ending at the science museum - we cannot recommend this enough). The flat is near dozens of restaurants, bars, and cafes, most of all it's a stone's throw to the Mercat Central, which you should absolutely absolutely positively go to! The flat itself is beautiful, spacious, and well accommodated. We were even able to cook a full meal (with what we bought at the Mercat) - a rarity in usual Airbnb travel.
Alberto is a great host - very responsive and made everything super easy for us to get around. If he is at work, you may be let in by his parents, who are absolutely lovely but do not speak English (side note: it's certainly possible to get by in Valencia without knowing any Spanish, but I would recommend brushing up or learning a few phrases). We had a minor problem getting the door open and his dad was at the flat within minutes of me texting him. Truly wonderful people, you can't really ask for a better stay. I hope that we will be able to stay here again!