Persistent air in system

I Recently cleaned my system using "Fernox DS40" (plus neutraliser). Drained + flushed 4 times as per instructions, then refilled + Fernox protector added.
Ever since, it continually fills with air and needs venting every day. Only happens when the boiler is hot, no problem when the stat is turned to minimum.

Any suggestions?

Is DS40 an acid cleaner? What you have is probably not air. If the system water was not higher than pH 7, and not more than pH 8.5 then you probably have CO2 generated in the boiler. You need to check the pH. A test kit is available at ironmongers or garden centres