Heavy Flamers are often used by specialist weapon operators in dense conditions, such as during jungle or urban warfare, or by Space Marine Terminators who make good use of them during cramped ship-boarding actions.

Is the flamer a heavy gun?

The flamer is considered a heavy weapon and as such benefits from relevant perks such as Heavy Gunner.

What Space Marines use flamers?

Flamers are most commonly used by Imperial assault forces such as Space Marine Assault Squads, though xenos races such as the Eldar, Orks and the Tau are also known to make use of similar weapons.