I have a gas fired forced air furnace that I heat my garage with when painting.
To control the firing of the burner I put a ball valve between the automatic gas valve and the burner.
Think of it as a gas grille on your patio, the regulator controls the pressure and the valve youturn controls the flow rate.
Works great to control the heat output level.
This modification works well for me but you should not do this modification as it negates any rating or listing that your insurance company may require, and if not done properly may kill you.