This is an article advising best proactive when your air inlet run is over 30 feet for a Focus Fireplace.
We advise 30’ maximum, (see manual snip below)
For a gas version there is no risk of smoking or soot. It should be ok until 40’.
**If you want to make the installation surer, you can increase the air inlet diameter to 8”**
This is not an official advice but this is what we know by experience.
For any further information needed, please contact European Home Service. 781-324-8383 ext3.