Does the air intake of a forced air heater need to draw outside air?

[Ostinato]

I currently have a Wallas system and am think of switching over to Webasto. The Wallas system draws some of its non-combustion air from the cabin and some from the outside. The Webasto only has one intake and is intended to draw from outside air.



What are the downside of drawing non-combustion air only from inside the cabin? Does the air become moister? Are there other issues?

[AB808]

Drawing air from outside decreases the moisture in the boat in a cold climate. Cool / colder air holds less moisture. So drawing in some outside air will reduce moisture

[SSgtPitt]

And it helps gets rid of the “boat smell” if you’ve been cooped up.
Actually, bringing some outside air in with the return side is healthier. All the plastics, varnishes, petroleum, foam cushions etc off gas into the boat. Not really a problem if you’re ( not “your” ) moving in and out and the hatches are open.

[Dymaxion]

Perhaps put in a "Y" section fitting in your intake piping so you can adjust ratio of fresh vs. recirculated air being heated. In colder weather admit less fresh air to aid heating up faster. Once warm you can admit more fresh air to help dehumidify as mentioned. 25% fresh air is a suitsble amount.