3” intake for 6” flue

[Greg egg]

Hearthstone Green Mountain 40
Stove has 6” flue, but only 3” intake hole at bottom. Preparing for outside make air ducting. 4” duct should be good so as not to slow air down any more than that 3” hole, right? My underlying question, though, is why is the air intake so much smaller? It has a nice burn when the damper is open, but when starting fires I usually have to prop open the door a smidge to get that extra air boost. Could this not be avoided by having an intake that matches exhaust? Not planning to modify the stove… just curious.

 

[wetwood4life]

My uneducated guess is that the exhaust gasses have more volume than the intake air. Air goes in, but air + smoke goes out. And it's hotter, so should take more space.

 

[gthomas785]

The volume of air you need for combustion is generally much less than the exhaust produced. If most of your exhaust is air, then you're running very inefficiently.

It's common to need to crack the door on startup because the flue isn't warm so less draft, which means not much air being pulled in through the intake. This is also when you want to give the fire more oxygen to get going. Once the flue is warmed up you should be able to shut the door. If the intake were designed for these startup conditions then it would be oversized the rest of the time and harder to dial in.

There are some stoves, especially the fixed burn rate stoves that have a startup/reload control where you push the button and it lets some extra air in and then shuts automatically after a certain amount of time.

 

[Greg egg]

That makes sense. Thanks for taking the time to explain.