Jotul F100 fresh air intake flowing in reverse...

[dbotos]

I had bought the fresh air intake adapter along with the stove about two years ago, but didn't get around to installing the fresh air intake adapter until just recently and I am not happy with the results...

Here's the background on the stove and fresh air intake adapter design:

The stove is all cast iron (no firebricks inside). Combustion air enters through a little slot above the door on the front where there's a slide lever for adjusting the amount of air that comes in. Simple enough and has been working well for about two seasons now. Exhaust is a straight shot of 6" double wall up to the ceiling (1 story), then it transitions to the stainless stuff to go through the attic and out through the roof. It has always drafted well and I've appreciated the straight exhaust run for ease of cleaning.

Now, the fresh air intake adapter. I must admit, I was a little disappointed when I saw what I got for my money when I opened up the box - it's basically a little 4" round takeoff with a slide gate that you attach to the heat shield on the back of the stove (after cutting a matching 4" hole in the heat shield). In theory, it seems logical - the stove is going to draw in air when operating and the fresh air intake basically provides a low-resistance path for fresh air to come in right at the stove instead of being pulled in through cracks around windows, doors, etc elsewhere in the house. And since that fresh air hits the back of the stove as it comes in (there's about a 3/4" gap between the heat shield and the back of the stove), it preheats that air some.

As for the ducting to the adapter, it runs from the adapter on the back of the stove through the wall into a bedroom closet on the other side. From there it shoots straight up through the ceiling and into the attic. Inside the attic, the end of the duct it about 2 feet up above the insulation. My logic in running it to the attic was that the attic is essentially outside air (front and back edges of the roof are soffit vented), yet with the benefit of being shielded from wind gusts, leaves, critters, etc.

After running the stove for a while with the new fresh air intake, it just seemed like it wasn't heating up the house as well as before. With the stove at operating temperature and a fire burning normally, I armed myself with a tissue (for flow sensing) and went up in the attic. Closed the attic door to make sure that wouldn't influence the results and went over to the open end of the duct. I placed the tissue over the open end to see if it would try to suck it in, but just the opposite happened - it wanted to blow it off. Now, when I say blow, this is no hair-dryer or leaf-blower kind of flow rate - more like an air hockey table where it wanted to float the tissue upwards. I even called my wife on the cell phone and had her adjust the gate on the intake adapter and it didn't help.

Apparently, the stove is doing a good enough job of heating the air in the fresh air duct that it's overcoming any tendency of (1) the stove wanting to pull in combustion air through that duct and (2) cold, dense air in the attic wanting to come down the duct.

The installation instructions that came with the fresh air intake adapter of course said nothing about where to pull the air from relative to the stove (above, level with, or below), so I'm wondering now if I screwed up by trying to draw fresh air from a level above the stove. Right now, I think the next logical option would be to run the duct down through the closet floor (into the basement) and then out the front wall of the house, a foot or two above ground level.

Does anyone else have any experience with this sort of thing? Just trying to get some feedback before I go cutting a bunch more holes in the house...

This is the stove we've got:

http://www.jotul.com/en-us/wwwjotulus/Main-menu/Products/Wood/Wood-stoves/Jotul-F-100-Nordic-QT/

 

[sunfish]

Yep, hot air rises. You need to run it out a wall, or under the house, if you have a crawl space.

 

[pook]

http://www.woodheat.org/outdoorair/outdoorairmyth.htm

 

[dbotos]

http://www.woodheat.org/outdoorair/outdoorairmyth.htm

I actually came across that when I was searching the other day. Some good points. Seems like you'd still want to have your house sealed up as tight as possible and then supply just the right flow rate of fresh combustion air right at the stove. Of course actually doing so would probably require drawing the fresh air from somewhere shielded from wind (what I was trying to do with the attic as a source location), a one way flap to prevent reverse flow, and maybe some sort of electronic flow control valve that would adjust intake flow rate based on appropriately located pressure sensors. I'm guessing you'd probably need some pretty sensitive pressure senors - maybe something in inches of water...

 

[pook]

i wonder if the OAK came off the stove, went down, & then up ? so the hot air from stove had to go down be4 rising

 

[Highbeam]

That woodheat article has long been debunked as rubish. Look around for the many opposing viewpoints.

The way you installed your OAK plumbing is pretty much the way you would install a chimney. The OAK chimney drafts as you would expect, upwards. If you were able to connect the stove to the OAK chimney using a sealed connection like many stove manufacturers then your OAK chimney would draft the proper way but I suspect overall stove performance would suffer.

Your idea about routing the OAK intake tube to the basement and then out through the rimjoist is an excellent one. This setup will also draft naturally. It will act to supercharge your stove and actually improve draft.

The ideal OAK setup is into a vented crawlspace. No wind issues, short run, and always some vertical gain beneath the stove.