Chimney Draft Fan

I bought a wood stove for my tinyhouse. The best place to put it has a bathroom drain directly above and a stovepipe through the shower floor is unconventional, begs for burns while showering, and also hard to seal. So I put a Chimney Tee through the downstairs wall to rout it outside. Alas, the extra routing made it draw badly before the flue got hot. Too much smoke is bad for lungs and stinks up the house, so I mounted a little fan under the Tee to push the smoke through, but it worked poorly. Cold stagnate air in the flue has enough weight to push fan draft back out through the fire door & into the house bringing smoke with it. The fan hole also let cold air into the bottom of the chimney which diluted the draft the chimney pulled even when the fire got hot.

Aiee! another dismal failure. What to do? ~ Capitalism to the rescue! ~ I bet a lot of people have smoky stoves in the morning and they'd send me money for designing a chimney fan that works, so I perservered and made the dandy design below.

Well they didn't because people using wood for heat typically don't have a lot of money, but it's still a working plan so this is how to make one for free (if you scrounge parts).


To get around the heavy cold air plugging the chimney, I put a fan on top the chimney that pulls a vacuum in the flue and draw air from inside the house. It even pulls some escaped smoke back into the stove. It takes a couple of seconds to build the vacuum so I wait a bit and open the door slowly. It works (yay). The fan is only necessary before the stovepipe warms up, turn it off once the fire is burning well.

For the venturi housing, I used a 24 inch section of Metalbestos cut down to 18 inches. The chimney drawing below shows 6 inch inside pipe dia, 8 inches outside. The cut section is 10 inches long, that might be too short because there needs to be some room for the air to flow smoothly. Turbulence wastes energy. Metalbestos is expensive but people throw it away if the insulation gets waterlogged (there's probably a reason but I don't know what it is, my suspecision is it's caused by chimney salesmen), and you can probably get sections for free if you ask around, because the shops have to pay to throw them away. Ask chimney installers, chimney sweeps, swap shops, etc. You don't need the (presumably waterlogged) insulation anyway, that space it occupies is where the Venturi goes. Still a good idea to check it for holes. You need to cut the outer tube in half to remove the insulation.

Metalbestos sections are designed to tolerate uneven temperature expansion. The inside tube is only connected at the top and the tubes can slide to accommodate heat expansion stress and let moisture out, so cut the outside tube using a surface grinder with 1/16 thick blade, go all the way around and pull the two halves apart. Hold it over a garbage can or sack because it's messy and contains asbestos. Do it outdoors wearing a Covid mask, and wash up immediately afterwards. Later, you put them back together by cutting slots and forcing the top over the outer tabs so it sheds rain water. Secure the tabs under the upper overlap with automative hi-temp gasket glue, and pop rivets or screws, to make it airtight.

Then cut the inside tube in half the same way, about an inch from the top, and cut a one and 1/2 inch ring off the piece that you just removed to allow for the overlap tabs and also leave an inch gap for the forced air from the fan to enter the flue pipe directing the airflow out the top (that gap is the Venturi). Secure eight spacers, shown red in the drawing, to the outside of that tube with rvits, sheet metal screws, or spot-weld. The spacers are "L" shaped, four are about 2 inches long with a one inch leg (the width of the insulation cavity) and four are one inch long.

The short brackets (red) are connected around the bottom of the inside tube so it will sit on top of the (outer tube) flange and won't slide down. This maintains the width of the gap at the top of the inner tube where forced air from the fan enters the flue pipe and creates up draft.

The long "L" pieces are attached around the top of the inside tube to hold it centered. They also probably reduce any tornado/coriolis vortex that might get started (Vortex = turbulence). I don't know if that's important here but having made wind generator blades for 15 yrs, my rule of thumb is laminar flow: good, vortex: bad.

Cut a square hole in the front of the outside tube that's as wide as the tube itself so it looks like a Mr. Coffee maker. Then make (or have made) some kind of metal box (stainless or galvanized) to hold the fan on the front with folded tabs around the curved surface of the outside tube, pop rivets, and hi-temp automotive gasket glue to keep it air & water tight, good to 700°F (beats solder).

Once the chimney is drawing, cold air is drawn in through the fan when it's not running, so it doesn't get hot enough to affect the fan or wiring. If you still worry, a fuse on that wire would simplify troubleshooting.

The Venturi fan has worked well for 3 or 4 yrs. This is the fan I used, $46 from Amazon. It has a speed control that's unnecessary, I just leave it on the highest setting.

Bill :)