How to add draught to simple box stove?

Question

I've designed and installed a small stove to burn wood scraps and bio-briquettes. The stove is really simple box-shaped stove as drawn here below.

However the stove does not work properly: the fire dies quickly, most probably because there is not enough draught. Opening the door a little helps creating a draft, but then smoke fills the room instead of running out through the chimney.

I would like to improve on the stove, so the fire may burn happily. I have been told the 45° elbow on the chimney reduces the air draught, but changing this now would be a bit hard.

I've thought of adding some openings:

• by perforating the bottom and adding a tray below. This would possibly create draft and allow to clean ashes more easily.
• by perforating the front wall at the bottom with some holes, adding a small grid to prevent sparks from sneaking out.

Do you think any of the two options here above would improve the stove do increase draught? Do you have another suggestion?

Answer 1

Regulations are pretty stiff over here about what you can and cannot do, however; out in the rural areas and backwoods of Oregon, we can totally relate. One of the things to search out on the Internet is Barrel Stoves. Many's the 55 gallon oil drum that's given its life for keeping a shop or house warm. There are quite a few plans available.

Stove design, the box is basically correct though for dimensions, I'd go with 18" wide by 18" high by 24" long, but:

1. The door placement has to be moved to the side opposite of the stove pipe outlet and the top of the door opening about six inches below the top.
2. You need controlled air intake (see below).
3. The stove pipe has to be 6 inches minimum and exit as straight and as high as you can go with it. It has to exit safely through the wall or ceiling.
4. With the larger suggested dimensions, if you can get firebrick, you can line the bottom and sides so the metal doesn't burn and warp. Otherwise, a 2" layer of clay soil in the bottom can help lengthen the life of the unit.

The simplest air control I've ever seen was on an Orly fireplace insert. You drill three holes in the door near the bottom, three holes in a long, thin plate of metal that can slide across in front of the holes in the door. The spacing on the holes is such that the plate can completely cover the holes in the door and be slid to completely open up when both sets of holes line up.

Some ideas for door attachment and air inlet control

Disclaimer: Always check your local codes for installing wood burning appliances. Due to the fire hazard of poor installation (many's the house or shop burnt down due to incompetence and at the absolute worst time of year for this disaster to happen), carbon monoxide leakage in super-insulated homes (secondary air intake needed) and insurance requirements, permits are often required and installation may also require certified contractors.

Resinous and damp wood materials will deposit creosote in the chimney if the stove pipe temperature within falls too low. Also, soot will build up from any fuel burned. Chimney fires are dangerous. There is a need for frequent maintenance and inspection to clean them out so this hazard can be avoided.

Answer 2

Draught is caused by hot, less dense, lighter gasses rising and creating a relative vacuum that causes the denser, heavier, higher pressure fresh air outside the stove to flow into the combustion chamber to displace the gasses you exhaust. This air contains oxygen that supports combustion. Try (safely) insulating the exhaust pipe, to keep the gases hotter all the way up and out to the end of the exhaust pipe. You waste whatever heat would be radiated from the pipe into your living area, but the draught will be stronger because the gasses in that pipe are hotter and lighter and will rise more strongly.

You also want that fresh air coming into the combustion box to be drawn across where the combustion is occurring, preferably low where the combustibles are situated so the fire can fully utilize that oxygen. So, air holes low and at the opposite end from the exhaust is effective.

Finally, since the oxygen for the fire is normally supplied from your living space, I like the idea of a second, smaller pipe coming from outside the building to feed oxygen directly to the front-bottom of the firebox, again, at some loss of heating efficiency because that outside air will be cold. More importantly, if the air is very cold, it will slow combustion. You could loop that air-intake pipe above or next to the stove to warm the air. This air-intake pipe needs to be adjustable and closable. The idea is that the oxygen to feed combustion has to come from somewhere, usually first your living space, and then ultimately from outside your building through cracks in your structure. If it were otherwise you would soon find it hard to breath. So, why not take control of where the air comes from with a dedicated intake? Reducing the low pressure you are creating in your living area by the combustion and exhaust will actually keep you warmer by reducing the amount of leaked air needed to equalize pressures.

If you have to keep that top door, definitely close the air intake before you open the top door. Can you tilt the box so the front is lower? That will help keep some smoke out of your living space.

Improved combustion will reward you with a warmer living space.

Disclaimer: I am no expert, so I gladly defer to more knowledgeable,safety conscious persons who might riff off these suggestions...

Answer 3

You have to change the stack size. I'm pretty sure there's a mechanical code requirement for solid fuel burning appliances have a 6" flue minimum. Only tested and listed appliances can have smaller. The stack needs to outlet well above surrounding roof. For adequate draft, it should be at least 10 feet high and have minimal horizontal offsets.

You cannot load from the top, it must be from the end opposite the stack exit. This is also where combustion air should enter. The air intakes must be controllable. This can be by any simple sliding or rotating plate arrangement that can be moved over the openings at any time needed.

I'm sorry for being blunt here, but you don't know what you're doing. You do not understand in the least anything about the physics of combustion. You stand a very real chance of either burning down your building and/or poisoning yourself and anyone with you with carbon monoxide. There's many little details you must know to install a safe solid fuel burning appliance. Too many to put forth in a forum like this.

If you choose to proceed anyway, at least invest in a carbon monoxide detector and a large fire extinguisher. PLEASE.

Edit 12/3: Nepal! That paints a different picture. While my concerns for your safety remain, your access to resources are severely limited. Here's a few more thoughts on making your installation safer and more usable.

Be sure there is plenty of clearance between components and combustible materials. At least 30" from the box to combustible walls. If there is less space available, place sheet metal heat shields on the walls with an open air space behind.

To minimize creosote build up, if available, use double or triple wall flue pipe. Consider constructing your own from different single wall sizes if necessary. In any case, ensure there is good clearance from flue pipe to combustible construction, especially where it goes through walls or roofs. 2" for triple wall to maybe 6" for single wall. Use sheet metal to fill the gaps. Obtain a proper size chimney brush to clean out the inevitable build up. Use it often.

The flue outlet needs to be well above the roof as I mentioned, specifically, at least 18" above any point of the roof within 10 feet of the outlet. The outlet should be screened to prevent large hot embers from flying out and igniting something in the area.

As Fiasco mentioned, fire brick lining is important. Besides protecting your box, it buffers the heat output to a more constant, comfortable level. Otherwise, your heater will perpetually be either too hot or too cold.

A good draft is your best insurance against carbon monoxide leakage. Do not use your heater unless you've determined that it is inducing a good reliable draft. While a little back puffing when you first open the door to stoke is acceptable, you should not otherwise observe any smoke leaving the box through any opening, ever. Except for the flue of course.

Be safe!