Presenting the theory and design of chimneys for the blacksmith's coal forge. Properly designed forge chimneys make the smithy comfortable to work in. Time and again I see or hear about beginners and first time forge builders having trouble with smoky chimneys, or worse yet, they have a smoky chimney and have begun to accept it as normal. Each time they make the same mistakes. Below are construction tips and some do's and don'ts.
January 22nd, 2011. Under construction. This page is undergoing a large rewrite (as are most of the Forge Design series) and more material will be added and pages will be cleaned up for easier reading and faster downloading as I have time to make the changes.
Chimneys draw more than just the smoke from the fire, a fact most often overlooked by many chimney builders. Ambient air surrounding the fire will be drawn towards the fire and flows into the chimney with the smoke. The higher or farther a hood or chimney opening is from the fire, the more ambient air that will displace smoke as it rises to the chimney. The cooler ambient air will be heavier than the smoke and will slow the movement of smoke in the chimney. If the fire is located too far from the chimney or hood, then the amount of ambient air moving over the fire becomes so great that, some of all of the smoke will be forced into the smithy.
My own preferences for flue size.
The closer that the chimney or forge hood is to the fire, the less ambient air will displace smoke entering the chimney. I prefer at least a 12-inch square or round chimney (inside dimension of the chimney flue). In the past I have accepted some claims made on the internet by smiths that claimed a lot of experience, that they build many chimneys with 8-inch and 9-inch flues. But I soon noticed that these guys spend most of their time on the internet boasting of their great forge building experience, and then when they actually work in their little smithies, they use gas forges. I have built a 10-inch chimney and it worked well. But this forge had a small hood and a brick fireback that helps guide the smoke into the chimney. I have also built forges with small chimney flues, and these forges needed to be completely enclosed to prevent ambient air from displacing any smoke in the chimney - or the smoke would actually blow out from the hood opening into the smithy! So beware of claims by others that they build smaller chimneys 'all the time that draw well'. These people might actually have a very little use for coal forges and in fact, use gas forges in their own smithies. Again, my preference is for a 12-inch (inner dimensions) round or square chimney flue. 10-inch will work, but the larger size is much better.
Keep the hood or chimney close to the fire.
Keeping the entrance close to the fire helps greatly with the smaller chimney sizes. Corbelling the brick or stonework above the top of the smoke entrance in the chimney also helps to guide more smoke into the chimney and at the same time excludes excessive ambient air entering the chimney. A small sheet iron shroud or hoodlet also works well.
The height of the top of the chimney flue above the peak of the roof is 4 feet or more to prevent wind gusts from pushing smoke back down the chimney. The 4-foot height above the peak of the roof is the recommended constant, no matter what chimney size is used. Slight deviations from the 4-foot rule may work also if kept small. Note the drawing at left.
It is common to see chimneys for modern fireplace inserts to be mounted well below the peak of the roof. Keep in mind that a forge chimney does not work like a wood-burning fireplace insert for a home and therefore is not treated as such. The blacksmith's forge chimney extends 4 feet above the peak of the roof.
Allow large space between chimney cap and top of chimney flue.
In the drawing at left (drawn somewhat close to scale) a 12-inch chimney pipe design was chosen. Note that the height or distance between the lowest edge of the chimney cap and top of flue, is equal to or greater than the width or diameter of the inside of the chimney pipe flue. To figure the dimensions or space needed between top of the chimney flue and the lowest edge of the chimney cap, the whole design must be drawn or considered as a two-dimensional form. This is because smoke can only exit one side of the chimney on windy days or even during days when a light breeze blows across the chimney.
The biggest mistake first time builders is to build a chimney cap with too short a distance between top of flue and lower edge of chimney cap. A first-time builder may assume at first that a 4 inch clearance for a 12 inch chimney cap would allow more than enough volume of smoke to exit the chimney, but this short gap creates a restriction in flow or path of smoke. To force smoke to make a right-angle turn anywhere in its path will cut the velocity of flow by more than half and fill the shop with smoke. The smoke needs to be encouraged to flow up the chimney as quickly as possible to help create draw at the smoke entrance. Always consider the gap between the top of flue and chimney cap as a two-dimensional area and design this gap to be equal with, or taller, than the width or diameter of the chimney flue. The chimney cap MUST NOT present any type of restriction to smoke passage. Forges do not force smoke out small chimney caps like wood burning stoves. If the smoke is not quickly drawn into the entrance of the chimney then it will flow out into the shop area.
Blacksmith's forge chimneys rise tall above the roofs of their smithies.
At left is a photo of my own shop showing the tall forge chimneys. For those new smiths who think they are mistaken to think forge chimneys should be so much taller than the roofline, this should clarify the point. The chimneys must be taller than the roof and no, there is nothing wrong with the way it looks. Notice also that the rain caps are tall above the top of the flues. Wider caps would be preferred but I didn't have enough sheet metal for two wide caps at the time these were made.
The old photo below left is of a restored chainmakers shop in the Black Country area of England. The photo is from a book called Chain & Chainmaking (Shire Album 69) published by Shire Publications. Anytime the forge chimney exits the lower area of the roof, it will naturally appear very tall. In this old photo the chimneys appear to be about 1-1/2 feet above the peak of the roof. Not exactly tall enough above the roofline to be completely trouble-free, but still functional and a good example of how common it is to build a chimney tall above the roof when it exits the roof at the lower end of the slope.
To anchor the metal chimney in place and allow for periodic service and replacement of parts, clamping brackets can be made to quickly attach braces and caps to the metal pipe. In this example the bands of the clamps are made of 3/16ths inch x 1 inch flat band stock. The braces and stays are then welded to the bands. A jig is used to support the mount pads (at the bottom of the stays) at the same angle as the roofline. By clamping the mount pads to the jig and setting up the chimney stays in place on the pads, the joints can be welded at the same angle as the roof. The jig is nothing more than a piece of flat iron about 2 feet long with a small piece of iron welded to it at the same angle as the roof measured from the horizontal.
Most important! The exit at the top of the chimney pipe must never be allowed to present any blockage or hindrance to the flow of smoke. Forge smoke builds pressure too low to push past obstacles or excessive angles or bends in the chimney flue. The path which the smoke takes from smoke entrance of the chimney, to the exit point, must be kept as straight as possible (not including smoke shelves in the lower chimney of side draft forges). Spark arresters must allow sufficient flow so velocity in the chimney is not hindered. If using a spark arrester it may be necessary to use a larger diameter chimney to compensate for the resistance to gas flow. Louvers in wood-burning stove caps create back pressure to forge smoke, again consider the chimney in a two-dimensional form and note that the louvers in a wood burner cap actually cover up a portion of the exit at the top of the chimney. Once again keep in mind that the blacksmith's forge chimney has nothing in common with wood burning stove chimneys and is therefore treated differently than wood burners. A simple rain cap such as suggested in the first drawing at the top of this page will work fine.
An enclosed forge with only a small opening for access to the fire will prevent most of this ambient air and smoke displacement while a larger opening or greater distance to the hood of chimney entrance may admit so much ambient air flow that smoke is entirely displaced into the building. A small pipe diameter of around 6 or 8 inches would work fine on a small enclosed forge if the access hole to the fire is small.
On small fabricated portable forges the hood fits closely or air tight around the forge with only the access hole in the front and maybe a small covered hole in the rear for long bars. A tapered style hood or transition towards the chimney pipe helps ease the smoke into the chimney. A barrel with a pipe extending above it will work though as long as the bottom of the barrel hood fits closely around the bottom of the forge.
This is the only type of forge and hood combination on which the 6-8 inch chimney flue pipe can be used with any real success. In this setup the amount of ambient air able to enter the flue along with smoke, is limited/restricted by the total enclosure of the fire within the hood. Despite that this chimney works almost like a wood burning stove chimney, bends in the flue should be avoided. The chimney should always go straight out the roof.
At left is an example of a small coal fired horseshoer's forge. This is my old horse shoeing forge which I once used to shoe draft horses in the 1980's. This forge was stolen in the late 1980's so I have no other photos of it. This forge is typical of small enclosed forges used by horseshoer's in shoeing shops and horse shoeing schools. The small closely fitting hood and tiny access door prevents a large amount of ambient air from entering the chimney which would only displace smoke going up the chimney. This forge used a 6 inch chimney pipe. There were occasional puffs of smoke to exit the front access door into the shop. So even this example shows that a 6 inch chimney pipe is only marginally adequate.
The small enclosed hood with the small flue pipe (6-8 inches) would make an easy and quick chimney for the little portable forges that are so popular among amateur smiths. The smaller hood takes up little room and will complement a small shop well. Important note - keep the small enclosed hood away from combustible walls as this tightly fitting hood gets very hot. This hood should be distanced away from combustibles and walls just like a large wood burning stove which is used in homes. By following building codes concerning wood-burning stoves and fireplaces, the smith can reduce fire hazard risk.
The YouTube video seen below, shows the small enclosed horseshoers' forges at the Midwest Horseshoeing School, that are used for heating horse shoes. This is a very nice professionally made video, and is a great video for anyone that is interested in the art of blacksmithing and horseshoeing. If you liked the small enclosed coal forges in the photos shown above, this video offers a great opportunity to watch these forges in action. Midwest Horseshoeing School has a new owner and a new building, located near Springfield, Illinois. For those of you that noticed the close resemblance of the small green forge in the photo above, the answer is yes, I did base the design of my small horseshoeing forge on the forges that are seen in this video. I attended this horseshoeing school in 1983 - when the school was still located in Macomb, Illinois. The video also introduces the viewer to making horseshoes, anatomy of the horse's foot, recent history and shoeing fads of the horseshoeing trade, styles and types of horseshoes. See how a horse's foot trimmed for shoeing, and also how the shoe is nailed on the horse's foot. Interviews include the new instructor - Steve Hedges, and the previous instructor - Roy Evans, and some of the horseshoeing students. This is a great video! Check it out.
At right is a drawing representing a large conventional style hood over a forge. Notice that there is only a slight difference in height of these two examples, above the forges. This minor difference in height is all it takes to influence the success of each hood.
A large hood needs a large chimney diameter of roughly 10 - 12 inches to allow smoke and ambient air to flow upward. The distance or height of the hood above the forge, directly effects how much smoke flows up the chimney or into the shop. Too great a height above the forge allows too much ambient air to enter the hood and most of the smoke goes into the shop. If the hood is too low then the hood draws well but, it is difficult to gain access to the fire when working.
During construction of a large overhead style hood for a ready made cast iron forge, I set the hood too high above the hearth (the bottom of the hood 35 inches above the sides of the forge.) I had mistakenly made the hood frame 12-inches taller than my original plan. This caused the shop to fill with smoke despite placing sheet metal panels around the forge and hood to try to prevent ambient air from entering the hood around the forge. It was as though none of the smoke was going up the chimney. Indeed, students in one of my classes noted that the smoke was going sideways and out from under the hood. So believe me when I say that excessive hood height above the fire is a problem.
I have now cut down the stand which supports the hood so that the bottom of the hood is 18 inches above the forge. All the smoke goes up the chimney now. When first constructing the hood stands, I was afraid of not making the hood tall enough above the forge to allow work to still be comfortable. Now with the hood in place and having had a chance to test the new arrangement, I wish I had made the height 16 inches above the forge. The18 inch height was comfortable to work with and lowering the hood just a couple more inches would have been ideal. The hood is adjustable with multiple sets of bolt holes in the rear frame so it can be moved lower later on if the new height is a problem.
Sheet metal panels hung on the rear and sides of the hood, which reach all the way down to the forge hearth would also help prevent smoke blowing out sideways from under the hood if a breeze is blowing through the shop.
The half hood is often mounted to the back of the forge hearth where it presents a rear firewall and two small side walls to help shield the smoke from slight windy days or drafty shops. The top halves of the sides of the half hood fold around in front where they overlap and fasten together around a piece of chimney pipe. The overlapping ends form a small hood which depending on manufacture, extend anywhere from half way to fully over the top of the fire. The half hood is cheap and light for easy transport. If the half hood is built too high above the fire, smoke often cannot be completely drawn into the chimney pipe and as a result, this forge hood should only be used in a shop with lots of ventilation. On the other hand a hood built more closely to the fire will tend to remove most or all of the smoke from the building. Many factory made half hoods are built with a chimney pipe size much too small for the amount of smoke and the height above the fire and add to the smoky shop.
If the half hood is home built, a 10-12 inch chimney pipe size will greatly reduce the amount of smoke released into the shop compared with the factory made version. My first working steel forge used a simple half hood with a 10 inch chimney. I recommend a 12 inch chimney but 10 inch works if the height of the half hood isn't too tall above the fire. The back wall of the hood was about 9 inches deep and was then lined with firebrick all the way up to just below the chimney pipe. The smoke clings to the wall of firebrick as it flows upward into the chimney and this reduces the amount of smoke blown sideways out of the hood and prevents excessive ambient air from displacing smoke in the chimney. The half hood forge is roughly 35-40 inches wide and about 25-30 inches tall from hearth to the lower overlapping edges in front above the fire.
Half hoods are cheap to make and very simple. But they cannot support the weight of a heavy chimney flue pipe. The pipe for this chimney was about 10 feet long and light enough that I was able to simply mount it on top of the half hood. The half hood was made of about 18 gauge sheet metal so it offered a small amount of rigidity for a light chimney. Heavier flues may need some type of wall or ceiling mounted support.
Smoke enters directly into the bottom or side of the side draft chimney rather than being directed into a hood which converges into the chimney. Large conventional hoods take up a lot of room and often block a lot of light in the shop, while the side draft forge is open and blocks little light through the shop. Small enclosed forge hoods are difficult to work inside of and cannot handle large ironwork, but the side draft forge offers a free and open view of the fire and work heating in the fire, and allows free access to large work in the fire. At left are some examples of a brick chimney in which the smoke entrance is long or tall above the fire. Corbelling of the brick structure allow the opening of the smoke entrance to extend over the middle the fire.
Why doesn't everyone build side draft forges?
With all these advantages the beginner might ask "why would a smith build a forge with a hood if the side draft if so good?" Many smiths often don't understand the principles of the side draft forge well enough to comfortable with building side draft chimneys. Another reason is that side draft forges require more planning prior to construction. A side draft chimney is placed very close to the fire. The smith might not be able to heat very wide objects such as armour because the chimney is too close to the fire and the chimney prevents the wide plate from being positioned over the fire for heating. Wide plates push the fire and smoke away from the chimney and may cause the smithy to fill with misdirected smoke. If the smith makes a lot of armour or must heat a lot of wide plates, then it would be better to build a large enclosed or medieval style forge hood with the fire placed farther away from the wall.
Small chimneys are found in shops where only a short chimney run is needed to reach the proper height above the roof. A more massive chimney structure is needed to support the weight of a tall chimney which must rise through the roof of a tall building. The taller a brick structure is, the wider it must be to support its own weight. It can be said that the massiveness of the chimney structure is a function of the height of the brick structure.
How close does the fire need to be to the chimney smoke entrance?
The closer the proximity to the fire, the stronger the smoke draw. For this lateral distance it works best to place the side of the firepot as close as possible to the entrance or opening of the side draft chimney. Vertical height of the chimney smoke entrance above the fire also must be considered. Again placing the fire as close as possible to the smoke entrance in the chimney. Above right is the brick forge with a short smoke entrance and a small sheet metal shroud mounted above the fire, at the Blacksmith shop at Mt. Pleasant Old Threshers Reunion grounds. At lower left is one of the brick forges at Colonial Williamsburg in Virginia.
My forges have a vertical height of about 2 inches above the level of the hearth and this is more a matter of construction details then design theory. My steel side draft forge hood is made of 2-inch angle iron so it isn't possible for smoke to enter any lower in the hood. However I also line the bottom of the inside of my brick side draft coal forge chimneys with firebrick which is 2-inches thick. In either case this is of no design problem since the blacksmith's fire is built at least two inches deep anyway so smoke originates about 2 inches above the hearth to begin with. At the Anderson shop forge in Colonial Williamsburg, the smoke entrance is short but it is located about 7 inches above the fire rather than level with the hearth, and a small sheet metal shroud helps guide smoke into the chimney. The height the smoke needs to travel before starting to enter the chimney may make the sheet metal shroud a necessity in this case. The masonry is corbelled in a decorative fashion above the smoke entrance.
What makes the side-draft chimney work?
Hot flue gases moving up through the chimney create a draft at the smoke entrance which continue to draw in more smoke once the draft is started. How does this work? The hot gases from the fire are lighter than the surrounding air, and therefore the hot gases rise. When the hot gases from the fire are directed into the chimney, the gases rise quickly because there is less resistance from surrounding ambient air and less cold air mixing or diluting the hot gases. With only hot gases rising up the chimney, a plume or head of hot gases forms inside the chimney- the plume of hot gases being substantially lighter than the ambient air. T. As hot gases exit the upper end of the chimney, more smoke and air must enter into the bottom of the chimney, thus a lower atmospheric pressure area exists at or near the lower entrance of the chimney. The smoke entrance to the chimneys vary in sizes and styles as numerous as the number of smiths who build them. The opening or smoke-entrance at the base of the chimney is located very near to the fire, usually beside and above the fire. Several photos above show examples of a tall chimney smoke entrance on the wood and brick forge from Matthew Edel shop in Haverhill Iowa, built in the late 1800's.
The size of the hole that the smoke enters in the bottom of the chimney varied in size to suit the preferences or prejudices of the blacksmith for whom the forge was built. Some chimney openings were quite tall and often taller than the internal width of the chimney (as seen on the forge at the Edel shop in Haverhill, Iowa, USA). The bottoms of the smoke openings in the chimney also varied in that the bottoms of some were level with the forge hearth while others were designed so the bottom of the chimney entrance was several inches or more above forge hearth ( seen on the side-draft forges at the Anderson shop in Colonial Williamsburg in Virginia, seen in the picture at left).
Most begin level with the fire, some begin slightly above the fire. Again, close proximity to the fire causes creates the strongest draft into the chimney. Corbelling (brick or stonework which tapers outward or inward) may be used to extend the smoke entrance outward over the fire somewhat to act similar to a small shroud on a half-hood. The entrance must be at least as large as the area of the flue. Some blacksmiths built a chimney completely open at its bottom next to the fire. See the brick forge above at the Old Threshers Reunion Pioneer blacksmith shop or the Matthew Edel blacksmith forge above also. Other forges used chimneys that were completely enclosed except for a smoke entrance built in its side next to the fire.
Some smiths install a small sheet metal panel above the smoke entrance to act as a shroud or hood. The extension of a sheet metal shroud, or corbelling of the brick structure of the chimney that extends outward slightly over the fire, cuts off the amount of ambient air drawn into the chimney and helps the chimney draw smoke more strongly. The greater the distance between the fire and smoke entrance of the chimney, the more important it is to install a shroud over the fire to act as a hood or to corbel the chimney opening over the fire.
Some smiths like to build the rear inside wall of the chimney behind the smoke entrance, with an inclined surface to act as a transition (see the photo of the Old Thresher pioneer shop forge above) for the smoke where it enters the chimney and turns upward into the flue. Others build the rear wall of the chimney straight believing an inclined rear inner wall has no effect on smoke moving into the chimney such as that seen on the Edel shop forge. I have found that when smoke is flowing next to a back wall entering a chimney, it tends to hug the wall and flow more in a straight path into the chimney. This was my purpose for adding firebrick to the rear wall of the half hoods of my steel forges. The tilted rear inner wall might offer similar benefits for smoke but this might also be just an aesthetic design chosen by some smiths.
The smoke shelf.
Smoke shelves in blacksmith's forge chimneys are a new development from the late 1900's. The smoke shelf is literally a shelf projecting about 4 inches deep into the chimney, about 6 to 8 inches above the top of the smoke entrance in the lower chimney. Smoke passes around the front of the shelf creating a rolling or swirling of gas flow over the shelf before passing out the top of the smoke chamber and up the chimney. During times of sudden lightly increasing back pressure such as a gust of wind passing over the shop, the shelf allows the swirling gases above the smoke shelf to continue to draw smoke into the chimney. The smoke shelf helps keep the chimney drawing strongly at all times.
The space between the front of the smoke shelf and the front inner wall of the chimney hood is larger than the lateral area of the flue. See the drawing of the side draft hood dimensions at http://www.beautifuliron.com/steelhoods.htm The area between the smoke shelf and inside of the chimney ends up being about 80% larger than the flue. This percentage of coarse is greater when the chimney flue is round. For an example of how well one of my side draft forge designs work go to http://www.beautifuliron.com/mysteel.htm. Keep in mind that the smoke shelf is no substitute for placing the top of the chimney well above the peak of the roof. The chimney flue is still placed 4 feet above the peak of the roof.
While building a new forge recently I found that a certain amount of height is needed to help make the chimney draw well. While stacking flue pipes temporarily on the chimney so I could test it and make bracing and parts for the final chimney, I noticed the amount of draw increased until reaching 10 feet in length. Adding a few more feet though and no more increase was significantly noticeable. So it appears that building about 10 feet of chimney on top of this type of forge hood is necessary for accelerating velocity of smoke in the chimney. I am unable to determine if more height would be a benefit.
Forge builders need to be aware that a large amount of heat is released when the smoke enters the side draft hood and then turns upward. Firebrick lining the floor and lower sides of this hood will reflect much of that heat upward into the chimney and make the hood slightly cooler to work beside. Brick and stone forges are cooler to work with since this heat is always reflected upward in the smoke, but steel forge hoods release this heat faster.
Brick and stone create the most attractive and most comfortable chimneys to work around. Brick and stone reflect heat into the chimney rather than shedding heat in the shop next to the smith. But brick and stone are the most expensive to work with and time consuming because of the extra work needed to build a solid foundation to support the massive weight of the stone structure. A typical chimney in a one-story shop will need at least 1-1/2 to 2 bundles of bricks to complete plus all the materials needed in the foundation. Flames and heat from the fire shoot downward on the bottom of smoke chamber, so firebrick is placed on the bottom of the side blast hood to prevent heat from damaging the bottom mortar of the chimney.
Steel offers a cheap solution to building side draft or hoodless forges. But steel is rather unattractive and sheds a great deal of heat in the shop next to the smith as he is heating the iron. And like the brick and stone chimneys, flames and heat from the fire shoot downward on the bottom of smoke chamber, so firebrick is placed on the bottom of the steel side blast hood to prevent heat from damaging the bottom steel structure of the hood. Firebrick placed inside the hood around the sides, help reflect heat away from sides of the hood making it more comfortable to work with, and reflecting heat up the chimney for a stronger draw.
Many people new to forge building attempt to compare a modern wood or gas fireplace insert for a home, to a forge chimney. There is no comparison. The fireplace insert chimney is often shorter than the peak of the roof of the house or other structures. There is no relationship between what is needed for a fireplace insert chimney to work compared with a forge chimney.
The reason a fireplace insert can use a chimney which is shorter than the roofline, is that both the air intake and the exhaust in the top of the flue, are outside the building on the same side of the house as the chimney. The flue experiences little or no pressure differential at all when a wind gust blows across it.
This is contrary to what a forge chimney flue experiences during periods of wind blowing over the building. A forge hood gets its air from inside the shop. No matter whether the fire is blown from a remote blower outside the shop or not. The smoke entering the forge chimney is entering from within the shop. The bottom of the flue is open to the pressures inside the shop. The top of the flue is open to the pressures it is exposed to outdoors when wind blows across the building structures surrounding the chimney. What this means to the blacksmith is that the pressures at the top of the chimney outside will increase when wind blows across it even the though the pressures inside the shop remain the same. If the pressure differential between the top of the chimney flue is too much greater than the pressure of the smoke and ambient air entering the chimney inside the shop, then the smoke might be blown back down the chimney into the shop. By placing the exit point of the chimney flue at least 3 to 4 feet above the peak of the roofline, we are attempting to gain some distance above the area of greatest pressure differential.
Page updated 16 July 2011.
This page under construction and lots more to be added.
The author can be emailed at address in picture below:
Updated May 23rd, 2001.