Hi all,

I'd like some advice on doing something incorrect, but functional.

So, I have a window in my shop about a foot higher than the level of my forge table. I used to have a flue coming out from it with a side draft and a chimney 1.5m above the roof and it worked really well. I have since moved away (so had to remove the chimney) and come back. My landlord is not allowing me to reinstall my chimney, or anything affixed to the building. I have kinda stopped caring that much about my workshop, I'm in the process of looking for alternative shop space. I need to reinstall my coal forge (Ive been using gas for the last 2 years) and just get things functional and the smoke out of the window.

I cannot add anything to the outside of the building, but i need to extract smoke out of the shop. I have a bouncy castle (bounce house) blower which is pretty strong and some thin sheet steel. Because I am not staying for long, i need to do it as cheaply as possible, not really invest anything in it, but I need a coal forge this summer to work on my qualification pieces (WCB).

My current idea is just a kinda hood, with forced air blowing across the top and, in theory, pulling the smoke via venturi. No idea whether that will work or not.

Note: this doesn't have to last for years

Any help would be awesome xxx

I've made a system with a blower feeding about a 2" nozzle inside a 6" flue/duct (in other words set up much more like a venturi/ejector burner, but on a large scale). It improved the draft, but the bathroom fan blower I had wasn't really sufficient. I'd worry with your set up that the air from the blower would slow (and the pressure increase) when it hit the opening at the top of the hood. Then it might not "know" to continue out through the window, and too much might end up blowing back down into the hood. There's only one way to find out, I guess.

Another possibility with your general configuration might be a true venturi -- something like a 6" vent from the blower that narrows down to 4" in a short section directly above the hood, with a lengthwise slot along the bottom to suck in air from the hood. That could take a lot of experimenting to get right, though.

Come to think of it, you could just use a tube about the same diameter as the blower outlet that continued across the top of the hood and ended just short of the window opening at the back. That would recreate a “venturi” burner inlet. You might need to add a horizontal length of stovepipe extending out the window to improve the flow.

I'm not sure how things work in the UK, but what you are proposing would be problematic here in terms of building code compliance. As usual these codes have been put together to help with health and safety, not just for you, but for your neighbors as well. While your venturi idea has merit and may function well for fume extraction, the issue is the penetration of the exterior wall and the location of the termination. I understand your situation, but feel I must advise of the potential insurance impact if there is an incident.

The terrible alternative that popped into my 6am brain, is taking a length of 4" flexible aluminum hose and fixing it directly to the intake of my blower. Putting the blower outside and hoping that the hose is long enough so there's not enough heat left in the air to melt the plastic bits ...