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Gas Forge Venting

Jordan LaMothe
Trusted Member Master Bladesmith (5yr)

I am upgrading my shop to an insulated, closed space, 20' by 25' in footprint, and both I and the fire inspector are concerned about making sure the gas forge is adequately ventilated. Does anyone here have a forge ventilation system that does not include opening a door or window to the outside? I want to keep my shop heated when I forge.



Topic starter Posted : 21/12/2017 7:46 pm
Robert Wright

Put an adequate size hood over it and vent through the roof using 10 inch stove pipe. Make sure you go at least 4 feet above the roof for proper draw. Also, install a damper above the hood to allow you too close off the air flow when not forging. Not my set up, but have seen many variations like this.


Posted : 22/12/2017 10:36 am
Karl B. Andersen

Any air volume removed from the room will need to be replaced with outside air. It's a law of physics you just can't avoid.

Karl B. Andersen

Journeyman Smith

Posted : 22/12/2017 10:45 am
Steve Morley
Estimable Member Apprentice Bladesmith (5yr)

You'd have to look at the combustion equation of propane to see what the gas balance is when burning propane.

There is actually a net positive of gasses produced. It's 6 molecules of gas in (1 propane and 5 oxygen) and 7 molecules of gas out (3 carbon dioxide and 4 water vapor). You produce more gas molecules through combustion than the forge consumes. Each molecule contributes equally to the pressure in your shop, so this combustion reaction actually creates positive pressure in your shop. Heating the shop with the forge also creates positive pressure simply because it warms the shop and heated gasses expand. If you have a tight shop you are slowly pushing gasses out of the shop through whatever openings there are in the walls, the water vapor and carbon monoxide displacing the nitrogen and oxygen...not good.

Burning coal would create even more positive pressure in a shop since the only gas consumed is oxygen but the two products (water vapor and carbon dioxide) are still produced. Even worse. Luckily the smoke (noncombustibles) makes people think about ventilation.

The real challenge is getting fresh (oxygen rich) air into your shop against this positive pressure. If you don't ventilate enough, you end up running your forge in an oxygen deprived environment and then you risk carbon monoxide poisoning (due to incomplete combustion). CO poisoning takes a while (sometimes weeks) to recover from...if you are alive to recover from it. I have other hobbies (running, skiing, the gym, keeping up with my kids) besides smithing, so I can't afford to have my hemoglobin tied up with CO, even a little bit.

In short, smiths shouldn't rely on the fire sucking fresh air into their shop. Most fires produce more gas molecules than they consume, so there is no "sucking." You need an easy way to convect those gasses out which will pull fresh air in through a window or open door. A hood would be great, but you still need an opening on the other side of the shop to let gasses in or the hood won't work well enough.

I just open my garage door and roll my forge outside or right up to the opening, regardless of how cold it is.


Note: Also remember that the propane is essentially being injected into the shop, so that's even more positive pressure. You must have an opening to let fresh air into the shop.

Posted : 22/12/2017 12:53 pm
Gary Mulkey
Prominent Member Apprentice Bladesmith


My shop is approximately the same size. What you need is to supply plentiful fresh air while forging while venting the gas exhaust and a way to shut this off when not running the forge so that the shop can be heated. During the forging you will generate more than enough heat to keep the shop heated even while venting. Example: I use a 24" exhaust fan mounted in the wall by my forge and keep a double door open to the outside while forging. The fan has louvers that automatically close when the fan isn't running. While not forging I simply turn off the fan and close the doors.

Posted : 22/12/2017 9:00 pm

If you use some sort of hood with a fan (oppose to just using the flew effect to create a draw) you could probably run it through some sort of heat exchanger to warm the fresh air entering the shop to replace the exhaust air you're venting out of the shop.

Posted : 24/12/2017 11:49 pm
Dean Pavia
Estimable Member Apprentice Bladesmith

Garage door open in the front of my shop, walk in door open, (4 feet elevated), at back of my shop. Gas forge in middle keeps me warm enough as long as I have good boots and heavy socks on.

Posted : 26/12/2017 2:24 pm
Jordan LaMothe
Trusted Member Master Bladesmith (5yr)

Thanks all for the suggestions. It looks like a hood and a vent for make-up air should be a good combination to not let too much of my heat out. Does anyone have a good hood design they can recommend?

Topic starter Posted : 28/12/2017 4:57 pm
Ed Caffrey
Master Bladesmith

I agree with R. Wright. A hood with a 10" vent pipe. I would recommend AGAINST any type of fan in the vent pipe..... in my experience, there isn't a fan out there that can withstand the temps in the vent pipe.... just make sure it extends beyond the peak of the roof, and it will create it's own "draw". My hot shop is 20' X 48' and even with the hood/vent, I have to have a window open, and a gable fan running to bring in fresh air....forges output a HUGE amount of carbon monoxide.

Posted : 29/12/2017 10:21 am
Seth Johnson
Active Member Apprentice Bladesmith


Thanks all for the suggestions. It looks like a hood and a vent for make-up air should be a good combination to not let too much of my heat out. Does anyone have a good hood design they can recommend?

You could look at using the double wall vent pipe as a way to introduce fresh air while reclaiming exhaust heat. It might not be enough air flow, but it would help.

Posted : 14/01/2018 11:59 pm
Roby (Rob) Schafer
New Member Apprentice Bladesmith

Hah, was going to say same - What goes out must come in!

Posted : 09/08/2022 2:39 pm
Thomas Franklin
Eminent Member Apprentice Bladesmith

For the air coming inside the space you could use a brick vent or louver. These are not cheap and you will need to size them according to your local fuel and gas code. This code will have an entire section dealing with combustion air. There will be several options. I suggest the two opening method (one intake next to ceiling and one next to floor). For example in Arkansas you need 1 sq inch FREE AREA per 1,000 BTU/h but not less than 100 sq inches. So you need to know what BTU/hr your forge uses. Lets use 400,000 as an example so that would be 400 sq inches total FREE AREA. Most intake louvers are about 50% free area. Now we need 800 sq inches of louver. I typically size these in dimensions of 8" in case we are dealing with brick siding but if not any dimension combination will do. Two (qty 2) 20" x 20" openings would be required in this example. Good thing with louvers is you can get them with automatic dampers to open when you are using the forge. This could even be tied into a switch on your forge blower. Stationary (always open) is cheapest. Downside is you now have cold air coming inside but you are safe. There are several methods and the code explains them all. You can do it mechanically but that is expensive. You could duct it in and have an opening next to the forge. Skin the cat however you want or can afford but follow the codes and be safe. 

Posted : 10/08/2022 11:31 am