# Firebox Door Vent Theory



## pianov (Jan 9, 2015)

I just built a new firebox door for my masonry offset smoker. I put a large vent low on the door for air to flow under the wood grate and another smaller vent up higher..... because I think that's a good thing. But I have to admit, I don't really understand exactly the function of the vents and how they might best work together.

I do of course know that the low vent is to provide air directly under the fire. But how best to use the upper vent - what conditions would prompt a knowledgeable operator to open or close it?

Any guidance would be greatly appreciated.  
	

	
	
		
		



		
			






Terry on Tampa Bay, Florida, USA

Picture of my firebox door below:













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## pianov (Jan 9, 2015)

DaveOmak has posted a lot of info on this, but I haven't found any direct answers to my questions. Digging through old threads on this subject I found info like (I'm paraphrasing): "before I installed an upper vent on my firebox, I had to open the FB door when I added any wood to a fire mid-smoke - otherwise it would produce tons of white smoke. Now I just open the top vent and it burns well."

Is that the main function of the upper vent - to make smoke more blue than white? Or is it used to control temperature more? Or both? Or something else?


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## daveomak (Jan 9, 2015)

The lower vent adds air to the fire to make heat....   Your firebox can overheat if that is the only vent you have....  The upper vent moves air from the FB to the CC, cooling the FB and heating the CC.....   learning to use both together, makes for more efficient burning of fuel and temp control in the CC.....   For all this to work properly and efficiently, the "system" needs to be airtight....


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## gary s (Jan 9, 2015)

I agree with Dave  Upper vents make a huge difference 













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## trucknchuck (Jan 9, 2015)

Well my small offset fire box has upper and lower dampers or vents and I use the top one to let heat out when temps are to high. I use bottom vent to control fire. More heat just open bottom up a bit. Less heat in cc open top vent. I keep my stack wide open   That's how I controll cooking temp.  It really don't make a diffrance to smoke color to what I can see.


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## pianov (Jan 10, 2015)

Thanks Trucknchuck, Gary S and DaveOmak. I smoked some salmon yesterday and experimented with the vents - trying to see how the two worked for controlling temperature. I think that I can see how the upper one has an effect and allows you to keep the bottom vent open as little as practical to conserve wood. They do seem to work well in tandem. I'll keep playing with them and learning. Thanks.

DaveOmak: One of the reasons I redid my doors and CC top was to seal everything well. I am thrilled at how well everything seems to be sealed. Even when I first start the fire in the smoker and the exhaust isn't warm enough to draw air/smoke and the fire is producing tons of white smoke - almost zero smoke leaks out any of the doors or top. I think that is really going to help me be able to control the fire and temps well. I'm very happy with how it is all performing so far.


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## daveomak (Jan 10, 2015)

Glad to hear it appears success is on the way.....    Thanks for the reply....  Others will benefit from this thread.....  

Dave


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