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Old 2015-09-26, 2:47am
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Speedslug Speedslug is offline
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Join Date: Mar 21, 2009
Location: Winnebago, MN
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1) The cap on the top of the out side end will keep the rain from funneling straight in to you bench.

2) If you can control the make up air coming to the back of you bench that will allow you to adjust how much air comes over your shoulder and into your breathing space and since you are up in Michigan that can make a difference in how much expensive heated air $ goes to waste going outside.
If you can duct the make up air to your bench then you can at least put something over the opening to control the flow. More on this thought below.

3) I found a ten inch "blast gate" on Ebay for some $40 some 8 years back when I set up my system and I have it between my fan (inside) and the duct work going through the wall outside.



This closes the vent ducting when the fan is off to keep the heat from flowing outside when I am not running the fan.

4) The 125 cfm per square foot of hood area is the suggested minimum and the 800 you mentioned is only going to give you a hood of 3 feet by 2 feet.

Those numbers are for the air flow right at the face of the hood opening and would only apply if you are putting your hands through the face of the hood opening like a barley box with the torch actually inside the hood enclosure.

If you are going to mount the hood horizontally ( like you would over a standard kitchen cooking stove ) You need to in increase the flow rate by ( and I am guessing here, hopefully one of the experts can chime in here ) at least a factor of 4 if not a factor of 8 to ensure capture of the combustion fume sphere.



The equations the safety folks work with use the idea that the air is moving through an opening in the side of a big box and that you are sticking your arms and your nose into that opening in the box and are doing work through the side of the box. The box you're working in will contain the combustion fume sphere and the 125 cubic feet per minute of air flow through the opening is enough to prevent the fumes from getting into the room you are sitting in.

If you then lift this box over your head and turn it so the opening is parallel to the floor then you are going to need much greater air flow to keep the now uncontained combustion fume sphere from leaking out of the column of air moving toward the opening and into the room you are in.


A barley box is the best method of ensuing you capture all the fumes but working through a large hole in the side of a box is annoying.

When I put the knuckles of my hands together my elbows are about 3 feet apart. Add another 6 inches on either side for a little wriggle room and that totals 4 feet.

Resting my elbows on the bench with my hands straight up reaches at least 1 foot and a half. Add another 1 foot because I don't like bumping my head on the box I am working in and that works out to an opening in the side of the box of 4 feet by 2 and a half feet.

The math totals that out to a 10 square foot opening that you want to move 125 cfm through each square foot of and that math gives a minimum of 1250 cfm needed to keep the combustion fumes from leaking into the home I and my family live in.

The 3 foot by 2 foot opening that 800 cfm works for would be a lot like trying to work through the face of medium sized LCD TV.


Having the ability to vary the speed of the fan can also be a problem source.
The size of the working space is not going to vary at all and that would be the only real reason you would want to vary flow rate of the ventilation fan.

What you would want to adjust is the ratio of A) how much heated / air conditioned air you want to flow past your shoulder into your breathing zone and then in to the fan and B) how much make up air flows from the back of your bench to mix with the combustion fumes and then into the fan.

You could do that with another blast gate on the duct work of your make up air but you will need to set up some kind of incremental mechanism to get discrete control of how much air moves in that duct.


When I set up my blast gate to close off the fan ductwork from letting heated air flow up my chimney I used a pulley and wire rope with a magnet tied to the free end. I got the wire rope used for picture hanging from the hardware store and I used some of those magnetic business cards as landing pads to prevent the magnet from making marks on my painted hood.

I put the magnet on my metal hood at one spot to hold the gate open and I move it to another spot to let gravity close the gate. I only use the gate all the way open or all the way closed.

But a similar method could be used to control just how much make up air you allow to flow to the back of your bench.
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