Frequently asked questions

  1. I’ve got a wall/window A/C unit. Isn’t that good enough ventilation? It has a fresh air option!
  2. Can I just vent my exhaust system into the attic space?
  3. I was at Home Depot/Lowes/Menards and could not figure out what to get for a hood/fan!
  4. Why can’t I tap into the existing dryer vent?

1. Window/wall A/C units do not really supply a lot of fresh air into a room. Usually, the switch opens up a baffle in the unit that allows the unit to bring in outside air. But it is unpowered in most cases, and usually will not support the high airflows that are required for glassworking ventilation.

2. Don’t just dump the exhaust duct into the attic! This is illegal in most parts of the country — chances are the exhaust byproducts will filter down into the living spaces of the house. All LEV (local exhaust ventilation) exhaust ducts **MUST** be ducted outside, by code and common sense. Now, that being said, you can certain duct *through* the attic and out the roof or gable end, but honestly? I don’t really recommend this (see other post on ducting out the roof here: https://mikeaurelius.wordpress.com/2008/02/05/venting-through-the-roof-or-through-the-side-wall/

3. Not surprising. Home improvement stores have absolutely no idea what constitutes “local exhaust ventilation”. Ignore the clerk. Purchase a range style cooktop hood, but be aware that you are going to have to modify it. They rarely have the proper sized fan for the size of the hood.

For overhead hoods, you need a minimum of 125 CFM per square foot of hood coverage area, and I’ve seen hoods for sale that barely meet half that. By one that covers the width of the area you need covered, usually 30″ to 36″.

Strip out everything in the hood (this is why it is a good idea to buy a basic model!). You can leave the light, but everything else has to come out.

You then have two choices: cut a round opening to fit your duct size, usually 8″ or cut slots like I have described in this article: https://mikeaurelius.wordpress.com/2007/12/25/ventilation-basics-part-3/ If you go for the slot option, you will have to build a plenum as described.

You will now have to purchase a fan to drive the system. You can find out how to do that here: https://mikeaurelius.wordpress.com/2007/12/25/ventilation-basics-part-two/

If you are under time constraints because of contractors, have the electrician wire a switch and leave additional conduit (or flex cable, his choice depending on code in your local area) that can be wired directly to the fan once your hood and duct work are installed.

4. Ugh. Do not tap into the dryer duct. Don’t even consider it!

The duct work from the hood to the outside needs to be the same diameter all the way down the line and must never decrease in diameter in the direction of the flow of air in the system. Typically speaking, your duct work is going to be at least 6″ in diameter, and most likely 8″. Most dryer vents are 4″, and some are corrigated (flex ducting) which is very bad for lampworking exhaust. In terms of square inch area, a 4″ duct is 1/4 the size of an 8″ duct, so not only will you be losing 75% of your air flow but you would also be adding turbulance from the size change (and corrigations in the duct). This is a very fast way to burn out a fan motor.

The exhaust duct needs to be ducted by itself straight through the outside wall and must not tap into any other duct work.

And even if it would work (which it won’t), you would not be able to use the dryer when lampworking or lampwork when using the dryer.

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