Early this morning, I came across this post on one of the glassworking forums.
… My hubby went to a furnace expert and asked about squirrel cage fans from a furnace and using them for an exhaust hood. That we wanted to use 8″ duct. They pretty much told him he couldn’t, we’d need 10″ or larger to do get anywhere near 1200 cfm. I was frustrated because hubby said it sounded like we couldn’t do it. I said that if science labs can have a fume hood with a strong draft there was no reason why we couldn’t. And since I worked in a lab once, I know how strong those fume hoods can be.
Honestly, I was so mad I told my husband to quit wasting his time and that you guys would know. If a furnace fan can blow hot air through a whole house, can it be set up for an exhaust hood with 8″ duct? We plan on making our own hood. We’re looking for a used fan, but may have to buy a new one.
One issue here is understanding the differences between a “Local Exhaust Ventilator” (or LEV for short), a “fume hood”, and a HVAC furnace for a house.
A furnace relies on a blower to push hot or cold air through existing ducting into various rooms of a house. Because of the design of the system, these blowers are often times very large and move a lot of air. Additionally, the air is first forced into a plenum which runs the length of the house off of which are routed smaller ducts (usually 6″) that then supply the hot or cold air to the various ducts in each room. Separately, there are cold air returns which also feed into a plenum system to allow the air to recirculate back to the furnace to be reheated or cooled, and the cycle begins again.
A fume hood used in science, industry and commercial applications is a device which removes poisonous or hazardous fumes from a very localized area and routes them either outside or to a conversion process to safely capture the fumes and render them harmless.
LEV is the hood/ducting/fan/fresh air supply that we as glassworkers use to safely ventilate our workstation areas to remove unwanted combustion fumes and hazardous glass and metal fumes from the molten glass.
Although it is possible to use a furnace blower in an LEV system, many times, it is far more work to adapt the blower to the design than it is to purchase a proper new fan designed for the application.
Most furnace blowers are designed with both ends open to allow free access to the cage-type blower wheel. This means that the entire blower assembly has to be enclosed in a box, with the exhaust opening ducted outside. Additionally, these large blowers are not designed to handle high static pressures, so closing the blower opening from 12″ square (144 square inches) down to a 8″ round duct (50 square inches) is going to have a significant impact on the back pressure and put strain on the blower motor (not to mention reducing the actual amount of air being moved through the system).
Don’t get me wrong here — some glassworkers have designed and built functional LEV systems with furnace blowers, it can be done, but it has to be properly designed, taking into account all of the factors necessary for the system to make maximum use of the blower. You cannot handicap the blower from the start by reducing its output area by 66%!!
It is far easier and much less expensive to build an LEV from scratch, using a single intake/single exhaust squirrel cage fan and properly sized ducting than it is to try to force something to do what you want that it was not designed for. Yes, the used furnace blowers may be free or very low cost, but that is no reason to assume that it is going to automagically work the way you want.