I'm not very familiar with 220V AC circuits and motors that run on them, so let me explain my question.

I'm building a soldering fume extractor for my workbench similar to what is shown in this video. It basically uses a simple ceiling exhaust fan along with some hose pipe to suck the fumes away from the desk to the window. He uses an exhaust fan which comes with a metal box enclosure and an outlet to attach the hose.

Unfortunately, where I live, I couldn't find the same type. So I settled for a simple exhaust and used the cardboard box it came in as the enclosure. I then connected 2 hose pipes which direct the air flow out to the window.

So this works all nice and dandy, but I noticed that when I close the box so that air only flows through the hoses, the fan speed slows down. I guess that's expected when the air flow reduces.

I'm sort of worried that this may put the fan motor under some additional stress and may cause it burn if I let it run for too long. Will this turn out to be some sort of safety hazard?

EDIT: I considered putting a fan regulator to the fan to control the speed, but I read online that adding a regulator to a single speed fan may lead to it getting burnt :(. So I guess that's not an option.

EDIT2: I also guess another option is to increase the dia of the hose or add more of them, but the ones I have are about 1 inch in dia and I'm not too keen on having more than 2 of them since it'll become unwieldy.

  • \$\begingroup\$ You're restricting the exhaust? Your making the fan work harder to push the same amount of air through a smaller area... \$\endgroup\$
    – Passerby
    Aug 29, 2015 at 10:36
  • \$\begingroup\$ @Passerby: Yes, pretty much - possibility of disastrous results? :( \$\endgroup\$ Aug 29, 2015 at 10:43
  • \$\begingroup\$ You should measure the power consumption from the AC input, under the worst case (lowest voltage level, highest load on the motor), and compare that value to the nominal value specified for the motor. If you're not exceeding the nominal value, then it is a reasonable expectation that the motor should not get burnt. There are electronics (called Motor Protection Circuit Breaker) which act as a safety device if the consumption increases to unsafe levels - for example when something blocks the motor. \$\endgroup\$ Aug 29, 2015 at 11:48
  • \$\begingroup\$ @LaszloValko - That's a great suggestion, thanks! I'm scouring to see if there's any watt meter I can get my hands on. Looks like the Kill-A-Watt isn't easily available in my area \$\endgroup\$ Aug 29, 2015 at 12:23
  • \$\begingroup\$ A 1 inch hose is too small for effective fume extraction, especially when coupled to such a small fan. You want a 3 or 4 inch hose. How about just using the fan to blow air across your workbench, so the fumes don't float up into your eyes. No need to pipe them out of the window unless you are in mass production, soldering all day every day. \$\endgroup\$
    – tomnexus
    Aug 29, 2015 at 18:18

1 Answer 1


Fans draw a load in relation to the amount of work they do. It sounds as if you are closing the discharge duct of the fan and the fan is working to compress the air in the discharge duct.

Set your system up so the discharge duct is left open. Close the inlet duct while monitoring the Motor load (current). You should see the Motor load go down and the fan speed remain the same or go up a little.

Restricting or blocking the inlet results in the fan not having to do as much work. It moves less air and any air it does move can be freely discharged in the exhaust duct.

  • \$\begingroup\$ Thanks for your suggestion! The problem is the fumes need to piped out to a window, so I do need the discharge duct open. Do you think it would be okay if I keep the inlet and the discharge holes the same size? Or make the inlet a little smaller? \$\endgroup\$ Aug 29, 2015 at 12:22
  • \$\begingroup\$ As compared to the size of the Exhaust duct - Same size or smaller on the inlet should be fine. \$\endgroup\$
    – Tinkerer
    Aug 29, 2015 at 12:40
  • \$\begingroup\$ Sorry - I meant to say "I do need the discharge duct closed" in my previous comment \$\endgroup\$ Aug 29, 2015 at 12:43
  • \$\begingroup\$ I tried making the inlet the same size as the discharge, but the fan speed didn't increase and the power consumption shot up. I think the next thing to try is get a bigger tube and try with that :| \$\endgroup\$ Aug 31, 2015 at 13:13

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