HVAC Question
 

Good Day

I'm looking at setting up a small work area in my garage and have asked a local HVAC/Sheet Metal contractor to give me an estimate to install a hood in my garage. I'm assuming he will need to know the BTUs per minute of the torch I will be using. Is there anything else I would need to provide. He will be on site to see the area so he can do whatever math he needs to in relation to the overall interior size of the space.

BTU is not the primary metric. It's duct velocity. You want a target of 2500 - 3000 feet per minute of air velocity in the duct so that dust, fine particulates, and fumes are all captured and removed.

A rule of thumb many people use is 125CFM per square foot of hood opening. So, a 4' x 2' hood (8 sq ft) would need a 1000CFM fan. After some math, an 8" duct provides about 2900 feet/min. and would be a good choice. The challenge here is to find some fans you can purchase and work backwards from there to see which one is best.

Also, don't forget about the make-up air. You need an opening that doesn't restrict incoming air or it will lower the fan's performance.

Thank you!

Would having a window open on the opposite wall provide adequate makeup air or does the makeup air also need to be pulled in using a fan? Or does the makeup air need to be brought in using the same cfm rate as what is being taken out?

A window (or vent) bigger than your exhaust fan opening should work well and not require an intake fan. Or just prop a door open while working

Good luck!

Thank you very much.