How do you cut holes in furnace duct to install a humidifier?

I use a hacksaw brace that allows the blade to fit in without attaching both ends like a conventional hacksaw brace. The open handled brace is available in most hardware stores.

Put the blade in the brace with the teeth pointing down to the handle and your hand. This seems backwards but you'll cut on the pulling stroke.

Attach some form of guide to the line that you want to cut. For example, drill a couple of holes in a metal ruler and attach the ruler to the line on the duct you want to cut. Attach the ruler using #8 sheet metal screws 5/8" long to the duct work. The purpose is to keep your line(s) straight and prevent the duct work from moving back and forth with the saw blade's action.

Drill a 1/2" hole or larger in the duct along the ruler's edge. This is necessary to insert the hacksaw into the hole to start your cut.

Here is an essential tip: Use a hacksaw blade with 24 teeth per inch. Don't use a more coarse blade like 18 or 22. Don't use a 32 teeth per inch unless it's a short cut or you don't mind sawing all afternoon.

Here's an crucial point: Cut your line by dragging the blade downward toward you; don't push the blade back up like sawing wood, just pull the blade down and out and then reinsert the blade for your next cut. If you do this, the blade won't jam and bend out of shape besides cutting the duct more easily and quickly.

Finally, be SURE TO WEAR safety glasses because the steel dust from cutting the duct will find its way into your eyes and cause a bunch of grief.