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Having some professional experience with controlled manufacturing environments, it occurred to me that being able to setup a negative pressure system in household rooms during renovations would be a great way to keep the construction dust away from the rest of the house.

This instructable focuses on fitting a filter to a ventilator fan.  To create a negative pressure in the work area you will need to put up a cloth or plastic sheet to block the doorway to the room, and will need to keep the windows closed.  I use a strip door like P/N 8447A11 from McMaster.com (it's a little pricy, but I've used it on enough rooms that it has more than paid for itself).  I've also made is a habit of using a sticky mat just outside the room (McMaster.com P/N: 6896T61 or similar) to help keep me from tracking out dust.


Step 1: Tools & Materials

Materials
8” 1,225 CFM Exhaust Fan, with 25' hose & connector
• Standard Pleated Panel Air Filters, 1" Thick, 12" X 12" (McMaster.com P/N: 2209K89 or similar)
• 8” round to 10” square duct adapter (You can get these at Lowes or Home Depot)
• ~16” X 8” 24Ga galvanized steel sheet metal scrap
• (20) 1/2” (13mm) pan head sheet metal screws
• (4) 2” drywall screws
• 4/4” X 14” X 6” softwood scrap
• (2) 10-32 X 2” pan head screws.
• (~2) 1/8" pop rivets.
• Caulking, like silicone, if needed.

Tools
• Saw (table saw works best, but you can do this with a hand saw or circular saw if you're careful).
• Drill
• 1/8 drill bit, with countersinking shank or a 3/8 drill bit too.
• No. 2 Philips Bit for you drill or a N0. 2 Philips Screw Driver
• Marking punch
• Tape Measure
• Adjustable square
• Pop Rivet Gun
• Hammer
• 18" or longer quick clamps
• 3/4" hole saw (optional)
• Chisel or Utility Knife
• Screw Cutter or Hacksaw and File


Step 2: Modifying the 8” Round to 10” Square Adapter

Drill out the rivet at the outer edge of the 8” round. Adjust the lap joint so that the 8” round become a shallow cone and slips into the inlet of the blower.

Step 3: Disassembly of the Blower

Loosen the screws holding the screen retaining clips on the inlet of the blower.  Remove the intake screen.

Step 4: Installing the Round-Square Adapter

• Press the 8” round down into the blower inlet until the bead sits on the screw retaining clips.  
• Drill a hole at the 8” round lap joint and install a pop-rivet to lock the shape of the round.  
• Install the intake screen just above the 8” round bead, and tighten the clips (this part can be a little bit of a pain, but be patient).

Step 5: Making the Filter Frame, Step 1

• Carefully measure the length of each side of the 10" square opening when held flat.  
• Cut four pieces of wood for the frame, 1” X 3/4”; 2 that are 2” longer than two opposing edges of the square opening, and 2 that match the remaining edges of the opening.

Step 6: Making the Filter Frame, Step 2

Mark and pre-drill holes to attach the frame to the 10” square.  Three mounting screws per side.

Step 7: Making the Filter Frame, Step 3

Pre-drill the holes for the drywall screws at the corners of the frame;lLong pieces only.  Make sure you drill deep enough to get at least 1” of full thread engagement. This will provide sufficient grip, given that the screw is engaging end grain.

Step 8: Attaching the Frame to the 10" Square

• Pre-glue the ends of the short lengths of the wood frame.  
• Pre-assemble two corners of the frame.  
• Clamp the frame to keep it in shape during handling (you don’t want to use the screws, as repeated tightening into end grain will strip them out quickly).
• Slip the frame in place on the 10” opening.
• Begin screwing the frame together, adjusting as needed to get a nice flat front face and square outer edge.

Make sure there aren’t any significant gaps. If there are, seal them with some sort of caulking.  You should end up with the frame slightly larger than the filter.

Step 9: Fabricating the Filter Retainment

Use the scrap sheet metal to fabricate edge clips to hold the filter against the frame:

• Clamp the sheet metal down, overhanging a stout bench top you don’t mind hammering on. Use an adjustable square to get the overhang uniform.
• Hammer the bend flat along the long edge. Repeat to get a rolled over edge (so it isn't s sharp edge).
• Fabricate a corner as shown.
• Pre-drill and pop-rivet the corners.
• If the back of the rivet is longer than 3/16” (~4mm), snip off at that height with diagonal cutters.
• Hammer the rivets flat enough to not interfere with the filter when installed.

Step 10: Attaching the Filter Retainment

• Fix the guides to the frame (3 screws each side) so that there is a nice tight fit. It should be just loose enough to be able to remove the filter with some effort, but not so tight you damage the filter during installation or removal.
• Make sure the gap between the filter and frame is closed all the way around. Any visible gap will allow dust to simply be pumped out of the room and into the rest of the house in great volumes.

Step 11: Adjusting the Blower Feet Height

The supplied feet on the blower may not be long enough to keep the filter from resting on the floor, which can lead to the square-to-round adapter coming loose.  To fix this I added some spacers to the front feet to tilt the blower up:

• Sawing out some feet extensions, 3/4” thick.
• Clean up the burs on the extensions.
• Remove the inlet end feet.
• Replace the existing rubber feet mounting screws with (2) 10-32 X 2”. Using a screw cutter to shorten the mounting screws so ~1/8” to 1/4” protrudes from top of the nuts when everything is reinstalled. All the rest of the hardware is reused.

Step 12: Final Touches

To make it easier to manage the duct when carrying this thing around I attached a 3' (~1 m) piece of paracord to the outlet screen before attaching the duct.  Then I attached the duct using the supplied hardware and sealed it with aluminum tape.

When transporting the ventilator, I compress the duct fully, reach in and grab the cord, pull it out the end of the duct and tie it to the blower handle.  Works great.

Step 13: Ready to Use

Whether you are sanding floors or doing demolition work, this tool is a great way to keep the dust from settling everywhere else in the house.  I've used is in the renovation of three rooms so far and it has worked great.  As a bonus it maintains air circulation, which helps keep the work area comfortable.
you are a saint! i have an old mold testing blower that's just waiting to be turned into one of these!
Glad to hear it. Renovations are not much fun, so I try to do everything I can keep the chaos contained.
This is great. I have to rig something up in my work shed. It's hotter than the bejeebus out there and has no ventilation. This would be perfect for getting the shop funk out of there when I'm working.

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