This project is continued from Part 6.
Bathroom Exhaust Fan – 4 Inch Vent Cap
The old economy bathroom vent fan used a 3 inch diameter duct and vent cap. The new Panasonic WhisperCeiling model FV-11VQ5 exhaust fan requires a 4 inch diameter duct and vent cap. I purchased a 4 inch vent cap with a bird guard and damper to prevent back-drafts.
This is the old 3 inch vent cap that will be replaced with the 4 inch vent cap:
The old 3 inch vent cap was mounted to the exterior wall with three screws:
- Two screws were set in the wood trim.
- A 3rd screw wood screw was loosely set in the stucco wall at the bottom right position.
A masonry screw should have be used here instead of a wood screw.
- No screw was present at the top right position on the vent cap, but there is a rather large screw hole in the stucco.
The surprise was the vent cap was not taped and sealed to the 3 inch vent duct! Another example of a job going to the low bidder with no quality control!
I pushed the old 3 inch flexible duct into the wall until it feel inside the interior soffit in the garage. I then looked and felt inside the old 3 inch duct hole to make sure no wires, plumbing, wall studs or joists were in the way before enlarging the hole to 4-1/4 inches with a hole saw.
4-1/4 Inch Bathroom Exhaust Fan Duct Installation
The old 3 inch hole in the exterior wall must be enlarged to 4-1/4 inches for the new 4 inch ductwork and vent cap. A 4-1/4 inch hole saw is perfect for the job. The extra 1/4 inch diameter translates to a 1/8 inch clearance (radius) around the 4 inch flexible duct.
The center pilot drill bit doesn’t help when drilling out the old 3 inch hole with a 4-1/4 hole saw because the pilot bit is inside the old hole with nothing to bite into. To accurately drill the new hole, I held the hole saw against the wall where I wanted the new hole and marked the hole saw outline with a red pen. I started the new hole by holding the drill steady with both hands and drilling at a slow speed until I had a groove cut in the wall. After a starting groove was cut to guide the hole saw, I could drill at full speed and force.
Continue drilling until the hole saw is completely through the exterior and interior walls. Note that I basically ruined the teeth on the wood-rated hole saw by cutting through cement stucco. The local hardware store didn’t stock a 4-1/4 inch masonry hole saw, so I wrote it off as a cost for getting the job done.
The hole saw did a fine job at making a perfect 4-1/4 diameter hole through the exterior and interior walls. A piece of the cement stucco broke off while drilling. I’ll fill this with stucco patch and caulk the seam next to the wood trim.
This is the view from from inside the interior soffit in the garage to the 4-1/4 inch hole drilled in the exterior wall. I’ve pushed a telescoping paint stick through the outside hole in preparation for pulling the new flexible vent duct through the soffit to the outside vent hole. Notice the drywall ring drilled out at the far end of the soffit.
Looking through the access panel I had previously cut in the interior soffit, the paint stick with a hook duct taped to the end is shown:
I installed the new 4 inch flexible duct through the interior soffit by:
- Attach the flex duct to the paint stick hook and secured it with a piece of duct tape so the hook won’t slide along the duct coil wire and tear the flex duct plastic liner.
- While my helper guided the flex duct into the interior soffit, I pulled the flex duct through the soffit with the paint stick.
- Guided the end of the flex duct with my hand through the 4-1/4 inch hole in the wall to the outdoors.
The new 4 inch flex duct extends a couple of inches past the exterior wall so I can tape and seal it to the vent cap:
The new flexible duct is slipped onto the 4 inch vent cap, then taped and sealed with two layers of aluminum foil HVAC tape. Do not use duct tape because regular duct tape will come loose. A metal worm gear band would be nice here for extra secure fastening, but there is no room for it in the hole.
I filled the old screw holes in the stucco with stucco patch, then fastened the vent cap with two wood screws to the wood trim. I only used to wood screws to mount the vent cap because:
- Two wood screws are more than sufficient for the light plastic vent cap.
- There is large gap of up to 5/8 inch between the right side of the vent cap and the stucco wall because the wood trim extends beyond the stucco wall and the vent cap is flush with the wood trim. Driving screws in the stucco would only make more holes and warp the vent cap. Due to the lack of planning by the home builder, I couldn’t relocate the vent cap to rest wholly on the stucco wall because the wall studs block the way.
As shown in the next part of this series, I filled the gap between the stucco wall and vent cap with foam backer rod then caulked the perimeter of the vent cap to make a watertight seal.
This project is continued in Part 8.
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