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Air Ducts, Removal and Replacement. one whole year of ductwork.
~mike gradziel.
December 2009 - December 2010:
Two home inspectors doing our pre-purchase checks in 2009 said these filthy air ducts were normal and required no immediate action. Never mind the frayed asbestos paper tape wrapped around gaping holes in the air intake, or the air drawn through unsealed joints from the dusty crawlspace littered with dessicated rat poo from another era - better not to disturb the asbestos-insulated ducts in the attic, they said. From the day we got the keys not once did we turn on that furnace blower until asbestos abatement contractors had removed the old duct, the interior walls were wiped clean, and new sealed and insulated ducts were in place. This took a very long time, on account of my ancillary cleaning tasks and a general shortage of time, but before the next season's cold weather it was all done!

Sized by careful calculations, designed to be whisper-quiet, and sealed up per code so well that Inspector Rick asked for photos to show as a good example, these ducts and the building materials used in their construction cost just $1400, not including another $1000 for the removal contract or supplies for the big attic and crawlspace cleaning jobs.
dusty horrors of the cold air return dusty horrors of the cold air return the attic and asbestos covered ducts
furnace ducts and gas flue the original furnace configuration white powder crystals of a zinc-sulfur compound in the gas flue
white powder crystals of a zinc-sulfur compound in the gas flue beginning the wall rebuild new insulation for the dining room
new fire-code wallboard in place framing the wall higher this section is finished
main duct installed and sealed original air intake the hole after duct removal
blocking out the hole new subfloor nailed and glued cut sheet metal
laying out the pattern bending riveting on a damper collar
finished hub, before sealing pre-plaster register box the distribution manifold, installed and sealed
each register box is fully sealed the main supply from the furnace four-inch duct to the bath
mastic seals the flex duct the mastic is UL Listed making duct connections with nylon straps and mastic
connecting the 12-inch duct ducts finished except for insulation penetrations through fire-rated wall board are sealed with fire-block caulk
closet with wall cut away for air return closet with wall cut away for air return the closet looking up, before project
framing up the closet wall new closet wall base view through the new duct opening
fabricating duct in the garage fabricating duct in the garage finished duct sections put together
the joints are riveted there are turning vanes in the big bend just a couple inches to spare fitting the duct into the bathroom so the door can be closed
bare closet with wiring in place bare closet with wiring in place the back side of the duct is fully sealed
finally, the duct is in place the inlet section is in place, but not yet riveted looking up at the shop-built rectangular duct
sections are riveted and sealed I designed the installation joint so I could access all sides to apply sealant view of the return duct in the attic
view of the return duct in the attic the duct system before final insulation the duct system before final insulation
the finished return duct in the bathroom closet the finished return duct in the bathroom closet the finished return duct in the bathroom closet
the duct walled over, with shelf supports installed attic ducts fully insulated attic ducts fully insulated
insulation installed on furnace air intake, plastered and painted, without filter air intake, plastered and painted, without filter
cutting flooring to patch where the old air return was arranging new flooring shaving pieces of flooring to fit tightly
finished floor: nailed, filled, sanded, and coated And now I proclaim this job complete, with the floor patched and the heater system fully operational. I plan to upgrade the thermostat sometime to a programmable unit, and neither the air registers nor the new air return have louvered covers yet, but those are decorative details. This project is finished!

January 2011:
The bathroom closet, new home of the air return duct, is finally finished. You would never suspect it was once nine inches deeper - it feels plenty big enough and the shelves still fit without trimming, except for the two that fit around the duct. I re-grouted and sealed the tile, caulked the perimeter, and installed the shelves.
finished closet finished closet finished closet

January 2015:
Years have passed and finally I have made a proper cover for the air intake. Up until now, we have seen the pleated paper filter bare in the intake opening but now it looks a little more finished with the white-painted expanded metal lattice from a screen door protector panel set into a wood frame. The paper filter stays in by friction and rests against a foam rubber weatherstrip, and the wood frame also fits by friction via felt pads stuck around the perimeter of the frame.
finished air intake cover, with filter finished air intake cover, with filter

Here are some notes I made along the way:

Apr 2010:
The ductwork was in terrible condition, yet through two home inspections (one by the seller, another by an inspector our realtor found) no one mentioned the horrifying black fuzz lining the ducts, the asbestos paper tape through which intake air was drawn, or the gaping holes all over the system. I knew I was going to replace it, but I am still discovering just how completely I will be rebuilding it. First, the octopus-tree asbestos-covered duct came out via a professional abatement service. Then, I decided to thoroughly clean the attic. Next I cut in new register box openings, and decided to wall up the garage with a fire-code floor-to-roof wall while I was running new duct. Finally, the cold air system turned out to be in even worse condition than the supply ducts, and while replacing that I decided to move the intake register too.

Part of the furnace flue was particularly old and corroded - white crystals formed inside and collected as a powder at the base of the flue. They are probably compounds of sulfur from the gas exhaust and nickel from the galvanized steel. This is interesting, but the parts are still serviceable though I have been tempted several times to install a new furnace while I'm at it. A typical furnace life is said to be 20-25 years and this one is 19 years old, but it would have been less heavily used than the national average here in temperate California. Plus, despite tempting government rebates, a replacement would still cost us money and then I would feel obligated to replace the flue, relocate its roof penetration, run new wiring to the furnace, and replace part of the gas line. Plus it would need a new air plenum. I'm saving that project for another three to five years down the road.

After removing the old ducts and air plenum, I pulled the furnace away from the wall, tore down the flimsy plywood, and framed up a new wall for 5/8-inch fire-code gypsum wallboard. This was rather difficult because I had to work around existing pipes and wires, and I didn't want to disconnect the gas or power lines into the furnace so the workspace was cramped. When I re-wire the house, I can extend the new wall to the right where wires now run. Once I move the flue roof penetration, I can extend the wall to the left. Also I will have to work around the water pipes there, and I should replace the galvanized steel cold water line with copper before I do the wall. Every project spawns others!

June 2010. I covered over the old cold air intake in the floor, intending to install a new one on the wall where dirt won't fall in and also opening up that floor space for our shoe rack. Down below, in the crawlspace, seventy years worth of dust, pebbles, leaves, and wood chips need to be swept up so termites don't move in and so I can work more comfortably on the ducts and plumbing. I pulled out all the old metal air duct, which was never sealed properly and so was drawing air from the crawlspace for decades. It was filthy. I am glad we washed down the house before we moved in.

For the attic ductwork, I installed a hub-and-spoke system rather than the more popular tree-and-branch design, for several good reasons: It is much easier to analyze airflow when all ducts receive the same input air pressure in the hub box, and if damper adjustments need to be made they can all be conveniently reached from that one spot in the attic. Placing dampers there, rather than at the register boxes in the walls, will minimize noise in the house, and it will look better, too. Lastly, I was going to have to buy 25-foot lengths of insulated flex duct, so I would have a lot of unused material if I was gradually stepping down a tree-trunk system from larger to smaller ducts at each branch. So I built this distribution hub, complete with dampers and an extra hookup for the future third bedroom.

July 2010:
Working evenings and early mornings while the attic was still cool, I installed the ductwork with plenty of mastic sealant and reinforcing tape. It is absolutely airtight. Now all that remains to be done is insulate the metal fittings, and of course install the cold air return duct.

September 2010:
The cold air return once ran through the crawlspace. The air filter was at the furnace, and the 20 foot long run upstream of the filter was lined with an inch-thick layer of black fuzzy lint. Huge gaps allowed air to be drawn in from the rodent-ridden crawlspace (rodents are gone, and the crawlspace is swept clean!). To make matters worse, those gaps through which air was being drawn were patched with fraying asbestor paper tape. Abatement contractors removed the asbestos, and I hauled out the dirty old duct and disposed of it at the scrap yard. It makes more sense to run the return through the attic, since the furnace is up there and it is easier to inspect the ductwork, and I wanted to move the air intake from its position in the floor where it caught dirt easily. I decided to make the run through the back of the 30-inch deep bathroom closet, since the space was bigger than we need. To do the job right, I pulled off the plaster and lath to embed the new duct within the wall, leaving maximum closet space. This was a dusty job!

November 2010:
The duct job is complete! Well, not really, but it is fully functional and Inspector Rick passed it with compliments. Now I just have to finish insulating bare duct, wall up the closet, paint the closet, patch the floor where the old return was, and start working on louvers to cover all the openings in the walls and ceiling.

I ended up having a sheet metal shop build me a piece of rectangular duct, which was rather costly, instead of putting in the old battered-looking but otherwise serviceable duct. I figured that since I had the wall torn open and would never again be able to access that piece of duct, it really should be of a quality that I can be content with. The 3D S-curve with turning vanes went together beautifully and is very quiet in operation, though I suppose there is always room for improvement; there is some flow separation at the vanes and rivets which makes a faint rustling. I am most proud of how perfectly the piece fit into the closet, with just an couple inches to spare on the width of the bathroom, and half an inch clearance floor to ceiling, and less than an eighth of an inch clearance during the several angular motions needed to take the duct into the shower, around the corner, and up into the attic. Then the adjoining piece fit perfectly into place and I riveted it securely, sealing all the joints with mastic.

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