Duct Work effects Air Flow in Homes

I purchased my house here in Utah about 12 years ago in 2007.  It was built in the very early 70’s;  many things have had to be updated over the last few years.   In prior years, I replaced all but 2 windows with double pane vinyl windows, and also replaced the doors.  This alone cut down the air leakage from outside and the dust in the house.     I  replaced all of the plumbing that was from the 70’s with PVC, copper and PEX.  In 2010, I updated the wiring in the kitchen and bathrooms and added GFCI receptacles.  I replaced the appliances, added a dishwasher, and painted all of the cabinets.   We tiled the kitchen and dining room, plus bathrooms and added heated floors there a few years back as well.  Two years ago I replaced the old carpet with hardwood in much of the house.  Two years ago I put a new furnace in and AC that is very efficient.    This year I replaced the breaker box and rewired several areas that were old aluminum wire ending in the wrong type of connections or just old wire.

When we moved in the house was set up for a furnace that was over sized to handle the windows and door leaks.   A small AC was added sometime to this house and the evaporative cooler was still on the roof not working.   The original duct work was set up for a small furnace that heated about 800 sq ft.  Very little return air and barely working.

This home made duct work was minimal in this house so I have replaced and added to bring it up to what I expect it to be.  I also have had very little air in about half of the house.   As I said, the furnace is 2 years old and I replaced the PSC motor for the fan with a constant torque motor that is set to put out about 1600 cubic feet per minute total air.  It is split between the kitchen, dining room, living room and large master down in the basement on one side and the other 3 bedrooms and bath rooms on the other side.  The bedrooms have had very little air coming through.

The ducts are in the basement which is finished.  I tore out sheet rock to find that three bedrooms were all fed from one 6″ duct that ran about 15 ft and then split three ways.  Now I know why no air on that side.

I took all the old 6″ duct that  began at the furnace on the bedroom side out and replaced it with two 8″ x 10″ ducts mounted side by side feeding a large 10″ x 20″ 10 ft duct across the same area.

         Looking up in the left picture you will see two ducts both 8″ x 10″ as this area has very limited room so one is 10 in wide the other is 10 in tall and 8 in wide.  The right picture is two 10 x 20 in ducts 5 ft long each.   Airflow went from two 8 x 10 ducts mounted side by side connected directly to the furnace output plenum and then combined into a 10 x 20 in duct to travel to the area where the bedrooms can be split off.

In the last 5 ft of duct, I distributed the air as follows: For the upstairs master, I put two 7″ round ducts up to 4 x 12 registers from a 7 in Elle on top;  I had already added a large return in that room and two others that needed it last year.  I then installed another 7″ duct from a Elle  for the down stairs bunk room which already had a good return.  I then added another 7″ duct to a smaller bedroom above that.   You can see the second 7″ duct for the master going through the variable ducting.

The airflow on this side has gone from hard to feel at 3 in away to very strong 3 ft away from each register.  With the 3 phase constant torque fan motor that provides good air and the changes to the duct system in the bedroom side,  it all works much better for this half of the house.  The other half was done last year and works very well also.

Now I will replace the sheet rock and texture it and get ready to paint all this.  This type of job takes two of us about 1.5 days to do if the area is open.  It took me about 3 days part time to do this as I took the sheet rock off and added good ducting here.   The cost to do this is about $2000 to $2500 depending on what we have to do.   I had already worked on the return system that helped the air flow as well.

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Terry

Professional Engineer with 30+ years experience

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