In the good old days HVAC duct work was small, metal and did not efficiently carry air because old, metal HVAC ducts are famously inefficient and leaky.
Because there are many joints, elbows and unions that either were left with gaps, or the gaps and holes developed over time.
Additionally, they ran the ducts inside walls and ceilings. Heated air escaped at every opportunity!
The ducts were not insulated and so they would emit heat everywhere, in addition to the heated air they blew into the house.
As time passes these same ducts would be used for air conditioning.
This was much less efficient still because air conditioning relies more on returning air than blowing lots of air out.
The returned air was not as much in volume as would be needed to efficiently cool.
That is why when newer, larger air conditioning units, and particularly heat pumps, are installed in older houses with metal ducts they are simply not as efficient as with the newer, larger, more insulated ducts and returns.
How did they solve this leaking problem?
Typically with the so-called "duct tape."
DUCT TAPE IS A MISNOMER! IT WAS ORIGINALLY CALLED "DUCK" TAPE AND DESIGNED FOR OTHER APPLICATIONS. ONE OF THESE APPLICATIONS WAS TO SEAL BULLET HOLES AND WRAP AMMUNITION CASES ON THE "DUCKS" (THE AMPHIBIOUS PERSONNEL LANDING CRAFTS) THAT ARE SO FAMOUSLY SEEN ON D-DAY FILMSTRIPS.
At some point the tape became "duct" tape and its mission to fix everything was begun! It is used now for everything! You name it - HVAC, plumbing, electricity, roofs, fences, gutters, downspouts, household repairs - well you name it, everything!
But how inefficient and leaky are these metal ducts?
In these thermal images warmer temperatures are demonstrated by orange and yellow.
The far right shows two ducts inside the living room walls to above separate and pass through the ceiling to service two third-level bedrooms.
The center image shows one of those ceiling ducts.
And the left image is the end of the line, where the duct is converted to blow into floor register.
They are leaky! Imagine those ducts passing air 30' or 40' or 50' or 60' from the furnace! That's a lot of heat wasted inside wall and ceiling cavities! The same would go for air conditioning. It is not efficient.
Air is escaping at joints and seams. Heat is escaping throughout the run of the duct.
The insides of the walls and ceilings are very comfy!
Today those seams and joints are better sealed. They tape them with aluminum tape, which is more sticky and lasts longer than the old duct tape. And many areas are painted with a latex paint, which seals everything nicely and permanently.
Inside walls and ceilings and attic spaces duct work is better insulated. Sure, they are glorified slinkies with plastic wrap and insulation all around.
BUT THEY AREN'T BARE METAL!
My recommendation: sometimes the good old days aren't as good as imagined. Especially with heating and air conditioning! Dramatically better efficiency is achieved nowadays! While today is not perfect, the industry uses a much Better Practice than in former times. And the insides of walls, ceilings and attic spaces are not so comfy!
Jay Markanich Real Estate Inspections, LLC
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560