When you have an older furnace and duct system, the older metal ducts end up heating and cooling everything on the way to the room you want heated or cooled.

Metal ducts are very leaky.  There are lots of seams and bends, corners and connections.  Each of those locations leak air, both heating and air conditioning.

Metal ducts are more efficient in that air flows more easily.  There is less corrugated surface area to impede air flow.

But they have to be treated or taped so as not to leak.

Modern building techniques paint metal duct joints with a latex slop to seal them off.  It works great!  And ducts in environmentally-vulnerable areas like attics and crawl spaces are insulated to not bleed off their conditioned air.

But not in the olden days!

I had a client who wanted to buy an older house.

As we spoke on the phone he was saying that one reason he likes the older home is because the HVAC air blows more efficiently in the metal ducts.

I said there are two ways that might not be true:

1.  For the reasons explained above, and I said that I could demonstrate it with a thermal camera.

2. Older duct work can be too small and confining for newer HVAC systems in that they want to draw in and blow out more air.  Particularly heat pumps.  The smaller, fixed ducts can create a bit of a blockage, which affects the efficiency of the system over time.

That number 2 can be significant!  It can wear down a heat pump, and kill it more quickly.

But, I walked around with my client and we could literally follow the ducts as they left the unit, went under the floor or up the walls, and then across a ceiling to service a bedroom two floors away.

Some of those ducts were 40', 50' and 60' long!  That is a lot of lost efficiency INSIDE THE FLOORS AND WALLS!

In this image you can clearly see two ducts that are passing up a wall.  In this case it's the living room wall, beside the fireplace. 

The orange and yellow areas show warmth!  You can see that even the floor molding at the bottom of each is getting warm!

The drywall at the warmest spot is 125F!  At this point the ducts are only about 10' from the unit.  From here they go up, turn left and right, and, passing above the main-level ceilings service a bedroom in the front and in the rear of the house.  Each of these is about 40' from the furnace to the bedroom floor.  Those walls will similarly cool down during the AC season.

My client was amazed.  Mighty Mo is amazing!  And yes, Mo was kind enough to snap a thermal image of the client for his scrapbook.  Using a thermal camera to teach is always a BEST PRACTICE!

My recommendation:  there are plusses and minuses to old and new construction.  Some things in older construction have to be lived with however.  The fix for this particular issue would be difficult at best, and expensive, if it is even possible!  The break even on this "remodel" might never be realized!