Properly Sized HVAC: Michele’s Fabulous New Duct System

Part 3 of (Still) A Lot More Parts

Most people only get intimate with their attic spaces about once a year – during the holidays to drag out the decorations. At best we give these spaces a cursory glance as we brave the chill and dust just long enough to remove (or replace) boxes of lights, wreaths, stockings, etc.

Most of us accept the fact that our attics are a mess because we spend so little time there. But a messy attic is one thing; a tangle of ductwork that is smashed, punctured, or sagging is another. This is not a mess we can afford to tolerate–for the ductwork in our homes is like the cardiovascular system in our bodies. It must be in good shape for our homes to perform as intended.

Case-in-point is my friend Michele’s house. I spoke in an earlier blog about the recent replacement of her HVAC system. As I alluded to before, she had a mess. Not only did she need a new heat pump, the distribution system was an ill-fitting mixed bag of components.

Here’s a quick overview:

  • Electric baseboards in each room provided heat. The baseboards successfully heated the bathrooms (where the doors could be kept closed) and less successfully heated the bedroom (where pets require a partially open door), and hardly heated any other rooms at all, either because they were turned off in unused areas or were insufficient to heat the room in which they were installed. Gas logs in the family room, therefore, were the primary source heat for the main living area.
  • A forced-air air conditioning system provided cooling to the house, at least where the ductwork (in the attic) wasn’t blocked off from certain rooms as it was to the laundry room and dining room. The latter is open to the kitchen and family room, so blocking the ductwork robbed a portion of cooling to all these rooms. It’s a little vague how or why this was the case as it had apparently been this way since they moved in the home.

(Did I mention she had a mess? It gets worse…)

  • It turns out that the duct system for Michele’s home was undersized. (Yes, ductwork should be sized just like the HVAC system. The difference is that the HVAC system should be sized according to the heat loss and heat gain characteristics of the house. The duct system should be sized according to the size of the HVAC system and the size and requirements of the rooms that it is serving.)

So, again speaking in terms of the human body, Michele’s house had the arteries of a child serving the body of a full grown adult. Basically, the undersized duct created a bottleneck between the cooling system and the rooms in her home. Not only does this interfere with conditioned air distribution, it drives up energy cost because the fan inside the central air handler has to work so hard to drive air through those tiny arteries… uh, ducts.

  • Then there were the leaks in the return air duct, which was basically sucking air out of the attic and putting it into the conditioned supply air. Such leaks add to a home’s heating and cooling requirement and consequently, energy costs.

Neither Michele nor her husband was aware of most of these oddities—but then again, how many homeowners would be? That is why it is critically important to choose a contractor who will properly size your HVAC system and is also conscientious about duct design. How can you be sure of your contractor? Tell him you want a copy of the Manual J load calculation. And if you really want to show him who he (or she) is dealing with, tell him you want a copy of the Manual D, a calculation HVAC contractors use to size duct.

Have You Looked at Your Duct Lately?
Michele’s new contractor did everything right, and after seeing her newly installed duct (which now provides both heating and cooling to her living spaces) I officially have duct envy. The contractor did a beautiful job installing a long trunk of 26-inch rectangular sheet metal duct, insulated with 3 inches of fiberglass duct wrap. This trunk line is installed in the attic where the previous duct system was, and runs the length of the house, with relatively short sections of flexible duct branching out from it to individual room vents.

The nice thing about having a main sheet metal trunk, especially in a house that has a fairly long axis, is that it provides a nice, smooth, unobstructed vessel for airflow. The opposite of this would be what I have in my home, which is a web of flexible round duct run all over the place like a giant octopus. Unfortunately, this is how most residential duct systems are installed these days. It’s cheap, fast, and easy.

Since building my own home, I have talked with a few contractors who say they always install sheet metal duct in a trunk and branch formation. It’s a more expensive installation, but the airflow is better and sheet metal is less vulnerable to crimping or punctures than flexible duct. If you think about how we approach our attic spaces, with a “get in and get out quick” policy of shoving our junk into whatever free space exists, then you can appreciate how easily long, leggy strands of flexible duct gets crimped and punctured. If I had it to do all over again, I would insist on the sheet metal trunk and branch method.

I’m pleased to report that Michele and her husband are enjoying the newfound comfort of their new heat pump and central heating and cooling system. They are also eager to see what the next few months show in terms of savings. But it’s not all about the dollars.

“For me bottom line is even if we end up paying the same amount to Duke Energy as we were before, at least we’ve fixed the ducting and we are finally warm in the whole house,” said Michele.

Now there’s a message for all the contractors out there who just think homeowners want down and dirty cheap.

1 Responses »

  1. Nice article, Trish. So many people in the HVAC industry want to focus only on the box, the heating and cooling equipment and ignore the distribution system that it’s created a huge opportunity for good contractors out there who want to go in and fix all the crappy ductwork in people’s homes. And yes, a good sheet metal trunk and branch system is usually an indication of a better system.

    I’m sure you know this already, but for your readers who don’t, the full HVAC design process has four manuals: J, S, T, and D ( These are protocols created by the Air Conditioning Contractors of America (ACCA), and you can’t do Manual D until you’ve gone through J, S, and T. Manual S, equipment selection, can be the most difficult and time consuming step because the designer has to try to match the loads and that requires going into the performance data of the equipment options. They also have to make numerous decisions – heat pump or furnace & AC; ductless mini-splits or conventional systems; ground-source or air-source heat pumps…

    We still have a long way to go, but some companies are catching on, though (

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