Duct Systems
When the building and HVAC industry looks at a high performance comfort system, the last piece of the puzzle that gets considered is the duct system. It should be the first. Unfortunately, too often the first thing that gets considered is the equipment (like the furnace or the AC unit), and frankly it should be the last.
Why is this the case? Well, it has to do with economics and investors. There’s simply much more money in selling HVAC equipment that there is in sheet metal. The HVAC manufacturers therefore need to go to market to guess who… the HVAC contractors. It’s not a bad thing, but most contractors are somehow led to believe the furnace they ally themselves with is oh-so important.
Our goal isn’t to minimize the importance of a good heating or cooling appliance but it is to stress how much more important a good duct system is.
The problem is most duct systems are one or more of the following: undersized, bad design (not streamlined), leaky, and lack insulation in non-conditioned areas. Lets go through the items one by one:
Undersized: Every duct installed on the planet owns what’s called a “Duct-U-Later” (sounds like calculator). Just about all of those installers understand what ‘should’ happen, but unfortunately theory and reality often don’t see eye to eye. Ducts are sized one of two ways: feet per second, (that is how fast the air travels), or the second is known as “friction rate”, which gets converted to static pressure measured in inches of water column. I realize these technical terms may sound scary. However, scary is a fitting expression for what we see out there. If the ducts are too small, it causes the blower motor to deliver less air. Less air means less energy transfer. Often times well purposely down-size the furnace to A: better meet the match between small duct system and new smaller furnace, and B: many of the now-retired heating installers for whatever reason installed rather huge capacity heating appliances back in the mid-part of the last century (well, we still see oversized equipment today, but that’s another page, click over to Manual J).
Bad Design: It’s true, there are duct systems that are created to be mechanical works of art, and duct systems that can be improved upon by even the most novice technicians. Here is the problem that is tough to get around: it doesn’t take a rocket scientist to put metal together. It’s pretty simple. There is certainly more experience for the fabrication of sheet metal. The crux of the matter is metal installation isn’t generally learned in the class room. It’s learned by some mentor-figure like in most every other building trade profession. To make a comparison to another industry, before smog emission equipment was required for automobiles, auto mechanics was generally learned by simply watching and doing and yelling across the garage when there was a problem. Today is different, cars are like rolling computers, and most every top-tech is certified by the ASE. This only means he or she has learned more than a non-certified technician. Pretty basic concepts in place here. The analogy is simply this: If the mentor-figure didn’t know some basic engineering skills, the ESP, or external static pressure, can be quite easily be way out of line. At the end of the day, the bottom line is the external static pressure must be within industry limits, usually .5 WC for furnaces and .3 for air handlers (please check with the manufacturers technical literature from your specific appliance). I’ve known self-proclaimed duct experts that owned the tool required to check ESP, but did he ever use it? No. But when checked with a manometer, the static was well above recommended standards.
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