HVAC ductwork: Set it and forget it, right?

Wrong, say growing numbers of technicians and HVAC educators who emphasize that calculated, deliberate ductwork design and construction are major factors in how effectively and efficiently a forced-air HVAC system works.

“The job of the duct system is to deliver the heated air and the cold air from the equipment into the living space, and there shouldn’t be anything that’s lost in between,” said David Richardson, a trainer and the curriculum developer at the National Comfort Institute (NCI). “The duct system determines how efficiently the equipment operates, more than anything else.”

Good residential ductwork, say Richardson and others, starts with careful thermal load calculations performed according to the procedures described in ACCA’s Manual J (the “J” is for joules, a measurement of energy).

“If people want premium results, they have to do the math upfront,” said Ed Janowiak, manager of HVAC design education at ACCA.

The guidelines in three other ACCA publications, experts say, should also be followed in connection with building out a system: Manual S (equipment sizing), Manual D (ductwork design), and Manual T (for “termination”), which contains formulas that help determine the types, sizes, and placement of the grilles, diffusers, or registers through which heated and cooled air flows into various rooms and return air travels back through the system. Janowiak compared a grille, register, or diffuser to having one’s thumb on the end of a garden hose: They control the direction and velocity of the air flow.

“When you have a process to follow, own the correct tools, and you have good training for what and why things happen in an HVAC system, no problem is unsolvable,” said John Boylan, general manager at Lakeside Service Co. Inc., an HVAC and plumbing business in Brighton, Michigan.


Don’t Guess

Testing, experts said, is vital. Static pressure testing, which measures the resistance heated or cooled air faces when traveling through a system, will help determine whether ductwork is properly sized, and should be as routine during service calls and system checkups as having one’s blood pressure taken during a visit to the doctor.

“If you don’t test, you’re just guessing,” said Richardson.

Richardson and Boylan say most existing duct systems are improperly sized; at Lakeside, Boylan said, static pressure tests are a regular part of service calls, and techs find that close to 80% of residential ductwork is too small.

“Usually the low-bidding contractor is the one who’s done the duct system, and it’s rarely right,” Richardson said.

An undersized duct system might mean, for example, that only 50,000 Btu are reaching the conditioned space in a furnace rated at 100,000 Btu, Richardson said.

“When the duct system’s undersized, you can’t get that air where you want it in the house,” Boylan said.

Static pressure testing can also other problems, such as a dirty coil or debris in the ductwork.

“Anything that can get in the way of that air moving ... that’s what’s contributing to external static pressure,” Richardson said.


Redoing Ductwork

But even the best duct system may need to be redesigned and modified when new equipment, like central a/c or a heat pump, is introduced.

Experts recommended the following steps in such cases: 1) Perform load calculations for each room. 2) Map the duct system for a simplified duct analysis. 3) Verify the size of the existing furnace fan and take static pressure readings. 4) Make recommendations for a redesigned duct system design based on Manual D guidelines.

“Typical heating-only systems aren’t adequate in size to move the air from an air conditioner or heat pump,” said Janowiak.

Similar steps should be taken, Boylan said, when a home is enlarged or a formerly unconditioned space, like an attic, is being finished for residents’ use and needs to be heated and cooled. Load calculations should be made for the new space as well as the existing space, said Janowiak.

“If you figure that 80% of residential duct systems are undersized, the existing duct system should almost never be ‘tapped’ off of,” said Boylan.


Detail Work

Richardson and Boylan made several other points regarding duct system design and construction, including:

  • The spaces in which ducts are installed can be cramped and inadequate, and installers sometimes will have to work around that. “Sometimes, the house, or the building, it’s going to beat you, and all you can do is the best you can with what you have,” Richardson said.
  • Climate and the ductwork’s immediate environment are going to affect its lifespan. Sheet-metal ducts in overheated attics, for example, are likely to deteriorate faster than those in cooler surroundings, Richardson said.
  • While testing often finds that ductwork is undersized for the home it’s in, techs often discover that the HVAC equipment is oversized, said Boylan.
  • Ductwork designers and installers should consider the type of duct material being used, and design and install accordingly. Flexible ducts, for example, should not exceed a maximum allowable proportion of sag per line foot, as too much sag will create excess static pressure, Richardson said. “Flexible duct is much easier to run, but it’s also much easier to install incorrectly without some attention to detail,” he said.
  • Properly designed and installed ductwork can result in considerable energy savings. “Fixing the duct system can sometimes pay for itself in less than a year,” Richardson said.
  • Good information about duct design has been easier to come by in the last decade or so, and more HVAC professionals are paying attention to ductwork.

“I’m happy that the industry is looking at this stuff,” said Boylan. “It’s important that we all learn not just how to change boxes out, but how to test (and) find solutions.”