What sparked the conversation?

A recent LinkedIn post by Kenneth Shultz, PE, lit up the MEP community. He shared photos from a project where the contractor used flexible ductwork in place of rigid metal duct to shave $3,000 off the budget. The flex duct was snaked through tight joist webs—but the bends were so severe, some air registers were barely getting airflow. To make matters worse, the contractor had removed Testing, Adjusting, and Balancing (TAB) from the project scope. That meant no one would know if each room was getting the proper airflow—until tenants started complaining.

That post hit a nerve. Engineers, installers, and commissioning pros flooded the comments with war stories. The theme was clear: when misused, flexible ductwork can quietly sabotage your HVAC system—and skipping TAB removes your safety net.

"That $3K savings just became a $15K+ tear-out special."
— Kenneth Shultz, PE

What Is Flexible Duct and Why Is It Controversial?

Flexible ductwork is a factory-made, bendable duct often connected to a rigid metal trunk to a ceiling diffuser or grille. It’s cheap, flexible, and easy to install—especially in tight spaces. But it comes with severe limitations:

📏 1. Keep It Short

Most codes and standards cap flex duct runs at 5 to 6 feet.

• Chicago Mechanical Code: “Shall not exceed 6 feet.”

• University of Washington Design Standards: Max 6 feet and only one elbow.

• You’ll see this in any halfway decent set of HVAC drawings.

🧱 2. Support It Properly

The American Society of Home Inspectors recommends:

• Horizontal supports every 5 feet

• Vertical supports every 6 feet

• Limit sag to ½ inch per foot

➰ 3. Avoid Compression and Kinks

According to installation manuals:

• Duct must be fully extended, not bunched or kinked.

• Each 90° bend acts like 20 feet of extra duct.

• A 180° bend can act like 40 feet of duct.

The Cost of Cutting Corners

That $3,000 in material savings can lead to:

• 🔻 Reduced airflow (rooms don’t heat/cool properly)

• ⚠️ Hidden performance issues (only show up after drywall is installed)

• 💸 Expensive retrofits (ripping out ceilings to fix airflow problems)

• 🔍 Poor air quality (flex ducts are harder to clean)

• 🔌 Higher energy bills (fans work overtime to compensate)

“We won’t know if these diffusers are getting 5 CFM instead of 150 until someone complains their room feels like a sauna.”
— Kenneth Shultz, PE’s

Why TAB (Testing, Adjusting, and Balancing) Matters

TAB ensures that every room gets the right amount of airflow. It’s a critical quality control step before ceilings are closed up.

Without TAB:

• There’s no proof that the HVAC system works as designed.

• You can’t catch imbalances or airflow issues early.

• Occupants are the first “testers”—usually by complaining.

“TAB isn’t a luxury—it’s insurance against invisible failures.”

Flex Duct: The Do’s and Don’ts

Length: Keep it under 6 feet—one bend max.

Support: Hang every 5 feet horizontal, 6 feet vertical. Limit sag.

Extension: Pull fully taut—no bunching or compression.

Bend Radius: Keep bends wide—at least 1 duct diameter.

Use Case: Use only as a short connector between rigid duct and diffuser—not the whole system!

What the Experts Are Saying

Here’s what MEP pros in the LinkedIn thread had to say:

🔧 “Flex isn’t the villain—misuse is.”

“It’s fine in the right hands, used the right way.” – John Chavez

Flexible ducts can work well if you follow the code, pull them taut, support them properly, and keep them short.

💰 “Low-bid culture leads to bad work.”

“Piece-rate installers get paid by the foot—not by quality.” – Darrell Hayes

Contractors often use flex duct because it’s cheap and fast. But poor installation leads to expensive callbacks—and the building owner pays the price.

🔍 “Commissioning and TAB need teeth.”

“Too many TAB reports skip key info like motor speeds and temperatures.” – Brad Miccio

Real commissioning means qualified professionals, robust reports, and active involvement from owners. It’s not a box to check—it’s how you prevent disaster.

🧠 “Tribal knowledge is gold—share it.”

“There’s more wisdom in comment sections than in most manuals.” – Nick Heim

The conversation inspired Kenneth Shultz to consider turning the best advice into a one-pager for clients. Every mistake shared can prevent future ones.

🏗️ “Better design avoids the problem.”

“Bulkheads and dropped ceilings are better than flex duct mazes.” – Kenneth Shultz, PE

Too often, architectural designs leave no room for proper ductwork. The fix? Coordinate early, plan for rigid ducts, and design for maintenance and performance—not just aesthetics.

Lessons Learned

• Flex duct can work—but only if installed correctly, and in short runs.

• Cutting corners on material and skipping TAB usually leads to higher costs later.

• Commissioning is critical. Don’t skip it.

• Good MEP design requires space, planning, and oversight.

MEP Tip to Live By:

“Cheap decisions are rarely free. Plan it right, or pay to fix it later.”

Final Thoughts

This article was adapted from a LinkedIn discussion led by Kenneth Shultz, PE. It combines code requirements, professional insights, and real-world experience to explain why flexible duct must be used carefully—and why commissioning and TAB are non-negotiable for quality HVAC systems.

For MEP for Dummies, it’s not just about ductwork—it’s about the value of thoughtful design, teamwork, and standing firm against the "race to the bottom."

In a recent LinkedIn post that garnered over 384,000 impressions, mechanical engineer Kenneth Shultz, PE, founder of PermitZIP, laid bare the hidden costs of “value engineering” HVAC systems. Facing a tepid system that delivered 60°F air at the registers, dripping condensation, and primed for mold growth, Shultz’s message was simple but powerful:

“This isn’t a design issue. It’s what happens when people pretend HVAC can be value‑engineered without consequence. Value engineering comes with a cost—this is just the part they don’t tell you when they promise to shave $100K off your HVAC budget.”

Tagged #DoNotValueEngineerHVACIfYouCantExplainItOnAPsychChart, Shultz’s post sparked 97 comments from industry professionals—and the consensus was clear: cutting HVAC features to save on upfront costs almost always backfires.

1. “Devalue Engineering,” Not Value Engineering

Wade Gauldin, HVAC Enthusiast, succinctly reframed the problem:

“I’ve always referred to it as devalue engineering. There is never value added, only value taken away. The end user is the one who has to live with these decisions…”

Shultz agreed, noting that asking proposers to share psychrometric studies often reveals whether they truly understand the system they’re cutting.

2. True VE vs. Scope Reduction

Several commenters pointed out that what passes for “Value Engineering” in practice is often just scope reduction dressed up in fancy terminology:

• Ed Trotter, OHSU Construction Project Manager, warned:

“Don’t confuse Value Engineering with scope reductions!!! True value engineering is a very good process. Scope reduction called VE is just a lie!”

• Rose Kennedy, Structural Engineer, added that cutting corners isn’t engineering at all:

“If doing something to ‘save money’ causes a failure, that is not ‘value engineering’ that is simply cutting corners.”

Shultz’s riposte? Encourage clients to demand psych charts before approving any VE change—saving millions by spotting charlatans early.

3. The Hidden Health Hazards

The post’s photo—registers sweating with 59.1°F air—drove home the risk of mold, bacterial growth, and indoor‑air‑quality disasters.

• Yunier Tarragona, Energy Efficiency Manager, emphasized the mold risk:

“Delivering 59°F air without controlling dew point… creates a breeding ground for mold, especially in drop ceilings and ductwork.”

• John Sweeney, Electrical Manager, recounted a retrofit horror story: oversized cooling without dehumidification led to 85%+ humidity, requiring electric heaters to re‑warm chilled air.

4. When VE Works—And Why That’s Rare

Not everyone dismissed VE outright. George Case, P.E. (AECOM Hunt) argued that, done properly, VE can save both first costs and life‑cycle expenses—if one has the expertise to evaluate equipment performance beyond “recycled specifications.” Shultz concurred:

“Done right, with a real understanding of system behavior, VE has merit. But too often, it means cutting features without grasping the consequences, usually by people who can’t justify their choices on a psych chart.”

He even challenged the community to share their examples of successful HVAC VE—citing ideas like bipolar ionizers, operable windows for natural ventilation, and demand‑control ventilation.

5. Practical Takeaways for Owners and Engineers

From this lively discussion, several best practices emerge:

1. Begin with Programming, Not Components

   • If budgets tighten, adjust usage profiles or square footage before hacking off HVAC features.

2. Demand Psychrometric Analysis

   • Require proposers to present a cooling/dehumidification study. No chart = no change.

3. Beware the “Invisible” Scope

   • HVAC is hidden, perfect for unseen cuts. Hold all stakeholders accountable for system performance throughout the warranty period.

4. Choose VE Allies Wisely

   • Work with engineers who take ownership from design through commissioning. A few extra dollars up front can save tens or hundreds of thousands in callbacks, tenant complaints, or worst of all, mold remediation.

Conclusion & Social Proof
Ultimately, the LinkedIn thread underscores a simple truth: you get what you pay for. Short‑term savings on HVAC almost always shift costs downstream—into mold‑related health risks, poor occupant comfort, expensive fixes, and reputational headaches.

“HVAC is almost always the first thing to get cut, yet it’s also the first thing people complain about,” sums up Shultz.

When you’re weighing system options with clients, point them to this discussion (🔗 Read Kenneth Shultz’s original post and comments). The collective voices of engineers, contractors, and industry experts offer powerful social proof: smart VE isn’t about shaving cost at the expense of comfort and health—it’s about optimizing value through rigorous analysis and collaboration.

Next time you face pushback on the HVAC budget, share these insights. Your clients will thank you for steering them away from “de‑value engineering” and toward sustainable, high‑performance solutions.

Site Unseen

Tips and tricks in construction, live from the field.

Welcome to Site Unseen, a newsletter series of short clips recorded live from the field by PermitZIP engineers. In each one-take video, experts identify and discuss various building components, offering insights into construction engineering without needing an engineering background.

Do you have a building component you’d like us to evaluate? Send us photos, and we'll find similar examples locally to film and share our thoughts.

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