When it comes to HVAC performance, guesswork isn’t just risky; it’s costly.
I recently had the opportunity to dive deep into a real-world HVAC mystery where 23 oversized systems caused catastrophic humidity issues in a 100,000-square-foot coastal facility. This wasn’t just another “slightly-off” install. We’re talking 90 percent humidity, water dripping from steel walls, and fine mist blowing from supply vents. You could feel it in your glasses; literally.
Let’s walk you through how my team and I tackled the issue using two of the most powerful tools in our belt: Manual J load calculations and the Digital TrueFlow® Grid.
By the end, we didn’t just dry out the building. We turned chaos into a model of how measurement and design should work together.
And by the way, if you want to watch this exact webinar replay, you can catch it now inside The Future of HVAC, our free digital community packed with expert training, Q&A, and support for HVAC professionals.
The Problem: Oversizing and Humidity Gone Wild
We were called out to a job with 23 four-ton heat pumps cooling a three-story storage facility near the Gulf Coast in Texas. At first glance, it seemed like a classic case of oversizing.
Humidity was through the roof; literally. Water was dripping off walls and ceilings. Units were spitting mist from the vents. You couldn’t stand 25 feet away without getting droplets on your glasses. Our initial thought? Shut off half the systems.
But the more we tested, the more it became clear: this wasn’t just oversizing. This was a layered disaster.
Step-by-Step Diagnostics: How We Broke It Down
Here’s how we approached the problem and what we found.
1. We started with a Manual J load calculation
Before we stepped foot on-site, we ran the numbers. Our go-to process is always equipment sizing first. The results confirmed what we suspected; the systems were 2x to 3x oversized for the space. That was our first clue.
2. We tested with the Digital TrueFlow® Grid
We swapped the filter with the Digital TrueFlow® Grid and immediately got a full picture of what was really happening. Despite identical systems, we were seeing wildly different airflow; ranging from 1040 CFM to over 2200 CFM. That’s not normal.
Fan curves gave us high readings, but the Digital TrueFlow® Grid told the truth: these units were underperforming. I even doubted the device at first (don’t we all?), but with calibration confirmed, the data didn’t lie.
3. The duct design was a joke
These systems had stubby little plenums that reminded me of Winston Churchill’s cigar; short, thick, and ineffective. There were no real branch ducts. Air was shooting straight out of the end, causing water to blow right off the coils. There was no way to balance or distribute the air properly.
4. Filtration was killing airflow
Each return had a steep-angle, four-inch filter. The pressure drop was off the charts. Combine that with inconsistent fan speeds and we had a recipe for airflow failure.
5. The condensate wasn’t draining
The pans were tilted the wrong way. The traps were the shallow kind you pick up at the hardware store. Water stayed in the pans while the units ran and only drained when they shut off. Negative static pressure from the pull-through coils was making things worse.
6. Refrigerant charge was a mess
Superheat and subcooling were way off. Long line sets, missing traps, no seal caps; you name it, it was wrong. We had to evacuate all the units, measure every line set, weigh in the charge, and then add sight glasses just to stay sane.
Turns out, the units needed a field-adjusted TXV (Thermostatic Expansion Valve) setting. This critical step wasn’t in the original install manual. Without that tweak, we were never going to hit the right performance levels.
7. Blower door testing revealed airflow chaos
We ran two blower doors at full blast. The data showed 23,600 CFM at 50Pa, which was actually decent for a building this size; but the way the air was moving inside the structure told a different story.
At one point, we saw a blower door fan spin without power—just from reverse stack pressure. Over 1200 CFM was flowing down a stairwell. We had serious air migration, even with a decent envelope.
The Fix: Measurement Meets Strategy
After adjusting TXVs, optimizing airflow using the Digital TrueFlow® Grid, fixing ductwork and drainage, and dialing in refrigerant charges, we still had one more step: dehumidification.
We installed four Quest dehumidifiers, two on the bottom floor and two upstairs. The result? Humidity dropped from 90 percent to a comfortable 40 percent at 73 degrees. Problem solved.
Lessons Learned (That You Can Use Right Now)
This job reminded me of something I see all the time: modern tools paired with medieval HVAC strategies.
If you’re in the field, here’s what you need to remember:
- You must do airflow before charge. Every time.
- Fan curves are often inaccurate. Use the Digital TrueFlow® Grid to know your real CFM.
- Never trust that equipment is set up properly out of the box; especially TXVs.
- Poor duct design can sabotage even the best systems.
- Drainage problems can look like refrigerant issues. Test both.
- Blower door tests reveal the real airflow behavior of the building, not just the numbers.
Most importantly: without a Manual J and without real-time measurement tools, you’re flying blind.
Want Hands-On Training? Join Us in Texas This November
If you want to go deeper and master Manual J load calculations like a pro, come join us in person for our upcoming Mastering Manual J Software 3-Day Course in Pflugerville, Texas from November 4–6, 2025.
This live event is built for HVAC contractors, designers, and anyone looking to level up their comfort system performance skills with real-world applications and expert coaching.
You can register here and grab your seat before it fills.
Final Thoughts: Why Measurement and Design Should Never Be Separate
You can have the best HVAC gear money can buy, but if your ductwork is wrong, your airflow is off, or your system is oversized, it won’t matter.
What we proved on this job is that you need both precision design and field-proven diagnostics. One without the other is incomplete.
This experience solidified for me how vital tools like the Digital TrueFlow® Grid and Manual J load calculations are for solving real-world HVAC issues.
And if you’re still relying on guesses or “the way we’ve always done it,” it’s time to upgrade.