Case Study: 182% More Airflow After Sealing a Severely Leaky Duct System
If your vents barely push air, the cause may be leaks you can’t see. This case study covers one Bay Area system, job H-04, that delivered only 238 CFM (cubic feet per minute) to its registers before sealing. After Aeroseal, it delivered 670 CFM, a 182% gain and the largest in MaxSeal’s 38-system data. Here’s what the test showed, what we did, and what it means for your home.
Key Takeaways
| Common Question | Short Answer |
|---|---|
| How much did leakage drop? | From 485 to 53 CFM25, an 89% reduction. How to read the numbers |
| How much more air reached the rooms? | Register airflow rose from 238 to 670 CFM, an increase of 432 CFM or 182%. |
| Is 182% typical? | No. It’s the top of our range. Across 38 systems, the average gain was 47% and the median was 38%. See all results |
| How much less does the system need to run? | In theory about 64% less runtime to deliver the same air, based on 1 − 238 ÷ 670. That’s a calculation, not a measured savings. |
| What does a job like this cost? | MaxSeal’s Aeroseal duct sealing is $3,000–$3,500 per system. See our price list |
| Which homes are most likely to see big gains? | Systems with high starting leakage and weak airflow. Those starting at 450–650 CFM25 gained 64% on average. Get tested |
1. The Starting Point
The baseline test showed a system losing a large share of its air before it reached the rooms. Leakage was 485 CFM25, which means 485 cubic feet per minute of air escaping when the ducts are held at 25 Pascals of pressure. That puts it in the severe range on MaxSeal’s working guide.
| Measurement | Before sealing |
|---|---|
| Duct leakage | 485 CFM25 |
| Total register airflow | 238 CFM |
| MaxSeal leakage band | 400+ (severe) |
2. What the Test Showed
A total of 238 CFM across every register is very low for a whole-home system. In homes with numbers like these, the usual pattern is weak airflow at most registers, with rooms at the end of long runs getting the least. Rooms like that are often slow to cool in summer and slow to warm in winter.
The leakage reading explained why. Much of the air the system moved was escaping through joints, seams and connections before it reached the registers. Adjusting registers or adding equipment would not have recovered that air. Sealing the leaks was the direct fix.
3. What We Did
We followed our standard process, testing first, sealing the leaks from the inside, and testing again to document the result.
Our Typical Aeroseal Process
- Baseline test. Measure total leakage with a calibrated Retrotec fan and airflow at every register with a flow hood.
- Hand repair. Fix any large gaps by hand, since Aeroseal seals holes up to about 5/8 inch.
- Prepare the system. Block the registers and isolate the furnace and coil so sealant only builds at the leaks.
- Seal. Inject the Aeroseal sealant while a computer tracks leakage live.
- Final test and report. Retest leakage and airflow and provide a before and after report.
4. The Results
| Measurement | Before | After | Change |
|---|---|---|---|
| Duct leakage (CFM25) | 485 | 53 | −432 (89% reduction) |
| Register airflow (CFM) | 238 | 670 | +432 (+182%) |
| Theoretical runtime to deliver the same air | 100% | ~36% | ~64% less |
Leakage finished at 53 CFM25, just above our standard target of under 50. Airflow at the registers nearly tripled.
The runtime figure uses MaxSeal’s simple formula: 1 − (airflow before ÷ airflow after). Here that’s 1 − 238 ÷ 670, or about 64%. It shows how much less time the system would need to deliver the same amount of air, in theory. Real energy savings depend on your equipment, thermostat settings and weather, and results vary.
5. What This Means for Similar Homes
H-04 is the strongest result in MaxSeal’s data, so treat it as the top of the range. Here’s how it compares:
| H-04 | Systems starting at 450–650 CFM25 | All 38 systems | |
|---|---|---|---|
| Leakage reduction | 89% | 88% average | 86% average |
| Airflow gain | +182% | +64% average, +47% median | +47% average, +38% median |
| Airflow added | +432 CFM | +291 CFM average | +225 CFM average |
The leakage reduction was close to average. What made the airflow gain so large was the starting point. A system delivering very little air has the most room to improve. Another high-leakage system in our data, H-10, went from 259 to 635 CFM, a 145% gain. If your system leaks heavily and your vents feel weak, a duct leakage test will show where you stand.
Conclusion
Job H-04 started with 485 CFM25 of leakage and just 238 CFM reaching the registers. After sealing, leakage dropped 89% to 53 CFM25 and airflow rose to 670 CFM, a 182% gain.
Most homes won’t see a jump this large, but every one of the 38 systems in our data gained airflow. Testing first tells you how much your home stands to gain.
Get a Free Consultation or call (408) 400-3630.
Related: Aeroseal results study · Two-system home case study · Three-system home case study · Case studies