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When Exhaust Fans Backfire

Adam Mufich ·
It's pressure, not equipment: why your registers sweat

By Adam Mufich, National Comfort Institute (NCI)

In the South, sweating ducts and registers are routine. In northern climates they are rare enough to catch a technician off guard. When condensation shows up, dew point is only half the story. The other half is what moves humid air toward the cold surface.

HVAC contractors in the South are familiar with sweating ducts and registers. In northern climates, condensation is less common, so it can catch technicians by surprise. When it happens, understanding dew point is only the beginning. You also need to understand what moves the humid air toward the cold surface.

When Air Gets Angry

On a previous job, a whole-house fan caused a natural-draft boiler to spill flue gases. That experience introduced me to the concept of MAD AIR: Mechanical Air Distribution and Interacting Relationships. The concept describes how mechanical airflow changes building pressures and creates unintended consequences.

Bath Fans for Dehumidification

Last summer, a homeowner called about sweating supply registers in her bathrooms after the family showered.

I recommended running the bath fans while showering and for 10 to 15 minutes afterward. Three weeks later, she called in a panic because the indoor relative humidity had climbed above 70%. She had been running all five bath fans and a large kitchen exhaust continuously to reduce the condensation. She heard my recommendation and thought that if running the bath fans reduced the sweating registers, then running more fans for longer should work even better.

The fans exhausted a large volume of air from the house. That depressurized the house and caused fan-induced infiltration. The fans pulled hot, humid outdoor air inside and raised the indoor relative humidity.

In this case, it was not an HVAC issue causing the registers to sweat. The problem was a double-edged sword. The homeowner went from not using the bath fans at all to running them constantly. Both extremes can cause moisture problems.

Lights Aren't Supposed to Drip

A few weeks back, I got a phone call from a longtime customer. He lives in a high-performance home where we installed a ground-source heat pump, energy recovery ventilators (ERVs), and a duct system. He said water was dripping out of a can light in his hallway.

The can light is on the second floor of a three-floor home. There is no roof above the light and no plumbing nearby. Directly above it is an uninsulated metal duct. This is common practice in the Midwest when ducts run inside the conditioned space.

At the house, I deployed my Fieldpiece psychrometers throughout the home and mapped them by floor in the measureQuick Probe Manager. It was important to measure temperature, relative humidity, and dew point on each floor.

measureQuick Probe Manager showing Fieldpiece psychrometers mapped by floor
Photo 1: Fieldpiece psychrometers deployed and mapped by floor in the measureQuick Probe Manager

I did not see any immediate red flags, so I drilled a hole in a supply boot that could be a culprit. I inserted my psychrometer into the hole and measured the air between the second and third floors. The space read 67.3°F / 69.8% RH with a 57°F dew point. That means any surface at 57°F or colder will become wet and can drip.

Where was the humid air coming from? This house is over 5,000 square feet with a 500 CFM50 blower door number. It is a tight building, but not 100% airtight. Why was this happening after years of the system running well?

Measuring Reverse Stack

Most of us know hot air rises. As HVAC professionals, it is easy to assume that is why the upper floors of a home are warmer than the lower floors. But if we are running air conditioning, where is that hot air coming from?

During cooling season, the location of air leaking into a home changes because of reverse stack effect. Cool, dense indoor air falls to the lowest level of the home and creates a positive pressure there. If there are any holes low in the home, that positive pressure pushes air out.

When that happens, the upper levels become depressurized and pull hot, humid outdoor air inside. The temperature difference between the house and the outside drives the pressure difference, which drives the airflow from outside.

I measured house pressure with reference to (WRT) outside through a third-floor window and found the top of the house at -2.8 Pascals (Pa). Next, I measured house pressure WRT outside on the first floor, at the base of the front door. The pressure bounced between -1.3 Pa and -2.2 Pa. That was a red flag.

Digital manometer at a third-floor window reading -2.8 Pa house pressure with reference to outside
Photo 2: Third-floor house pressure WRT outside, reading -2.8 Pa
Digital manometer on the first floor reading negative house pressure with reference to outside
Photo 3: First-floor house pressure WRT outside

Knowing how reverse stack effect works, I expected the first floor to be positive. Why was it negative?

Fans Can Have an Impact

I had to investigate the depressurization. This house did not have bath fans; it used two ERVs for all bathroom exhaust. I went outside to inspect the ERVs' concentric vent kits and found the bug screens on both intakes blocked. The ERVs were exhausting air from the home but could not bring in outside air.

Concentric vent kit for the first-floor ERV with its intake bug screen clogged with debris
Photo 4: First-floor clogged ERV intake
Concentric vent kit for the second- and third-floor ERV with its intake bug screen clogged
Photo 5: Second and third floor clogged ERV intake

After cleaning both intakes and restoring airflow to the ERVs, I remeasured first-floor house pressure WRT outside. It was now +2 Pa, much closer to what you should expect during cooling season.

The ERVs had been pulling hot, humid air through holes in the house that connected to the cavity where the duct ran. I did not fix the holes; I fixed the pressure problem, which reduced the air movement through them.

Hindsight in 2026

Looking back, I would have insulated the ducts. More importantly, 10 years ago I might have treated the wet duct as the problem instead of measuring the pressure conditions that moved humid air toward it. A deeper understanding of pressurization and depressurization, and how they affect relative humidity, lets you diagnose the actual problem instead of treating the symptom.

The house is part of the HVAC system; the two go hand in hand. When you focus on one over the other, it is easy to miss how they interact, and that can lead you to misdiagnose the issue.

About the Author

Adam Mufich is an instructor, a member of the curriculum development team, and a podcast host for National Comfort Institute (NCI). Adam owned and operated a residential HVAC company in the Chicago suburbs that specialized in High-Performance HVAC™ contracting. If you're an HVAC contractor or technician interested in learning more about NCI's training, contact Adam here. NCI's website is full of free information to help you improve your professionalism and strengthen your company. Visit HVAC Today to read more articles like this or listen to the High-Performance HVAC Podcast.

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