Sweating on one cold duct run
Beads of water or damp insulation on the fresh-air or exhaust duct closest to the HRV, usually worse in humid weather.
Start here: Start with insulation condition, loose outer wrap, and air leaks at joints and collars.
HRV / ERV Condensation Problem
HRV Duct Sweating
Direct answer: HRV duct sweating usually means warm humid air is hitting a cold duct surface, or cold outdoor air is leaking where it should not. The most common fixes are restoring airflow, replacing dirty HRV filters, correcting loose or missing duct insulation, and clearing a blocked condensate drain if the unit is producing water internally.
Most likely: Start with the easy stuff: check whether the sweating is on the cold fresh-air duct, around the HRV cabinet, or on nearby house ducts. Dirty filters, low airflow, disconnected insulation, and air leaks around the unit are more common than a failed core or motor.
Look at where the moisture is showing up first. A little surface sweating on one cold section of duct points you one way. Water dripping from the cabinet, frost inside the unit, or widespread wet insulation points you another way. Reality check: some light sweating during very humid weather can happen on poorly insulated cold duct runs, but steady dripping means something needs attention. Common wrong move: wrapping over soaked insulation without fixing the air leak or airflow problem underneath.
Don’t start with: Do not start by replacing the HRV core, fan motor, or controls just because you see water. Most sweating problems come from airflow, insulation, or drainage, not a dead major component.
If the wet spot is mainly on insulated metal duct near the HRV,check for torn vapor barrier, loose seams, or missing insulation before assuming the unit is bad.
If water is coming from the HRV cabinet or drain area,check the condensate line, trap, and internal frost buildup before buying parts.
Last reviewed: 2026-04-06
What the sweating looks like matters
Water dripping from the HRV cabinet
Moisture is collecting under the unit or running from the bottom panel instead of just sweating on the duct.
Start here: Start with the condensate drain, trap, and any frost buildup inside the HRV.
Frost inside the unit, then water later
The HRV ices up in cold weather and then drips when it defrosts or warms up.
Start here: Start with filter condition, airflow restriction, and whether the defrost cycle seems to be working.
Nearby house ducts are wet too
Sweating is not limited to the HRV connections and may show up on other metal ducts in the same area.
Start here: Start with basement or mechanical-room humidity and whether this is really a whole-space moisture problem, not just an HRV problem.
Most likely causes
1. Damaged or missing insulation on cold HRV ductwork
Cold incoming-air duct surfaces will sweat fast when warm humid room air touches bare metal or wet, torn insulation.
Quick check: Look for exposed metal, split outer jacket, sagging insulation, or wet fiberglass near seams and elbows.
2. Dirty HRV filters or restricted airflow
Low airflow makes parts of the unit and ductwork run colder than normal, which encourages condensation and frost.
Quick check: Pull the HRV filters and check for dust loading, pet hair, or a matted surface that blocks air.
3. Condensate drain or trap problem at the HRV
If the unit is making normal condensate but cannot drain it, water shows up at the cabinet and nearby duct connections.
Quick check: Inspect the drain pan area, hose, and trap for slime, kinks, standing water, or a disconnected line.
4. Air leakage or high humidity around the unit
Even with a working HRV, humid basement or utility-room air leaking into cold duct joints can create sweating that looks like a unit failure.
Quick check: Feel for air movement at taped joints, collars, and access panels, and note whether the room itself feels damp or musty.
Step-by-step fix
Step 1: Pin down exactly where the moisture starts
You need to separate duct-surface sweating from cabinet drainage or frost problems before touching anything else.
- Turn the HRV off at its service switch or breaker before opening panels.
- Use a flashlight and trace the first wet point, not just the lowest drip point.
- Check whether the moisture is on the outside of a duct, inside the cabinet, at a duct joint, or under the unit.
- Press on the duct insulation jacket. If it feels soaked, look for a split seam or loose tape nearby.
- Look for frost, ice, or heavy water staining inside the HRV access area if you can open it safely.
Next move: If you can clearly tell whether this is outside-duct sweating, cabinet drainage, or internal frost, the next checks get much faster. If everything is wet and you cannot tell where it starts, dry what you can reach, run the unit briefly, and watch the first place moisture returns.
What to conclude: A single cold duct sweating points toward insulation or air leakage. Water starting inside the cabinet points toward drainage or airflow trouble. Frost points toward restricted airflow or a defrost issue.
Stop if:
- You see damaged wiring, burnt insulation, or signs of arcing inside the HRV.
- The unit is mounted where opening panels puts you on an unsafe ladder or cramped platform.
- There is enough leaking water to threaten ceilings, drywall, or electrical equipment below.
Step 2: Check the HRV filters and basic airflow first
Filters are the most common service item, and restricted airflow is a frequent reason ducts get colder than they should.
- Remove the HRV filters and inspect both sides under good light.
- If the filters are washable and the label or frame indicates cleaning is allowed, rinse with mild soap and water, then let them dry fully before reinstalling.
- If the filters are disposable or damaged, note the size and style for replacement rather than forcing them back in.
- Make sure supply and exhaust grilles in the house are not blocked by furniture, dust mats, or closed dampers.
- Restore power and listen for both HRV fans. A normal unit should not sound like one side is struggling or stalled.
Next move: If airflow improves and sweating drops off over the next day or two, the problem was likely restriction, not a failed major component. If filters are clean and both air paths still seem weak, move on to insulation, leaks, and drain checks.
What to conclude: Dirty HRV filters support the strongest DIY fix on this page. Weak airflow with clean filters raises the odds of a deeper fan, core, or control problem, which is usually a service call.
Stop if:
- A fan is humming, grinding, or not turning even with clean filters.
- The unit trips a breaker or loses power when restarted.
- You have to reach into moving fan sections or live wiring areas to continue.
Step 3: Inspect duct insulation and air leaks around the HRV connections
Most visible duct sweating is simply humid room air reaching a cold metal surface through bad insulation or leaky seams.
- With the unit off, inspect the first several feet of cold HRV duct on both sides of the cabinet.
- Look for missing insulation, torn vapor barrier, loose foil tape, open seams, or crushed insulation that no longer covers the metal evenly.
- Feel around collars and joints for gaps where room air can get in when the unit runs.
- Re-secure loose outer insulation and vapor barrier with HVAC foil tape if the insulation is dry and intact.
- If insulation is soaked, remove only the ruined outer wrap or wet section as needed so it can dry and be replaced properly instead of trapping moisture.
Next move: If the sweating was limited to one or two exposed spots and sealing them stops new moisture, you found the issue. If the duct is fully insulated and still sweating heavily, check for drain trouble, internal frost, or unusually high room humidity.
Stop if:
- The duct insulation appears to contain unknown older material you should not disturb.
- The duct joints are badly rusted, collapsing, or need major rebuild work.
- You find moldy, saturated insulation spread through a large concealed area.
Step 4: Check the condensate drain and look for internal frost
If water is coming from the cabinet, the unit may be making condensate normally but failing to drain it, or it may be icing up and shedding water later.
- Turn power off and inspect the HRV drain pan area if accessible.
- Follow the condensate hose from the HRV to its drain point and look for kinks, sags, slime, or a disconnected line.
- If there is a trap, make sure it is present and not clogged with debris.
- Clear a simple blockage by flushing the disconnected drain hose with warm water at a sink, then reconnect it securely.
- Look inside the unit for frost on the core area or around the air path. Heavy frost after clean filters usually means the problem is beyond a simple homeowner fix.
Next move: If the drain starts flowing and cabinet dripping stops, you likely solved the immediate water problem. If the drain is clear but frost keeps forming or water keeps returning, schedule service for airflow, defrost, or control diagnosis.
Stop if:
- You cannot access the drain or pan without removing major internal components.
- Ice is packed around fans or electrical sections.
- The drain line runs into a finished area where more testing could cause water damage.
Step 5: Decide between a simple maintenance fix and a service call
By now you should know whether this is a filter-and-insulation job or a deeper HRV problem that needs testing.
- If the only confirmed issue was dirty or damaged filters, replace the HRV filters with the same size and type.
- If the only confirmed issue was torn or missing duct insulation, repair or replace the affected insulated section and seal the outer vapor barrier well.
- If the drain was blocked, verify it now flows freely and that the cabinet stays dry during operation.
- If sweating continues with clean filters, intact insulation, and a clear drain, book HVAC service and report whether you found frost, weak fan sound, or cabinet water.
- Until the root cause is fixed, protect nearby finishes from drips and keep the area around the HRV dry and ventilated.
A good result: The duct should stay dry or nearly dry in normal operation, with no steady dripping from the cabinet or soaked insulation.
If not: Persistent sweating after these checks means the unit likely needs professional airflow, fan, or defrost diagnosis rather than more guesswork.
What to conclude: The supported homeowner fixes here are filter replacement, minor insulation sealing, and drain clearing. Ongoing frost, weak airflow, or repeat cabinet water usually means internal service is next.
Stop if:
- You are considering replacing the HRV core, fan motor, or control board based on moisture alone.
- The unit still ices up after basic maintenance.
- Water is affecting ceilings, framing, or electrical equipment and needs immediate containment.
Replacement Parts
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FAQ
Why is my HRV duct sweating in summer?
Usually because warm humid air in the basement or utility room is hitting a cold duct surface. Torn insulation, loose vapor barrier, or air leaks at duct joints are the first things to check.
Can a dirty HRV filter cause condensation?
Yes. A clogged HRV filter can reduce airflow enough to make parts of the unit and ductwork run colder, which encourages sweating and sometimes frost.
Is sweating ductwork always an HRV problem?
No. Sometimes the HRV is fine and the real issue is high room humidity, damaged duct insulation, or leaky duct seams. If nearby house ducts are sweating too, look at the space conditions as well.
Why is water dripping from the HRV cabinet instead of the duct?
That usually points more toward a condensate drain problem or frost melting inside the unit than simple outside-duct sweating. Check the drain hose, trap, and signs of internal ice.
Should I replace the HRV core if I see condensation?
Not based on condensation alone. The core is not the first suspect for this symptom. Filters, airflow, insulation, air leaks, and the drain are much more common causes.
When should I call an HVAC technician for HRV sweating?
Call if the unit ices up repeatedly, one fan is not moving air, the breaker trips, cabinet water keeps returning after drain and filter checks, or the moisture is causing damage in finished areas.