Water under the unit
A puddle forms below the cabinet or near the drain connection, usually after the unit has been running for a while.
Start here: Start with the drain pan, drain hose, trap, and any low spots in the tubing.
Ventilation leak troubleshooting
HRV / ERV Dripping Water
Direct answer: If an HRV or ERV is dripping water, the usual cause is condensate not getting out the way it should. Start with where the water is showing up: from the cabinet, from the drain connection, or from nearby ducting. A clogged or sagging drain line, dirty filters reducing airflow, or frost and thaw inside the unit are the most common causes.
Most likely: Most often, the drain pan or condensate drain line is partially blocked, disconnected, or pitched wrong, so normal moisture backs up and spills out of the unit.
Look for the simple clues before you open anything up too far. Clear water at the drain area points one way. Water dripping off insulated duct or sweating on the outside of the cabinet points another. Reality check: a little condensate can be normal, but water on the floor is not. Common wrong move: cranking the unit to high speed before checking the drain and filters often makes the leak worse.
Don’t start with: Don’t start by replacing the core, motor, or controls. Water at these units is usually a drainage, airflow, or cold-weather issue first.
If water is coming from the drain fitting or bottom seam,check the drain pan, trap, and hose slope first.
If water is forming on the outside of duct or cabinet surfaces,treat it as condensation and insulation or airflow trouble, not a bad part.
Last reviewed: 2026-04-04
Where is the water actually coming from?
Water dripping from duct or fittings
Drops form on metal duct, seams, or collars near the unit rather than from the drain outlet.
Start here: Start with sweating and insulation checks, then look for low airflow or very cold incoming air.
Leak happens mostly in cold weather
The unit may drip during freezing weather or when outdoor temperatures swing and the unit thaws.
Start here: Start with frost buildup clues, blocked filters, and whether the drain line may be frozen.
Water appears after cleaning or filter changes
The leak started after the door was opened, filters were removed, or the core was handled.
Start here: Start with filter seating, core seating, door gasket contact, and whether the drain hose got bumped loose.
Most likely causes
1. Condensate drain restriction or bad drain slope
This is the most common reason for water under the unit. The pan fills, then spills from the cabinet seam or around the drain connection.
Quick check: With power off, inspect the drain hose for slime, kinks, sags, or an uphill section before it reaches the drain.
2. Dirty or mis-seated HRV / ERV filters reducing airflow
Low airflow lets the unit run colder and hold moisture where it should not. In winter that can lead to frost, then dripping when it melts.
Quick check: Pull the filters and look for heavy dust loading, collapsed media, or filters not fully seated in their tracks.
3. Frost buildup inside the HRV / ERV
In cold weather, restricted airflow or defrost trouble can let ice build on internal surfaces. When it thaws, water can overwhelm the pan or leak from odd spots.
Quick check: Look for ice, wet insulation, or water marks inside the cabinet after a cold snap.
4. Exterior condensation on cabinet or ductwork
If the outside of the unit or nearby duct is sweating, the water is not coming from a failed internal part. It is warm humid room air hitting a cold surface.
Quick check: Wipe the surface dry and watch whether droplets reform on the outside of the metal rather than appearing from inside the cabinet.
Step-by-step fix
Step 1: Pin down whether the water is internal drainage or outside condensation
These look similar from the floor, but the fix is different. You do not want to chase a drain problem when the real issue is sweating duct or a cold cabinet surface.
- Turn the unit off at its service switch or breaker before opening panels.
- Dry the cabinet bottom, drain area, and nearby duct surfaces with a towel so you can see fresh water clearly.
- Check whether the water trail starts at the drain fitting or bottom seam, or whether droplets are forming on the outside of metal duct or the cabinet skin.
- Look for clear signs of sweating: evenly spaced beads on the outside surface, damp insulation jacket, or water only on the room side of cold metal.
Next move: If you confirm the water is only forming on the outside of duct or cabinet surfaces, focus on insulation, air sealing, and airflow instead of internal parts. If the water is coming from inside the cabinet or from the drain connection, keep going with drain and airflow checks.
What to conclude: Outside sweating points to condensation on cold surfaces. Water from inside the unit points to drainage backup, frost melt, or a panel or gasket issue.
Stop if:
- You see damaged wiring, scorched insulation, or water reaching electrical components.
- The unit is mounted where opening panels safely is not practical.
- There is heavy mold growth, standing water inside the cabinet, or soaked building materials around the unit.
Step 2: Check the condensate drain path from pan to household drain
A partially blocked or poorly pitched drain line is the likeliest cause and the least destructive thing to fix first.
- With power still off, remove the access panel if it is easily accessible and locate the drain pan and drain outlet.
- Inspect the HRV / ERV drain hose or tubing for kinks, slime, debris, a loose connection, or a sag that can hold water.
- Make sure the tubing runs downhill continuously to the drain and has not been pinched behind the unit.
- If the drain line is removable without forcing anything, disconnect it at an accessible point and flush it with warm water into a bucket or sink.
- Clean the drain pan gently with warm water and mild soap if it has slime or debris, then reconnect the line securely.
Next move: If water now drains normally and no fresh dripping appears during operation, the problem was a blocked or poorly routed condensate path. If the drain is clear and pitched correctly but the unit still drips, move on to airflow and frost clues.
What to conclude: A backed-up pan or slow drain lets normal condensate spill out. If the drain path is clear, the unit is likely making too much water from airflow restriction, frost, or cold-surface condensation.
Stop if:
- The drain connection is cracked, brittle, or glued in a way that will break if you force it.
- You cannot access the pan or tubing without removing major duct sections or electrical covers.
- Water has been leaking long enough to damage framing, drywall, or ceiling materials below the unit.
Step 3: Inspect filters, core seating, and door closure
Restricted airflow is the next most common reason these units drip, especially in cold weather. A filter installed crooked or a core not seated right can change airflow enough to create frost and water.
- Remove the HRV / ERV filters and inspect for heavy dust, collapsed media, or damp buildup.
- Clean reusable filters only if the unit uses washable filters and they are in good shape; otherwise replace them with the correct type and size.
- Check that the heat or energy recovery core is fully seated in its guides and not tilted or left partly out after cleaning.
- Inspect the access door and gasket contact. Make sure the panel closes evenly and latches fully without gaps.
Next move: If cleaning or correcting the filter and core setup stops the dripping, the unit was likely running with poor airflow or bypassing air where it should not. If filters are clean and seated properly but the leak continues, look for frost, freezing, or a defrost problem.
Stop if:
- The core is damaged, torn, or crumbling when handled.
- The door will not seal because the cabinet is warped or the latch hardware is broken.
- You are unsure whether the core orientation is correct and the unit has no clear panel markings.
Step 4: Look for frost, ice, or a frozen drain in cold weather
When these units drip mostly during or after freezing weather, frost and thaw are strong clues. The fix may still be simple, but this is where you stop short of guessing at controls or motors.
- Open the access panel and look for ice on the core edges, inside surfaces, or around the drain area.
- Check whether the drain tubing or trap runs through an unconditioned space where it could freeze.
- If the unit is iced up, turn it off and let it thaw naturally with towels in place to catch water. Do not chip ice off the core.
- After thawing, restore power and watch one full run period to see whether water now drains normally or starts building again.
Next move: If thawing and correcting a cold or frozen drain path stops the leak, the immediate problem was freeze-related drainage or frost buildup. If frost returns quickly, or the unit keeps making water with clean filters and a clear drain, service is the right next move.
Stop if:
- Ice is heavy enough that panels are stuck or internal parts are frozen solid.
- The unit trips a breaker, hums without moving air, or shows obvious electrical trouble.
- You suspect a failed defrost sequence or fan problem but cannot verify airflow safely.
Step 5: Put the unit back in service only after the water path is controlled
Once you have corrected the simple causes, you need to verify the leak is actually gone and not just delayed until the next cold cycle.
- Reinstall panels, filters, and the core exactly as they were meant to sit, then restore power.
- Run the unit at normal settings and watch the drain area, cabinet bottom, and nearby duct for 15 to 30 minutes if conditions allow.
- If the drain is carrying water cleanly and no new dripping appears, keep monitoring through the next cold or humid period.
- If water still appears from inside the cabinet after a clear drain and clean filters, schedule HVAC service for airflow balance, defrost, or internal component diagnosis.
A good result: If the area stays dry and the drain handles moisture normally, you have likely solved the problem without replacing major parts.
If not: If the unit still leaks from the cabinet, stop there and get it serviced rather than guessing at motors, controls, or the core.
What to conclude: A dry test run confirms the fix. Continued leaking after the basic checks usually means a deeper operating fault, not just a dirty unit.
Stop if:
- Water reappears near wiring or electrical compartments.
- The leak is above finished ceilings or walls where another test run could cause damage.
- The unit sounds abnormal, airflow seems weak, or frost starts returning during the test.
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FAQ
Is it normal for an HRV or ERV to make some water?
Yes. Some condensate can be normal, especially in cold weather. What is not normal is water dripping onto the floor, out of the cabinet, or off nearby ductwork.
Why does my HRV or ERV drip more in winter?
Cold outdoor air can create frost or heavier condensate inside the unit. If the drain is slow, the filters are dirty, or the drain line freezes, that moisture backs up and spills out.
Can a dirty filter really cause water to leak?
Yes. Low airflow can make the unit run colder and build frost or excess moisture in the wrong places. When that melts, it can overwhelm the drain pan or leak from the cabinet.
Should I pour bleach or strong cleaner into the drain line?
No. Start with warm water and a simple flush. Strong chemicals can damage components, create fumes, or leave residue in a small mechanical space. Do not mix cleaners.
When should I call for service instead of keeping at it?
Call for service if the drain is clear and the filters are clean but the unit still leaks, if frost returns quickly, if airflow seems weak, or if water has reached any electrical area.