Heat Pump Leaking Water? Where It’s Coming From and What to Do

Here’s the thing most people don’t realize about their heat pump: it’s a dehumidifier that happens to also move heat. Every hour it runs in cooling mode, moisture from your indoor air condenses on the cold evaporator coil and runs off. On a humid summer day, a residential system can produce anywhere from several gallons to twenty gallons of water — sometimes more in very humid climates. All of it has to go somewhere.

The system for handling that water is unglamorous and easy to ignore: a sloped pan under the coil, a drain line running out of it, and gravity. That’s it. When any part of that chain fails, water finds the next lowest path, which is usually your ceiling, your subfloor, or the insulation in your attic. Understanding that chain is most of what you need to diagnose a leak.

Water that’s supposed to be there

Before you panic, check whether you’re looking at a fault or at the machine doing its job.

Condensate dripping in cooling season. A steady drip from the end of the condensate line outside the house, or a trickle into a floor drain, is exactly what should happen. If you can see water leaving through the intended path, the system is working. The volume can be surprising — a humid day in the Southeast will produce a puddle at the drain outlet, and that’s fine.

A puddle under the outdoor unit in cold weather. This is the one that alarms people every winter, and it’s almost always normal. During heating, the outdoor coil gets colder than the outside air, so moisture condenses and freezes on it. Several times a day the system runs a defrost cycle to melt that frost, and the meltwater runs out of the bottom of the cabinet. Wet ground, a small puddle, ice around the pad on a cold morning — all normal. Our guide to frost and ice on a heat pump covers when winter ice crosses over into a real problem.

Steam and dripping during defrost. Water running off the outdoor coil while steam rises is the defrost cycle working. It looks dramatic and it’s fine.

A small amount of moisture at a ductless head in humid weather. Ductless indoor units drain through a small line inside the wall sleeve. A little condensation on the outside of a head on a very humid day isn’t unusual. A steady drip down the wall is.

Stop first: when to cut the power

Some water is a maintenance chore. Some water is an electrical hazard. It’s worth being clear on the difference before you go poking around with a flashlight.

Shut the system off at the breaker — not just the thermostat — if any of the following is true. Water is dripping onto or pooling around the electrical compartment of the air handler or furnace. The unit sits in an attic and there’s a stain spreading on the ceiling below. Water has reached a condensate pump, a control board, or wiring. You see or smell anything scorched. Or the unit is in a basement and standing water is approaching it.

The reason is simple: an air handler contains line-voltage wiring, a blower motor, and on many systems a control board and electric resistance heat strips. Water and any of those is a shock and fire risk, and a soaked board is an expensive part that didn’t need to fail.

If the leak is mild — a damp spot near the drain, a slow drip at a fitting, water in the secondary pan but not overflowing — you can safely shut the system down at the thermostat, put a towel and a bucket under it, and work through the sections below.

Where it’s leaking, why, and how to fix it

Find the location that matches what you’re seeing. Each row links up with a section below.

The clogged condensate drain line

If you take one thing from this guide, take this: the drain line clogs, it clogs often, and it’s the reason for most indoor leaks.

Here’s why it happens. The drain pan under your evaporator coil is a permanently damp, dark, room-temperature environment. That’s an excellent home for algae, mold, and bacterial slime. As biofilm grows, it combines with the dust and lint that inevitably get past the filter and forms a sludge. That sludge migrates into a drain line that’s often only three-quarters of an inch across, and at some point — usually a low spot, an elbow, or the trap — it stops moving. Water backs up in the pan, reaches the rim, and spills.

The symptoms follow a predictable order. First, water in the secondary pan under the unit, if you have one. Then a float switch trips and shuts the system off — which people experience as “my AC stopped working” rather than as a drainage problem, and it’s actually the system protecting your house. If there’s no float switch, the first symptom may be a wet ceiling.

Climate makes a big difference in how often this happens. In humid regions, annual clearing is a reasonable habit and twice a year isn’t excessive. In a dry climate with less condensate, lines clog far less. Systems in attics and closets tend to be neglected simply because nobody looks at them.

How to clear a drain line safely

This is one of the few HVAC repairs that’s squarely in homeowner territory. It takes about half an hour, and the main tool is a wet/dry vacuum.

1. Turn the system off

   At the thermostat, and at the breaker if there’s any water near electrical parts. Don’t work on a running system.

2. Find both ends of the line

   The indoor access cap near the air handler, and the outdoor termination point. You’ll be pulling from the outdoor end and flushing from the indoor end.

3. Vacuum from the outdoor end

   Take the paper filter out of your wet/dry vac — you’re pulling water. Wrap a rag or a piece of duct tape around the joint between the vacuum hose and the drain pipe to get a decent seal. It doesn’t need to be airtight, just close. Run the vacuum for one to two minutes. You’ll often hear the clog break and see a slug of dark, foul water come through.

4. Check what came out

   Dark slime, grit, and a strong musty smell mean you got it. Clear water with no debris means the clog may be further in, or the problem isn’t a clog at all.

5. Flush from the indoor access port

   Remove the cap on the T-fitting. Pour about a cup of distilled white vinegar slowly into the port, wait 20 to 30 minutes, then follow with a couple of cups of warm water. Watch the outdoor end — water should run out freely.

6. Confirm flow and reassemble

   Replace the access cap so it seals. A missing or loose cap lets the blower pull air through the drain and can stop the line draining properly.

7. Run the system and watch

   Turn it back on in cooling mode and check after 15 to 30 minutes that water is coming out of the outdoor end and there’s nothing in the pan. Check again the next day.

Two more cautions. Don’t blow compressed air into the line hoping to push the clog out — high pressure can blow a joint apart or force water back into the pan and out onto the floor, and on some systems it can damage the pan. And don’t run a plumbing snake in from the outside; a stiff snake in thin PVC can crack a fitting or puncture the pan at the far end.

If the line clogs again within a few weeks, the clog wasn’t the whole story. Look at slope, at trap design, and at whether the pan itself is holding standing water — the section below on drain plumbing covers why.

Drain pans, level, and cracks

Two pans matter here, and they do different jobs.

The primary pan sits directly under the evaporator coil, inside the cabinet. It catches condensate and channels it to the drain connection. It’s usually plastic on modern equipment and sheet metal on older units, and metal pans rust through eventually — decades of standing water will do that. A rusted or cracked primary pan leaks straight down inside the cabinet, and replacing one generally means pulling the coil, which puts it firmly in professional territory and makes it a reasonable moment to weigh repair against replacement on an older system.

The secondary or emergency pan is a separate, larger pan under the whole unit, required by code in most jurisdictions when the equipment sits above a finished space — an attic over bedrooms, for instance. It’s a backup. Water in the secondary pan means the primary drainage failed and the safety net is holding. Many secondary pans have their own drain line that terminates somewhere conspicuous, like over a window, precisely so you notice. Some have a float switch that shuts the system down instead.

Then there’s level. An air handler that isn’t sitting level drains to a corner instead of to the drain connection. Water pools on the high side of the pan, overflows the rim, and drips inside the cabinet. This happens when a unit was set carelessly, when a platform settles, or when someone moved the equipment during another repair. It’s diagnosable with a torpedo level laid across the base, but correcting it means supporting and shimming a heavy unit without stressing refrigerant lines or duct connections — which is why it belongs to a contractor even though the diagnosis is easy.

When a condensate pump fails

Gravity is the preferred way to drain condensate, but plenty of installations can’t use it. A basement air handler below the level of the nearest drain, a closet unit in the middle of a house, a ductless head on an interior wall — all of them need a condensate pump, a small box with a reservoir, a float, and a pump that pushes water up and out through narrow tubing.

Pumps fail in predictable ways. The reservoir fills with the same slime that clogs drain lines, and the float sticks. The check valve gums up so water flows back into the reservoir. The narrow discharge tubing kinks or clogs. Or the pump simply wears out — they’re mechanical devices with a finite life, generally measured in years rather than decades.

The symptoms are a full reservoir, water overflowing the pump box, and often a system that’s shut itself off because the pump’s safety switch tripped. Before assuming the pump is dead, check three things: whether it’s actually plugged in (it’s more common than you’d think for a pump to get unplugged during other work), whether the outlet has power, and whether the discharge tubing is kinked or blocked. Cleaning the reservoir with a diluted vinegar solution revives a fair number of pumps.

If it’s genuinely failed, replacement is a modest job. The pumps themselves are inexpensive parts, and a handy homeowner can often swap one, though anything involving the electrical connection or the safety switch wiring is better handed to a technician.

A frozen coil that melted

Sometimes the leak isn’t a drainage problem at all. It’s a thawing problem.

When an evaporator coil ices over, it accumulates far more water than the pan and drain were designed to handle in a short period. Then the ice melts — all at once, often when you shut the system off — and it sheets off the coil, overwhelms the pan, and runs onto the floor or into the ductwork. People find a puddle, clear the drain line, and are baffled when it happens again, because the drain was never the problem.

The tell is ice. Look at the indoor coil if you can access it, and at the larger insulated copper line where it enters the air handler. Frost or ice on either means you’re dealing with a freeze, and the causes are the familiar list: a badly clogged filter, closed or blocked registers, a failing blower, a dirty coil, or a low refrigerant charge from a leak.

The thaw procedure: turn the cooling off but set the thermostat fan to ON, so room-temperature air moves across the coil and speeds up melting. Put towels and a shallow pan underneath. Give it several hours — a heavily iced coil can take most of a day. Only once it’s completely clear should you start the system in cooling again, and if it ices up again, stop and call. Repeated freezing on a clean filter almost always means refrigerant or airflow, and refrigerant work is licensed, EPA-certified territory. Our guide to a heat pump that isn’t heating and our icing guide both go deeper on why coils freeze.

Water on the floor is a symptom with two very different parents: drainage that failed, or ice that melted. Clearing a drain won’t fix a freeze, and thawing a coil won’t fix a clog.

— Why the first step is finding the source, not mopping

Sweating line sets and bad insulation

Here’s a leak that isn’t a leak, and it fools a lot of people.

The line set is the pair of copper tubes running between your indoor and outdoor units. In cooling, the larger of the two — the suction line — carries cold refrigerant vapor and runs well below room temperature. Warm, humid air touching cold copper does exactly what it does on a glass of iced tea: it condenses. That’s why the suction line is wrapped in closed-cell foam insulation.

When that insulation is missing, split, compressed by a zip tie, gapped at a joint, or degraded by years of UV exposure on the outdoor run, bare copper gets exposed and sweats. In an attic or crawlspace, that produces a steady drip that soaks insulation and eventually shows up on a ceiling — with no fault anywhere in the drainage system. Inside a wall, it can go unnoticed for a long time.

The signs: water tracking along the copper lines rather than pooling under the unit, drips that follow the line’s path, and damp insulation along the run. Where the line set is accessible — an attic, a basement, or the short run at the outdoor unit — re-insulating is reasonable homeowner work. Use closed-cell foam pipe insulation sized for the tube, butt the seams tightly, seal them with the manufacturer’s tape or adhesive rather than leaving gaps, and use UV-resistant covering or paintable jacketing on anything exposed to sunlight outdoors. Don’t compress the foam with tight zip ties; crushed insulation is the same as no insulation.

Where the line set runs inside a finished wall, you’re into a contractor conversation. And if line-set sweating is happening throughout an attic, the underlying issue may be that the attic itself is far more humid than it should be, which is a building envelope problem rather than an HVAC one.

Outdoor leaks and base pan drainage

Outdoors, water is mostly normal — but there are two situations worth checking.

Blocked base pan drain holes. The bottom of an outdoor unit has holes designed to let defrost meltwater and rain drain out. Leaves, grass clippings, pine needles, seed pods, and mud plug them. Once they’re plugged, water stands in the base pan. In summer that mostly accelerates corrosion. In winter it’s worse: standing water freezes, ice builds up around the bottom of the coil and the fan, and you can end up with ice damming that blocks airflow, prevents defrost from clearing properly, and in bad cases lets ice contact the fan blade.

Clearing them is homeowner work. Cut power at the disconnect, look at the base of the cabinet, and clear the drain holes and the surrounding debris with a gloved hand, a stick, or a shop vac. While you’re there, clear leaves out of the bottom of the cabinet generally.

Poor drainage around the pad. An outdoor unit should sit on a pad raised enough that meltwater drains away rather than pooling around and under it. In cold climates, mounting brackets or taller stands are common for exactly this reason — snow and repeated freeze-thaw cycles around the base are hard on equipment. If your unit sits in a low spot that collects water, or if a downspout discharges next to it, correcting the drainage is worth doing. Our roundup of the best heat pumps for cold climates touches on why elevation and clearance matter in snow country.

Why the drain plumbing itself matters

Two installations can have identical equipment and wildly different odds of a leak, purely because of how the drain was plumbed.

Corrugated flexible tubing is the problem child. It’s cheap, it’s fast to install, and it bends around obstacles without fittings, which is exactly why installers reach for it. But it has two properties that make it a poor long-term drain. First, the corrugations mean the interior wall is a series of ridges rather than a smooth bore, so slime and debris have somewhere to catch and build. Second, flexible tubing needs continuous support along its whole length, and it rarely gets it — over time it sags between supports, forming bellies. Each belly holds standing water, and standing water is where biofilm grows fastest. So the material that clogs most easily is also the material most likely to develop the low spots where clogs start.

Rigid Schedule 40 PVC with continuous slope is the better answer. A smooth interior bore gives sludge less to grab. Rigid pipe supported at proper intervals holds its slope instead of sagging. And the standard practice — a minimum fall of about a quarter inch per foot of run, in the direction of flow, all the way to the termination — means gravity is doing consistent work instead of fighting a belly halfway along. Rigid pipe also lets you install a proper cleanout tee near the unit, which is what makes the annual flush a five-minute job instead of a disassembly project.

The trap matters too. A condensate drain needs a properly sized P-trap, because the blower creates pressure that will otherwise either push water back or pull air through the line and stop it draining. A trap that’s too shallow gets pulled dry; one plumbed backward doesn’t work at all. Trap depth is calculated from the blower’s static pressure, which is why this is a design detail rather than a guess. If your drain has never worked right from day one, an improperly designed or missing trap is a leading candidate.

None of this helps if the drain is already installed badly, but it’s exactly what to specify if you’re getting a new system or having a drain redone. Ask for rigid PVC, continuous slope, a cleanout tee, a correctly sized trap, an appropriately located secondary pan, and a float switch. It adds very little to an installation and prevents a whole category of problem. Our seasonal maintenance checklist covers keeping it clear once it’s in.

What each fix typically costs

Typical 2026 national ranges for residential equipment. Regional pricing, after-hours rates, and equipment access all move these numbers.

The last row is the argument for taking a small leak seriously. Clearing a drain line costs you an afternoon. Replacing a section of ceiling and dealing with mold remediation costs considerably more, and insurance treatment of gradual water damage from a maintenance issue is not always favorable.

6 mistakes that turn a leak into damage

Frequently asked questions

Your final checklist