While not unusual, it’s always unwelcome to find water dripping from an AC vent. That water is always an indication that something is wrong and could be causing damage, while trying to figure out how to stop it. It is unlikely that this water leak will cause major damage to furniture beneath it or plywood around it, but it’s just one more thing to have to fix, after finding and fixing whatever is causing the leak.
It is important to figure out just when the leaks are happening, so you know what to repair. There are several different root causes to consider. If leaks only happen when it is raining then there’s a good chance that you have a water leak in the roof. But if the water leak is present under clear skies, that’s not likely to be the cause.
Finding the cause and fixing it quickly is critical to reduce damage to the home. In addition, whatever is causing the problem can prevent the air conditioning from working, because of the leak. Fortunately, most of these problems are fairly easy to fix.
Possible Causes for Intermittent Water Leaking from AC Vent
Most leaks from AC vents are intermittent problems, which could show up every couple of years or so. Proper preventative maintenance of the HVAC system, especially annual cleaning and regular changing of the filter, will go a long ways towards eliminating these problems.
The immediate thought when water starts leaking from an AC vent is that there is something wrong with the air conditioning unit. That is a definite possibility, but it is probably not true. There is no water run to the air handling unit in the house, so any air leaking is coming from a secondary source of one sort or another, even if that source is associated with the HVAC air handling unit.
Dirty Air Filters
The first thing to look at, when trying to find the root cause of water dripping from an AC vent, is the AC filters. While dirty filters cannot directly cause water to be dripping from the vent, they can be the indirect cause. Solving the cause, without changing out those dirty filters can ultimately lead to the problem coming right back in a couple of days.
There is always some quantity of dust in the air, which the air filters try to filter out. But when the filters are dirty, they block the airflow through the system. This allows moisture to stay on the evaporator coils, rather than dripping off. The cold Freon in the evaporator coils can freeze this condensation, turning it into ice. The longer the ice sits there, the thicker it gets, blocking air flow and allowing the home to become hot.
It is also possible that a dirty air filter will become disconnected along the edge, allowing air to pass around the filter, rather than through it. When this happens, the dust that the filter should catch can become stuck to the evaporator’s fins, building up there and blocking air flow. This blockage also helps more dust and condensation to accumulate on the evaporator, making it easier for it to ice up.
Frozen Evaporator Coil
Condensation forming on the evaporator coils is a normal part of the air conditioner’s operation. But when that condensation freezes, it impedes air flow, cutting the efficiency of the unit. At the same time, it is possible for the new condensation to be blown downstream, rather than dripping into the drip pan, because of the access to the drip pan being blocked by the ice.
Keeping the evaporator coil free of lint and ice is an important part of keeping any air conditioner working properly. Some regular maintenance is required, especially the replacement of filters. In addition, annual cleaning of the evaporator coil is recommended.
Clogged Condensate Line
As a normal part of how air conditioners function, they remove moisture from the air, dehumidifying the home. This condensate needs to drain somewhere. So the air handling unit sits over or on a drip pan, which collects this moisture. That moisture is then drained out through a small PVC plastic pipe. The pipe may T into an existing drain line or it may run to the outside of the home, draining out onto the ground.
It is not uncommon for this drain line to become clogged with dirt, dust and debris, especially since the air handling unit usually sits in the basement or attic, where there is an abundance of dust. When this happens, the water that should be draining out through that line can back up into the drain pan and overflow. When it does, it is not uncommon for it to leak out through the return vent, especially if the air handling unit is in the attic.
Rusted Drip Pan
The drip pans used for capturing that condensate are made of galvanized steel, much like air ducts are. While the galvanization process of coating the steel with zinc protects the steel from rusting, if it becomes scratched before or during installation, that scratch can allow rusting. Additionally, rusting can begin at the cut edge of the metal that is used to make the drip pan, because the cut edge is not galvanized.
If caught early enough, a rusted drip pan can be repaired with normal household caulking. Acrylic painter’s caulk can be used, but silicone caulking is even better. It is important to apply this caulking to a clean, dry surface, in order to get it to adhere well.
Broken Condensate Pump
Depending on the design of the system, a condensate pump may or may not be used. If the system can be installed in such a way that allows for gravity draining of the condensate, then the pump might be left out. But in all other cases, a condensate pump is installed, to empty the drip pan and send the water down the drain line.
Either the condensate pump or the water level switch that is attached to the pump can be broken. Since the switch is less costly, it should be checked first, verifying that it opens and closes with the ohm setting on a multimeter. With many switches, the opening and closing of the switch can be heard, but this should not be counted on. If the switch tests well and water is not draining, the pump might need replacing.
Possible Causes for Chronic Water Leaking from AC Vent
Some homes might have a chronic water leak from the AC system. This is different from the intermittent leaks mentioned above, in that these problems happen regularly, probably more than once a year. They also stand out in that they will happen in a new home, whereas the intermittent problems mentioned above will not.
For the condensation to drain properly from any air conditioning system, it needs to be installed properly. There are a number of things that can be done incorrectly in the installation process, which will prevent condensate water from draining properly and all but guarantee a chronic leak. These problems include things like forgetting to install a pan, not connecting the condensate pump to power, and poorly installed drain lines.
Because of the difficulty in locating the cause of any such problems, it is best to have the air conditioning system checked by a professional, should an improper installation be suspect. While the unit and its installation probably have a warranty, paying a separate AC professional to make this inspection helps to ensure that an honest inspection is performed.
Insufficient Insulation Around the Ducts
attic-mounted air conditioning systems are prone to condensation forming on the ducts, if they are not properly insulated. It is easy to determine if the air conditioning system is attic-mounted, because the vents will be in the ceiling or in the wall, close to the ceiling. If the unit is installed in the basement, the vents will be mounted in the floor or in the wall, close to the floor.
Most ducts today are made of a material that looks like a bubble pack, coated with aluminum foil. This has largely replaced galvanized steel ducts, due to its lower cost and ease of installation. At the same time, the insulation properties that this type of duct offers reduce the chances of condensation forming on the outside of the ductwork, due to moisture in the attic (most attics are not insulated).
For homes with galvanized steel ductwork, the solution to this problem is to install insulation around the ducts. A heavy layer, like that used in the attic, is not required. A thin layer of “duct wrap” or a “duct sleeve” is sufficient.
Air Leaks Around the Vents
Air escaping around the edges of the vents can cause dripping as well, as it will allow for condensation to form at the vent. To determine if there are any such leaks, merely feel for air escaping around the edges of the vent, while the unit is operating. These leaks can be sealed with painter’s acrylic caulking or by adding foam insulation strips, of the type used for weather-stripping windows and doors, applied to the underside of the vent flange.
Maintenance – Cleaning the Evaporator Coil
As mentioned above, the evaporator coil in the air handling unit will accumulate dust over time. Changing the air filter regularly will greatly reduce the amount of dust and other debris that accumulate on that evaporator; but it really can’t be stopped fully.
Most people say that AC filters should be changed monthly. That depends largely on how big your home is, how much dust there is and how much your unit runs. People who live in a hot climate, where their air conditioning runs continually, need to realize that chances are their filter will not last a full month and will need to be changed more quickly.
It is a good idea to install a gauge or indicator to tell when the filter needs to be changed. This operates under the principle of negative pressure. When the air pressure downstream of the filter is too low, because the filter is clogged and the blower is still trying to suck air through it, it will indicate that the filter needs to be changed.
Air conditioners should be serviced every spring, before the weather turns hot and they are in heavy use. HVAC contractors offer this service, which includes checking the function of the system, the amount of Freon in the system and should include cleaning the evaporator coil, although not all contractors include this in their service.
It is actually not all that difficult to clean the evaporator coil yourself. There is always an access panel somewhere on the air handling unit, which makes it possible to reach in and access the evaporator. In many cases, this access panel is large enough to get one’s upper body through. In closet mounted air conditioners, such as those installed in apartments, the bottom of the unit might be open, once the filter is removed, offering access to the evaporator.
All that is needed, to clean the evaporator, is an old toothbrush and some water. Turn off the air conditioning and gain access to the evaporator. Wet the toothbrush and rub it over the evaporator, running it in the direction of the fins on the coil. Never rub it perpendicular to the fins, as this can bend them. The toothbrush will get dirty quickly and need to be rinsed frequently.
If bent fins are encountered while cleaning the evaporator, they should be straightened with an Evaporator Fin Comb. These are readily available from AC supply houses, or online from sources like eBay and Amazon. They range from $4 to $16 or so.
Cleaning out a Clogged Drain Line
There’s a good chance that the AC drain line will need to be cleaned out sometime, quite possibly every few years. The accumulation of dust, mold and algae can eventually restrict the inside of the drain pipe to the point where water can no longer drain out.
Cleaning a clogged line is a multi-step process, as the line can be clogged at the ends or at any elbow or other thing which could cause an obstruction. A round brush, like a bottle brush (better yet, a test tube brush, which is smaller), a wet/dry vacuum, duct tape, bleach and a wire snake are all useful in cleaning out the drain line. It might also be necessary to have PVC unions and cement, if it is necessary to cut the line in order to clean it out.
To clean out the line, start by shutting off the air conditioning while working on it. It may also be necessary to disconnect and remove the condensate pump, as it may very well block access to the drain line. When removing it, check to see that it is not plugged and clean out the water channel, if necessary.
It may be necessary to empty water out of the drain pan, before removing the pump or cleaning out the line. This can be done by sucking the water out with a shop vac.
Run the brush into both ends of the drain line, cleaning out any debris that is found right at the ends. Then attach the brush to a wire snake, if available, so that the brush can reach farther. At the most, it will be possible to reach to the next elbow or T in the line.
Attach the hose for the shop vac to the end of the drain line with duct tape, sealing it all the way around. Then turn on the shop vac, allowing it to suck out the debris that has been loosened. Allow it about a minute to suck out any debris, then attach the hose to the blower side of the shop vac and blow out the line.
If the drain line empties outdoors, rather than connecting with a T into a larger drain line, repeat these steps from the outside.
It may be necessary to cut the drain line and run the brush through from the cut, if the blockage is beyond an elbow. This is really not much of an issue, as PVC is easy to cut and can be spliced back together with a union. Nevertheless, if it is possible to clean out the line, without having to cut it, that saves some work.
To test whether the drain line is clear after these actions, pour a gallon of water down it. If the water doesn’t back up, then the line is probably clear. If it does backs up, suck out the water with the wet/dry vacuum and seek out a location to cut the line after an elbow and clean out the line further.
Once the line is fully cleared, it is a good idea to pour some 10% diluted bleach down the line. This will kill mold and algae, helping to prevent the line from clogging back up quickly. Make sure that all unions or other connections are properly glued and there are no leaks left to cause further problems.
Fixing Damage the Leak Causes
Unless there is a leak dripping vent that is left unattended for a prolonged period of time, about the only damage that can be caused by water leaking out of an air conditioning vent is staining of the drywall. This can be rectified by priming and painting the affected area.
Whenever dealing with a water stain on drywall, wood or plywood, the best primer to use is a white-tinted shellac, such as Bullseye primer/sealer. The shellac is fast-drying, reducing chances of the stain migrating through the shellac and the white primer covers up the discoloration, so that extra coats of paint are not required.