Charleston Home Inspector discusses Air Handler Condensate Lines

Submitted by Ray Thornburg on Thu, 08/23/2012 - 16:03


All About Air Handler Condensate Lines

Have you ever crawled up into the attic and noticed the pan under your air handler. Well this pan should be dry. It is only there in case the primary pan drain clogs (the one inside the unit).  If you see water in this pan call for service because your primary drain line is clogged. (They typically drain outside). Sometimes the primary line has a trap and that is where it usually gets clogged. The trap should be cleaned every year at least once. There should be a clean out there with a removable cap to facilitate cleaning it.

Here is a short synopsis of the condensate drainage issue and how condensation from the air handler is handled.

As air flows over the AC coils condensation develops and drips into a primary condensate pan. The primary pan is inside the unit under the AC coils.  This pan has a drain which typically exits to the exterior of the home. The primary pan is not required to be trapped by any of the codes to date. However;some manufacturers require a trap for their designs and if they do it should be done. The condensate line has to exit to an approved place (this is usually outside the home). It cannot or should not terminate directly into the sanitary sewer system because sewer gases could enter the home. It cannot terminate under the home in the crawl space. The primary drain should be insulated.


hvac unit drain line preferred method   

 Click the image to enlarge. In this picture we see the preferred method with all three lines piped separately outside the home. We recommend this method based on the problems we see. You won't always see three lines because some units are not designed for it and other methods are allowed. Notice that normally water should only be coming out of the primary line. The other lines are there in case the primary line clogs up. If water is coming out of the other lines then you have a problem.





If the primary drain is trapped it is good practice for it to drop 2” from the pan and then have a 2” trap seal. The clean out of the trap should have a removable cap. The cleanout is not a vent. If there is no trap but there is a cleanout then the cleanout should have a removable cap. The main reason for traps in primary drain lines is to keep the unit from sucking unconditioned air back into (or out of) the system. The manufacturer typically includes instructions in their manual as to how the trap should be constructed.

Drain lines should be well supported through out because a sag is actually another trap and double trapping will cause a back up. The primary and secondary drain lines must be piped separately.

A secondary pan is now required if a leakage could cause damage. This pan should be at least 3” bigger than the unit on all sides. The drain line for this does not need to be insulated or trapped but should exit in a place outside where the occupants of the home can see it dripping should the primary drain back up. There should not normally be water in this pan as it is there in case the primary pan clogs up. If there is water in this pan call a service person.

There may be overflow float type switches installed on the secondary pan or in the drain line or both to cut power to the unit in case of overflow. If this device is on the secondary pan then a drain is not required for it.  Some types of designs (i.e. down flow units) may not have a secondary pan and in that case a high water cut off is installed in the primary pan.

During the short period of time that a typical home inspection takes place condensation is unlikely to present itself in sufficient qualities to show up in a secondary drain pan (unless the AC has been running a while). However there may be signs of a problem like water stains and rust that will help alert the inspector to potential problems.

ok.... on to our discussion

below is a diagram from the lennox install manual..... I'm citing fair use here.

Ok so you can see from the photo above that the primary pan (on this model) has two outlets to handle condensate. One is the condensate drain; the other is the overflow. You can see from the photo that the overflow doesn't come into play until the primary pan starts to fill up. So what happens if you plug up the overflow is that (if the primary fails) condensate will continue to fill up the primary pan and overflow in places that they don't want it to go. Naturally if something happens you want the water diverted to where you want it to go. Common sense would dictate that the overflow should either be piped separately out, turned down to discharge into the secondary pan or have a switch installed to de-energize the unit. 

There is some confusion in the manual. Now the manual states for the model that if the overflow is not used it must be plugged with provided cap. But the diagram above states that the overflow should always be connected. Of course the latter statement makes more sense.


Some Rules about Air Conditioning Condensate Disposal


·         Condensate may not drain to public way.

·         Drainpipe min ¾” with 1/8” slope per foot.

·         It may drain to an indirect was receptor ( i.e. laundry tub, lavatory tailpiece, tub overflow pipe)

·         It may not be directly connected to waste or vent pipe.

·         If condensate blockage would damage building components then install one of the following.

*   Secondary drain to a conspicuous point of disposal.

*   Auxiliary pan with a drain to a conspicuous point of disposal. Auxiliary pan must be separately piped. M1411.3.1 #1

*   Auxiliary pan with a detector and drain fitting.

*   Water level detection device in primary, secondary, or pan with interlocked cutout.( shuts equipment off in case of an overflow.

·         Down flow units with no secondary drain and no way to install auxiliary drain pan requires an internal blockage detector with interlock cutout.

·         No drilling saddle type fitting for discharge into drain, waste, or vent stacks to accept condensate disposal.

·         Auxiliary drain pans should be 1.5” deep minimum and 3” larger than the unit or coil dimensions in all directions.

·         Supports inside of pan to support equipment shall be water resistant and approved.

References IRC M1411-M1411.6

Ok so let's look at some photos to see what it's like out in the field.


a coil with pan Here is a pic of a downflow unit with an a coil on top and a plastic pan underneath. This pan is actually a square trough with a hole in the middle so air can flow through it to cool the home. You can see the single drain line at the far left bottom behind the copper line. This was acceptable at the time it was built. I rarely see backup problems on units like this because the condensate line goes straight down.




condensate overflow  Here is a pic of the primary overflow turned down to discharge into the pan. This makes sense as the secondary pan has its own drain. (Although we prefer it be piped separately out.) What you don't see in the pic is insulation around the primary drain pipe. Remember since traps are not required you would have to thumb through the installation manual to see what's needed here. On the pan you can see an overflow device installed.




 Here you see one that is properly trapped, insulated; the clean outs are properly installed and theres a protection devise located in the primary pan overflow. Good Work Men!






turned up  The primary overflow in this pic is turned up and capped which negates it purpose. If it was turned down to flow into the pan it would make more sense but air would escape which is one reason for the traps. It should be piped separately out.(although this method is not prohibited) It does have a trap and a "clean out" but the clean out really should be on the other side of the trap so you can blow the trap out downhill. If the trap is on the uphill side it is easy to occasionally pour a capful of bleach into it to help keep it clean. The "clean out" on the downhill side should be a vent with no cap. The rust stains is a concern so they're already having troubles.



not insulatedIn this pic you see a trap, no clean outs, no secondary and it's not insulated all the way. It's also not supported well at the joint




extra pan   

     Here is one that is having trouble. See the rust in the pan and the wonderful auxiliary pan to the right. Rust in the pan means they have been having trouble. In this case the ceiling finish was damaged. Look closely and water stains are visible on the plywood under the pan. I suspect the trap was too deep but a call for service is definitely in order. Wood is not a suitable support for an air handler either (under unit inside pan).



 condensate pan with efflorescence

     Here is a air handler pan which is white with efflorescence. Efflorescence is the white powder like salt which is left behind when water is constantly evaporating. It's a sign that the primary drain line is was clogged. So much so on this unit that microbial growth is now a concern due to the extra humidity evaporating water in the pan creates. Note that they chose not to use the secondary overflow drain line (too bad). I suspect that the primary trap is too deep. I see a lot of backups on units that have traps so if traps are used they should be done per the installation instructions.




condensate must be piped separately

    Here is a set up that's not allowed. The separate pans must be piped separately. This one has a T connecting them together. Rust and efflorescence can be seen in the pan so they have already been having problems.






water in air handler pan     In the picture at left is a lot of water and rust in the condensate pan. The primary drain line is clogged and the auto cut out switch pictured as not activated yet. This leaves a certain amount of water in the pan waiting to rise enough to drain out the pan drain. This is why cleaning those traps are so important.




good method of disposing of condensate    Deciding what to do with the condensate after it leaves the air handler can be a struggle. Most of the time it just drips on the ground and kills the grass due to over saturation. Here is a picture of a good way. The owner dug a post hole about 2' deep inserted a 4" pvc pipe and filled it with rocks...... no more dead grass! This picture shows a package unit where the air handler and compressor is outside and heat and cooling is pushed to the home through ductwork.




One thing to think about is that the primary overflow can be capped off (if the specific installation manual says you can).  If it is... then if an overflow happens it spills out into who knows where. If it is turned down to drain into the pan it creates another hole that the ac will either blow through or suck through essentially reducing its efficiency. This is one reason they put traps on the primary drain in the first place.... and another reason to say we don't live in a perfect world. Having said this I feel that the amount of air escaping this hole is insignificant (esp. if it is piped separately to the exterior) and the benefits of the secondary overflow outweigh any perceived drawbacks. Of course the manufacturers install instructions take precedence.

To trap or not to trap. Well it depends on which way the wind is blowing. If your AC coil is before the blower(coming from the return air side) (the air that returns to the AC unit) then there will be suction and most people now a days trap it to keep from sucking outside air into the main drain line. On gas units and others where the coil is after the blower then there will be positive pressure and cold air will blow out through the drain pipe no trap is needed.  Often the pipe is so long not much air goes through it anyway. But you certainly could put one there and you certainly should if the installation manual says to. I often see problems on units which are deeply trapped so beware.

So what's the bottom line. Condensate drain lines are not required to be trapped but should be if the manufacturers instructions say so. Secondary drain is allowed to be capped. The secondary pan needs a drain line unless there is a device installed to detect an overflow and cut the unit off if it does overflow.

We recommend that all three condensate lines be installed if possible. If a trap is required then the instructions should be followed. Line should drop sharply if possible.


typical ac condensate drain connection    

     Click on the image at left to enlarge into a new window. At left is an diagram from a lennox install manual. I'm citing fair use here. It shows the typical condensate disposal recommendation and instructions for their unit. 




The Trap Vent

The question arises occasionally on whether the vent is necessary or whether a cap should be installed on the vent. To thoroughly understand this consider that in a home plumbing system the trap serves to keep sewer gasses out of the home. The vent on this system serves to keep the trap from siphoning dry by equalizing the air pressure. While sewer gases are not an issue on a HVAC drain line; in theory at least the trap could siphon dry. In practice however the slow dripping of a HVAC drain line is unlikely to develope a siphoning force enough to empty the trap. However; if the system has a trap it is best to have a uncapped vent after the trap This is what the diagrams above show. In the winter time though the traps are likely be evaporate dry anyway as the AC is not used during those months. The recommendation remains to put the vent downhill of the trap with no cap.


high effeciency furnace condensate drain Here is a condensate line which is a little different. It's from a high efficiency (HE) furnace. A HE furnace can have PVC vent pipes because very little heat is lost up the flue (so they don't get hot). As a result an acid like condensate is collected (instead of rising out with the exhaust gases) at the bottom and in this case the instructions show that it is ok to tie the two condensate lines together (from the evaporator and the exhaust flue). Sometimes these are routed over to the washing machine stand pipe which is an excellent way to handle the condensate from these systems.



One final note. Determining conformance to a particular manufacturers installation requirement is beyond the scope of a home inspection. However;  knowing what's required will help eliminate possible future problems.

References IRC M1411.3, M14113.1-4, M1412.1-4

A word about the primary pipe insulation. Some people feel that the code doesn't directly address insulating the primary condensate line. Around here I've seen many homes under construction where only the horizontal part of the drain line in the attic is insulated. Obviously anywhere there exists a possibility of a temperature difference condensation could occur and should be protected. As a home inspector I've seen the problems created by a lack of insulation on the primary condensate line and therefore will always recommend insulating it. Here are some pretty pictures that show condensation forming on the lines in the crawlspace in our discussion on piping support. I cite the following references that seem to allude to the necessity of insulating the primary ac condensate drain line.  

M1412.3 Insulation of piping. 

Refrigerant piping, brine piping and fittings within a building shall be insulated to prevent condensation from forming on piping.  

N1103.3 Mechanical system piping insulation. Mechanical system piping capable of carrying fluids above 105°F (40°C) or below 55°F (13°C) shall be insulated to a minimum of R-3.

503.2.8 Piping insulation from the energy code

All piping serving as part of a heating or cooling system shall be thermally insulated in accordance with Table 503.2.8.


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Submitted by Ray Thornburg on Thu, 08/23/2012 - 16:03