Condensate disposal locations (C) Carson Dunlop Associates Improper HVAC Condensate Piping & Disposal
Condensate drain SNAFUs

  • CONDENSATE DRAIN PROBLEMS - CONTENTS: Air Conditioning Condensate Handling defects lead to condensate spillage, leaks, mold. Where should the air conditioner or heat pump condensate drain be connected - where are we permitted to dump condensate? Examples of Model Building Codes Condensate Disposal Regulations & Recommendations
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Improperly-installed A/C condensate piping, drains & pumps:

This article describes undesirable and improper methods for disposing of condensate from an air conditioner or heat pump or similar device.

This air conditioning repair article series discusses the inspection and repair or un-clogging of air conditioning condensate systems, including Air Conditioning condensate drains, condensate pumps, and their proper installation as part of our review of condensate piping, traps, drains, condensate pumps, and the detection and hazards of air conditioning system condensate leaks in buildings.

Condensate leak health and safety concerns are reviewed.

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Examples of Improper Disposal of Air Conditioning or Heat Pump Condensate Disposal

Photograph of condensate spillage in a crawl spaceCarson Dunlop's sketch at page top shows the proper locations and one improper one (the plumbing vent) for disposal of air conditioner or heat pump condensate. Condensate disposal by connection to a plumbing stack vent pipe, is not recommended and is prohibited by building code in some jurisdictions.

Our photo of condensate piping below a crawl-space mounted air handler shows another ugly condensate disposal method: dump it on to the dirt crawlspace floor.

This article describes that and other air conditioner or heat pump condensate disposal mistakes, troubles, problems. [Click to enlarge any image]

Article Contents

Condensate Drain Leaks & Clogging: Improper Slope

Air conditioning drain line installation (C) D FriedmanOur photo (left) shows a white flexible tube used as condensate drain tubing for a split system air conditioning system being installed in a New York Home. (click photo to see an enlarged, detailed version). Photo courtesy Galow Homes.

Even now the drain is not perfectly sloped (note it's a bit high at that second cripple stud from left) but it was much worse before we re-routed the drain. The air conditioner installer had the drain line sloping up-hill in the area I've circled in the photo.

Having already had condensate drain line clogs and backups and leaks from the indoor air handler into the building wall at another split-system air conditioner where the condensate drain was improperly sloped and clog-prone, I was not going to let it happen again at this installation.

The installer thought I was being unreasonably demanding.

He was ignoring the plumbing code (1/8" per foot slope for condensate drain lines) and apparently he didn't recognize the potential costs in rot, insect damage or mold if we simply let the condensate drain clog (due to an improper slope and dust that will enter the line) followed by leaks into the building wall up at the air handler.

Or this was a plumbing job "... guaranteed until my truck reaches the end of your driveway".

See SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS for complete information about the installation, routing, insulation, & protection of condensate drains for split system cooling or heat pump units.

CONDENSATE DRAINS - Air Conditioning or Heat Pump Condensate Drains Connected to a Building Plumbing Vent Pipe?

Photograph of  improper condensate drain connected to plumbing vent line

Sewer Gas & Bacterial Hazards at Air Conditioners

[Example air conditioning system inspection report language]:

*** Safety Recommendation: this condensate line is connected to the house drain/vent piping - risking possible bacteria or even dangerous sewer gases entering the building air handling system.

Good practice (and some building and mechanical codes) includes a moisture trap (just as with other plumbing drains) and routing of the condensate to a wet drain line or preferably outside to discharge into the gutter system or to the ground.

Sewer gases include methane which is an explosive gas. We don't want methane nor bacteria in our air conditioning system.


Below is a second example of improperly connected air conditioner condensate drain lines to a plumbing vent: the condensate line is connected to the house drain/vent piping; according to some experts and plumbing codes this is an improper plumbing connection, and for some lines there also is no condensate trap in this plumbing arrangement, risking possible bacteria or even dangerous sewer gases entering the building air handling system.

Photograph of  improper condensate drain connected to plumbing vent lineGood practice includes a moisture trap (just as with other plumbing drains) to help prevent this problem.

Our understanding is that despite this very common installation found in our area, this is an improper plumbing connection which is dumping liquids into plumbing lines intended for dry-use only.

Correction by a qualified plumber does not usually involve significant expense.

Watch out: HEALTH NOTE: Condensate drains should not be connected directly to a house drain (without an air gap) as bacteria can grow back up the condensate line to contaminate building air, or sewer gases may be drawn up the drain and into the building air when the blower fan is operating.

Reader Comment: traced sewer gas odors to HVAC condensate drain connection to vent stack

22 May 2015 T. W. Straub said:

We just found the source of our septic smell in the East Wing of our home. It appears that the HVAC contractors who installed the system on this wing plumbed the drip line from the air handler into the vent stack without using a P trap. That might have eliminated the problem, but I doubt it would be code compliant.

The way we found the problem was by using a smoke bomb in a shop vac and blowing the smoke down the vent stack from the roof. While doing that, we looked for smoke in the walls where the stacks were and then looked in the attic. When we opened the access to the attic, we could smell the smoke (Superior #2B - 8,000 cu. ft. smoke bomb) and could see the smoke billowing out of the seams of the air handler. I hope this helps someone else out there --- Tom

It's helpful to have a "real world" report confirming what experts have been telling us for a long time and that some installers or building owners simply don't accept. The convenient shortcut of connecting the HVAC condensate drain to the plumbing stack is overwhelming for some people even though it's a bad idea.

This comment was posted originally at SEWAGE ODOR SOURCE LOCATION. Also see SEPTIC / SEWER ODOR SOURCE TABLE.

A/C or Heat Pump Spillage in Building Crawl Spaces

Photograph of condensate spillage in a crawl space

Air conditioner condensate spillage in crawl spaces: is sometimes seen, especially if it's a dirt-floor crawl space.The installer probably figures the condensate will just "go away" through the soil exposed in the crawl area.

What s/he failed to consider is the risks of a legionnaire's infection or a mold problem caused by spillage of water into an indoor, and in this case confined and rarely-inspected space.

In our photo, the air handler itself was mounted in a crawl area, making service and repair more difficult and thus more costly. And spilling A/C condensate on the crawl space floor is asking for a building mold or insulation mold problem too.

Photograph of condensate spillage on the wall

Air conditioner condensate spillage down building walls: such as the condensate from this attic air handler can stain the building walls and is simply ugly.

An expert HVAC technician might also have something to say about those rust stains themselves - we may be looking at rust from inside the air handler, indicating that A/C condensate is spilling and leaking around inside the unit - perhaps we're actually looking at a hidden mold problem in this building - more investigation would be appropriate.

Safety Hazards of Air Conditioning or Heat Pump System Condensate Leaks Onto a Furnace Heat Exchanger

Photograph of A/C condensate damage to a furnace heat exchanger (maybe) Photograph of A/C condensate damage to a furnace heat exchanger (maybe)

Air conditioner condensate leaks into a furnace as we can see in these two photographs, can be dangerous. If the air conditioner condensate leaks cause rust holes in the furnace heat exchanger there is risk of dangerous flue gases, including carbon monoxide, leaking into the building air supply when the heater is running.

The rust seen in the bottom of the blower compartment tells us that this problem has gone on for some time. Further inspection of the heat exchanger is needed for damage, and on most systems, further inspection for mold contamination in the air handler and duct work may also be in order since the system has been spilling water into the air handler and perhaps the ductwork.

Disposal of Air Conditioning or Heat Pump Condensate Outdoors

Photograph of outdoor air conditioner condensate disposal This photograph shows a typical point of disposal of air conditioning condensate outdoors, onto the ground.

This A/C condensate line originated at the air handler in the building attic, though at some installations we could be looking at condensate from a condensate pump located in the building basement.

There's basically no issue with disposing of condensate at this location, though this particular photo shows two more subtle points to watch:

  1. There is some soil subsidence going on where the condensate is dripping, telling us that the soil around the foundation is fresh, loose backfill.

    Watch out that the air conditioner compressor/condenser unit does not begin to tip as this backfill settles further from the soil subsidence that will accompany rainfall, snowmelt, and time. Concerns for tipping A/C condenser units are discussed at Air Conditioner Compressor & Condenser Installation Errors.

  2. Foundation leaks: The second more subtle point to watch that has evidence in this photo is that little shrinkage crack we see in the poured concrete foundation right where the A/C condensate is dripping.

    We might see water leaks into the building interior at this point if the foundation is visible indoors, or if the wall has been covered with finish materials we might have a mold problem. I wouldn't go on to cut open such a wall to investigate this particular case further without other corroborating evidence. More about foundation crack diagnosis is available at How to Diagnose & Evaluate Vertical Foundation Cracks.

Photograph of outdoor air conditioner condensate disposal

This photograph shows what seems to us to be a sloppy installation of air conditioner condensate drainage.

The installer has sent the condensate drain line outdoors (fine) through the building eaves or soffit (OK) but left the condensate drain pipe terminated where it drips onto a lower roof, splashing up and staining building siding, possibly creating a wear spot on the roof shingles, and thus perhaps a roof leak before the rest of the shingles are ready for replacement.

Air Conditioning or Heat Pump Condensate Drains Routed to Hidden Locations

Photograph of drains to a hidden location Condensate drains routed to hidden locations: What about installers who route a condensate drain to some hidden location? The drains in this photo might be ok, or maybe not - it depends. If they disappear into an inaccessible or not readily visible location such as a crawl space the system is asking for trouble - such as a wet moldy crawl space.

If the drains appear outside or at some other visible location we're in good shape. In this particular case, the air conditioning system for the computer center of a large college was draining onto the floor of a utility room where condensate ran along drywall and then across to a floor drain.

The drywall gave us a little area of mold to clean up but luckily nothing of any consequence. We could see the ends of this drain if we looked long enough.

The discharge point of all of the air conditioning system condensate drain lines, that is the system condensate drain and the air conditioner condensate overflow pan drain, must be readily observable. (Ref. Uniform Mechanical Code Sec. 1205 and Sec. 510. Condensate overflow pan is suggested for attic space per UMC (Uniform Mechanical Code) Section 1205.)

HVAC Condensate Articles


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