The AC ran constantly for a day. Then next day air will not blow. I shut down the system. A lot of water poured into the pan under the air handler in attic. Somehow it overflowed. I collected at least a gallon.

A picture of brand specs, pan, and what is visible from the 4-inch filter access opening is below. The unit is a variable speed Lennox installed in 2005. (The outdoor unit is a 3.5 ton Lennox.)

What could be the causes? Is it safe for me to turn it back on?

Under what conditions should I consider replacement? Would they use same tonnage as before or do they need new calculation?

This looks like a disaster waiting to happen! My system is missing a water detection system.


The new diagnosis is that the air handler motor is failing. It does not run fast enough. The moisture in the air passing over coils freezes there. When you shut down the system this ice melts down and overwhelms the drain.

If the coils freeze you can thaw them by opening an access panel, or using a hair drier. You may thaw partially frozen coils by occasionally turning the ac off and running the fan on ON position (assuming the coils are not completely frozen already.) If the air flow is low it means the coils are partially frozen. Also frozen outside line means your coils are frozen already.

The air blower motor eventually did fail completely and was replaced with a temporary one. The unit is working now.

The reason for water getting outside the pan did not become clear as it happened a second time, after all drain pipes were cleared.


A pro came and his conclusion was that this is a one-off-event. In short, we don't know what happened. Refrigerant was at a normal level (45), blower motor worked, the drain line attached to the unit seemed not clogged. The other one attached to the pan was blown out, but no clogs could be detected.

I remember seeing the thermostat entirely blank before the event. Not sure if it means anything. The pro said something about the blower might have stopped working temporarily, as in a glitch.

I learned that (1) if you see frost in the outdoor unit cold line then the indoor coils are likely to be frozen too (2) you are to put a couple of blue pills before the P-trap to prevent algae build up in summer time (3) invest in a shop-vac to blow the pipes regularly.

Label of AC

water damage, outside of unit Drain line issue?

enter image description here evaporator coils

ice-over-coils ice over evaporator coils

enter image description here Frozen refrigerant line on outside unit

  • 3
    collecting that much water AFTER shutting the system down tells me the evaporator coil probably frose. This could be an air intake restriction or low freon or yet another cause. Best bet is to have the pros look at the system. It may be as simple as placing a new filter in the return duct.
    – RMDman
    Commented May 13 at 14:25
  • Is there a trap under all the black tape? And, what is that extra PVC 90 next to the primary drain?
    – jay613
    Commented May 27 at 17:13
  • @jay613 Yes there is P-trap. I am guess the other pvc is an overflow drain into the pan. However water drips from all seams.
    – Maesumi
    Commented May 27 at 18:06
  • Totally off topic but that looks like poison ivy on the outdoor unit
    – zelinka
    Commented Jun 6 at 21:33

3 Answers 3


Sounds like: The condensate drain Is clogged. That's always made out of small pipe (too small for drain use otherwise) that's prone to clogging, by AC installers who enjoy being called back for more money to clean them out when they do clog. We can see that this is the case here. Or because homeowners won't accept bigger pipes, but I'll go with the "makes more service calls" explanation until proven otherwise. It also can be seen to lack an access port to make rodding it out easier on the elbow pictured (you want a wye or tee with a plug in the part lined up with the pipe rather than elbows for easy cleaning, and they DO need cleaning, all too regularly.)

In addition: "you shut it off and then a lot of water poured out" might be the coil is icing up, which is often a sign that either airflow is too restricted (dirty filters, fan not running right) or refrigerant is low and the evaporator coil is running too cold (low refrigerant generally meaning a leak in the system somewhere.)

Neither is a sure sign of time to replace, though a 19 year old system may well be leaking in a place that's not economically sensible to fix. But it might just need a clean filter and a cleaned out drain line.

If the old unit is the right size (runs all the time on the hottest days only) then one the same size works. Many installers fudge-factor heavily on the "too big" side "to be sure it's big enough" so if it's only running half or 2/3rds time on the hottest days, might want to recalculate. Also if you have improved insulation, or do improve insulation, recalculating taking that into account is worth doing.

If it's been a while since you replaced the filter, do that. Clean out the condensate drain line and make sure the pan actually drains through it when you are done. Run it, and then take a look in the access hole right after you shut it off to see if the coil is iced over. If it is, then it's time for a service call. It should be wet, it should not be frozen.

  • 1
    Could also be a function of the water collecting in the trap and forming a sludge, kinda the consistency of partially set Jell-O, if you know what I'm talking about. That would stop the pan from draining.
    – Huesmann
    Commented May 14 at 13:57

I should note that your pan should have a fill sensor that cuts the unit off if the pan ever fills with water. If you don't have one, I would get one installed. You could have had a lot of water damage here.

There's a couple of common reasons a condensate line becomes clogged


Lots of cool water and a dark place for it to grow. Algae can easily plug a condensate line. Once in a while you should run some bleach down the line to prevent it from taking hold.

Old pipes

I had a tankless water heater installed, and it had a tube hooked to the old overflow pipe from the tank water heater. Code requires that you need to be able to take that potentially scalding hot water and dump it outside. The plumber used this, however, for the condensate pipe (newer high efficiency natural gas devices recapture some heat from the exhaust, thus producing condensate). The catch here was that it was copper than ran down to a galvanized pipe.

One night we had water on our bathroom ceiling. Investigating found this pipe was overflowing with water. The old galvanized pipe had corroded over the years and this steady stream of moisture was causing it to shed rusted material, clogging the pipe. I used my compressor to blow air down the pipe and there was a deluge of rusty material and water. Had to do this on the regular until my father-in-law found out and re-plumbed the condensate from the heater and the nearby HVAC unit to dump the water outside.

If you decided to re-plumb, I would suggest 3/4" PVC pipe (less prone to clogging). Find a path to the nearest edge of your roof and slope the pipe so the water continually falls until it runs down the pipe and outside. This solution won't work if you live far north, where hard freezes (i.e. you get below freezing and stay there) are a more common experience, but works fine for southerly areas where such freezes are rare.


Reading what you wrote:

  1. "I remember seeing the thermostat entirely blank before the event".
  2. "The AC ran constantly for a day".
  3. "Next day air will not blow".
  4. "I shut down the system".
  5. "A lot of water poured into the pan".

You probably have a thermostat problem which caused 2 and froze up the air handler, then you had 5 which showed up the problem with the drain on which other posters have commented.

I used to live in a town where one day every summer we had such high humidity that all the AC froze (except those units which went into defrost mode, and then we had steam coming out of the computer room air vents).

  • So assuming a thermostat malfunction can cause all of this what is the proper "reboot" process?
    – Maesumi
    Commented May 16 at 1:17

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