Hi,

I am having an unusual problem with the drain line on my Central A/C (evaporator unit) inside my home.

Water will not drain out of the unit when anything is connected to the drain pan outlet pipe. I have back flushed the line and confirmed that it isn't clogged and I have blown through it and can confirm that there is no clog.

After the last clog, (last year,) I installed an T junction pipe with a screw-off fitting about 8 inches from the unit so I could easily drain it in an emergency.

Well then, I unscrewed the plug in the T-fitting (which is well below the lowest level on the drain pan,) and no water drips out. I then snaked the 8 inches of line between my emergency Drain T-fitting and the rubber fitting connecting the PVC pipe to the outlet for the drain pan and found no obstruction. During this time I left the A/C unit on and observed a steady drip of water out of the pan. I re-connected the pipe and guess what - No water dripped out of the emergency t-fitting even after several minutes.

I then disconnected the rubber fitting from the drain pan and a large amount of water came spilling out. I observed the drainning water for a few more minutes and checked the drain pan with a flashlight and determined that there appeared to be no obstruction in the drain pan.

Just for kicks, I put my pinky finger in the hole to feel around for problems and found none. However, I noticed that my little finger closes about 60% of the opening the drain pipe. Even though there is plenty of room for water to drip out while my finger is in the top section of the pipe, no water comes out!
As soon as I removed my finger a large amount of water comes out.
Am I imagining this?

A neighbor said I need to install a pipe that allows air to flow out of the drain line between the drain pan and the P-trap, (or J trap.) I could believe this but how come I never needed this before?
(He also said that an old rubber (furnco ? ) fitting could also cause this.)

Any thoughts anyone might have would be greatly appreciated.

Thanks,
Jim

 

It's hard to imagine how you could have a problem like you describe. You could have a suction problem with the air pressure inside the evaporator much lower than ambient. If that's the case you should feel a flow of air INTO the condensate drain line when it's disconnected from the drain line. As an experiment you could provide a vent line so see if that would help. At our house we have a construction project going on. The plumbers have just been hooking up the floor drains in my new garage before they pour the concrete. There is a vent line that will vent the drains to the atmosphere. You would think that the level drop going to the main line in the street (about 15 feet) would be enough to assure proper drainage but a vent is still necessary. As they always told us in school, water always flows down hill, but sometimes air pressure can exert a stronger force on the water than does gravity. Otherwise you could never get most pumps to work. At least that's the only thing that I can think of that would fit the description of your problem.

 

The problem is caused by the fact that the evaperator coil is installed in the return air duct and is under negative pressure and air is being drawn into the drain.
You need to install a p-trap directly to the coil outlet.

 

Hi GregH and Jughead.

Thanks for the tips. I will implement them.

I did notice that there is a large volume of air coming out of the Drain Pan outlet pipe.

I am not sure what the coil outlet is.. Is this what I have been calling the drain pan outlet? If not can you give me an idea what this is ?

Thank you,
Jim

 

I meant drain outlet.

If you are feeling air pressure at the drain fitting your coil is downstream of the fan.

It is more common for water not to flow when the air is being drawn in as when installed upstream on the suction side of the fan.

I have seen water not flow when air was blowing out the fitting and I would suggest you make a trap that is at least three inches deep.

If you are unsure what I mean by this just post back.

 

Hi GregH,

Thanks for the clarification.

I just realized that I haven't given you all the pertinenet information: I added the P-trap earlier this year on the advice of my neighbor who said it would keep out critters.

Since this P-trap was never in the original design, would it be suggestable to remove the p-trap first and retest before I add another deeper p-trap?

I appreciate your insight...

Thank you,
Jim

 

As im not sure if the air blowes out or sucks in, there at the drain out let .Are we sure that the coil is clean??

ED

 

It's also a little puzzling to me as well. My A coil is on the discharge side of the furnace blower so there is a strong positive presssue that would tend to blow the water out the drain line. One of the other posters implies that you could have the A coil on the return air line (or the suction side of the blower). I've never seen an installation like that, but that could explain the problem. There would be a strong negative (relative to atmospheric) pressure on the drain line and you would need some kind of vent or trap in order for the condensate to flow to the higher pressure outside world.

 

Yes -the coil is clean. (I assume you mean the evaporator coil.) I just cleaned the it earlier this year and noticed a marked improvement in the cooiling inside the house and lower energy bills since then.

The air at the coil blows out of the drain pan tube.

The evaporator coil is on the discharge side of the blower as far as I know.
(The flow is: Cold air return to heater to evaporator coil to the duckwork for rest of the house.)

FYI: At the time I cleaned the evaporator I added the p-trap and about 20 feet of PVC pipe outside my home to stop the build up water around my foundation.

Thanks,
Jim

 

Are you 100% sure the drain isn't clogged? The outlet isn't covered with mulch or anything like that is it?

 

As I said in my other post it is possible that if you have a high static pressure in the supply duct it is possible for the water to be held back by the velocity of the air exiting the drain fitting.
A way to confirm this is to allow the airconditioner to run untill there has been a lot of condensate dripping and then have someone kill the power to the fan while watching the drain. If water begins draining when the fan stops then you will know that this is the problem.

Also, you have to make sure the trap has no T's in the line between the drain fitting at the coil and the downstream side of the trap.
The height of the trap has to be deep enough as well to allow the water that sits in it to not be blown out.
You can check this by having the fan turned off and pouring water into the drain pan of the coil until the water goes through the trap and out the drain.
If you then start the fan you should not have any air come out the drain.
You can use a match to check for airflow.
If air blows out of this opening then either there is a leak and the trap empties or the length isn't long enough.

 

HI GregH,
I think I am now understanding your first post...

I am experimenting with the drain configuration now and have noticed some improvement.

I think you are right the the volume of air going into the drain pipe is keeping the water from flowing out.

I appreciate your patience re-explaining some of the details that I didn't understand.

I'll let you know how its progressing tomorrow.

Jim

 

i commonly install a tee with the side opening attached to the evaporator. Under normal circumstances I install a female thread adapter and a plug on the top to permit a cleanout.

in instances where the evaportor is on the negative air side of the fan I install a two or three inch riser on the tee, leaving the top open as a vent for the drain.