AC drains through primary but blown water to secondary drain?


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Old 09-15-12, 11:27 AM
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AC drains through primary but blown water to secondary drain?

I recently moved to a ~9 year old house with Carrier units in the attic. Long story short: the units have a history of draining water to the pan even though primary drain is not clogged. Trying to figure why this is.



What you see here is:

- Primary and secondary drains
- The pan with some rust in it - evidence that water was sitting there for a long time and draining into secondary drain
- The secondary drain pipe that was rotated "up" by the AC company that I called in; this was their "fix" for water coming out of secondary drain pipe.

So now, here is my thinking:

Since the AC guys "fixed" this (LOL) - the unit has NOT been pouring water everywhere or into the pan; so it must be draining through the primary drain.

The "fix" is messed up, as I am pretty sure that water will never go "up" this secondary pipe if primary gets clogged, unless it is under pressure. It'll go somewhere else instead.

The question is:

Why would the until be spitting out water through the secondary drain pipe into the pan if primary drain is (evidently) not clogged?

Am I overthinking this and having some water in the external pan is OK?

Thank you!
 
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Old 09-15-12, 01:16 PM
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If water is not draining out of the primary drain fast enough it builds up and starts coming out of the secondary. This could be due to many things ie unit not level, improper or double P trap, Partially clogged drain line, dirty coil among other things. If this "fix" is working it could be better than what it might cost to fix it assuming someone would even figure out the real cause.
 
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Old 09-15-12, 09:14 PM
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Totally messed up, that secondary drain is useless, better fix it right now, otherwise it will cause more damages later.
 
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Old 09-16-12, 06:17 AM
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cut the primary drain before the trap on the horizontal run and blow through the drain to clear it. Look through the pipe attached to the coil to make sure the pipe is clear. Reattach the drain together using a union so that the drain can be taken apart again in the future.
 
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Old 09-16-12, 08:41 AM
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You could check for a drain line blockage but if there is not then the p-trap or bump in the drain line on the left may not be deep enough.
If the a/c coil is on the upstream or suction side of the fan air may be sucking into the air handler preventing draining.

You first should check to see if there is any unusual restriction on the return air side.
If you CAREFULLY try to partially remove the fan access door and it almost falls away when the fan is running you are ok but if you have to nearly pry it off then you may have too much restriction on the return side which could cause your problem.

If there is not excessive return air suction pressure you could rework the drain line with a deeper trap.
Can't say exactly how deep to make it because it depends on your normal static pressure but four inches deep should be enough.

If you measure the between the bottom of the drain where it exits the air handler and the secondary pan you will see if there is enough space or if you have to make the trap outside of the pan.
 
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Old 09-16-12, 09:58 AM
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Thanks everyone, some food for thought, huh. I'll have to get up there and do some experimenting / measuring!
 
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Old 09-16-12, 10:22 AM
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I just re-read your original post and now noticed you said:

Why would the until be spitting out water through the secondary drain pipe into the pan
Are you referring to the upturned pipe as the secondary drain because you labelled the emergency drain pan as the secondary drain as well?
If so my ASSumption was wrong as the only way water would spit out that pipe was if the evaporator coil was mounted downstream of the fan.
You seem to be referring to two different pipes as the secondary drain.

For clarity you could refer to the upturned pipe as the secondary drain and the lower pan as the emergency drain pan as it is designed only to have water in the event of an overflowing drain pan.

Is your a/c coil up or downstream of the fan?
 
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Old 09-20-12, 09:56 PM
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Hmmm been a few days... thank you for your reply.

You are correct, of course. Both the home inspector as well as AC people that came referred to the drain that comes out of the house above the window as a "secondary". But I can see that this is likely not correct, as you say...

Water was essentially spitting out of the upturned pipe. let's call that a real seconday, and then it would sit in the pan and drain through the emergency drain over the window. I have actually noticed this behavior on BOTH units.

On the down or upstream question - I will have to admit that I do not know. Is this something that I need to open the units to see? :| I am a total AC unit newbie. I saw that manuals are on one of the units so I will locate them tomorrow and see if it says how to open it up.
 
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Old 09-22-12, 08:13 PM
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Wen up there today and, it seems like the coil is indeed downstream of the fan. Anything I should know about that setup?
 
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Old 09-23-12, 06:28 AM
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Because of being downstream the pressure in the duct will definitely cause water to blow out of the open drain ends.

You have an improper installation.
That slight curve in the main drain should be much more pronounced.
The air pressure in the duct where the drain exits the cabinet could be under a couple of inches of static pressure.
The measurement of inches air pressure is actually inches of water column and is a convenient measurement to helping you fix this problem.

The main drain line has to be removed and reinstalled so that the trap in the line is deeper than the static pressure in the duct.
IOW if you have a trap that is four inches deep and your cabinet has 2 inches of static pressure the water will not blow out the opening.
Also, the only open vent that you should have on your drain line must be after the trap.
Any openings before the trap will blow water and prevent it from going down the drain.

I would suggest at least four inches of trap depth.

Here is what you need to do:

 
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Old 09-28-12, 12:56 PM
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(came back from a week of business trip)

Thank you, what you say makes sense to me.

What I am confused about is the following:

There is actually quite a bit of space between the drain pan / emergency drain and the floor, so I should be able to do this.

What is unclear to me is if I should "deepen" the trap for the primary (it sounds like it) and then create the trap for the secondary, placing the trap before the secondary drains into the emergency pan?
 
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Old 09-29-12, 06:26 AM
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Ok
The primary drain which is labelled correctly in your diagram needs a trap as in the pic above.
4" in depth SHOULD be enough.
Just make sure that the pipe that exits the trap is below the line that enters it and be sure to allow 4" of water depth.
IOW if your pipe that exits the trap is 1" below the entering drain line, the first drop should be about 6' deep.

The purpose of the secondary drain which is mislabeled on your diagram and is the open stub beside the primary is so that you can pipe it to a visible location to be able to see if your drain is blocked.
If you are going to pipe this to the emergency pan then just remove this pipe and plug the hole.

The emergency pan does not need a trap.

Once you are done you will need to make sure that once the trap is full of water that it is able to drain without passing air through the water column.
If it does pass through the 4" depth of water than you have other problems.

There is a way to simply measure the static pressure if you wanted to go to the trouble.
Turn off the air handler and temporarily plug the primary drain so no air can escape and connect a clear hose to the secondary drain,seal the connection with tape and leave the hose end open.
Tape the clear hose to the side of the air handler so that you drop down from the secondary drain and back up so there is at least an 8" trap formed in the loop and the end of the hose is well above the trap.
Then add water to the clear trap so that you have water sitting to a depth of 8" or so.
Use a marker to indicate the height of water in the tube and then turn on the air handler.
Once the water finds a new level with the fan running mark the water level.

The difference in height is the static pressure in inches of water column ("wc) and what you use to determine your trap depth.
The trap should be at least 1" deeper than the static pressure.

or, just make it 4" and you should be ok.
 
 

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