Intermittent clogged drain issue

Hi,
I have a small condo, approx 25 yrs old, where in the past year has developed an intermittent drainage problem. on occasion, the drain pipe on the airhandler fills and the tray begins to over fill. I have had an HVAC technicians out three times to fix the issue. once they forced water thru the piping and we saw the water coming out with sludge buildup. another time he put some "pills" that I think where to disolve the buildup. on the third occasion he forced water again but it came out the other end without any buildup. all three times it seemed the problem was cleared up but eventually would return in two weeks or so. I have an electric water heater under the airhandler that I've replaced twice becasue of the ewater overflowing onto it and frying it.
The condo is on a first floor of a three story bldg. and the three units on the 1st, 2nd, and 3rd floors share the drainage pipe but the they are not experiencing problems I am. I am in florida and need to use the AC most of the time. like I said, the problem is intermittent. about every two weeks or so this occurs.
Do the following make sense?
1. hook up a shop VAC and suck out waterever may be trapped in there.
2. could and animal be crawling up into the pipe, explaining why this is intermittent?
3. This sounds crazy but, could this be a vacuum problem where maybe the pipe is not clogged but instead the pipe is acting in tha same manner as how a straw retains water when you have you finger covering one end?
Or any troubleshooting ideas?
Thanks in advance for your replies.
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This is Turtle.
Sounds like you don't have a P-trap in the drain line or you have 2 P-Traps installed someway in the drain line. this is a very common occurance in the Condo and apartment HVAC system when they are not installed correctly.
And no don't tell me it can't be this for it has worked for years and just not started it. ome system will work for years with out the P-Trap but out of the Blue it will styart doing it.
P-Trap is a part of the drain that look like the S drain section under your sink to retain water to keep vapor from coming back out of sewer line.
TURTLE
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It's also possible that there is an inadequate P-Trap. I've seen countless P-Traps that are more of a token attempt at imitating what some "tech" thinks is supposed to be there but has no idea why or what it does. It seldom hurts to have too much of a trap but can be useless to have too little of a trap.
Steve www.refrigerationbasics.com
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this is Turtle.
I see what you are saing all the time around here and is the major cause of the problem.
My ideal was the P-Trap was not there or you have 2 --- P-traps installed in some unknowing way like a big run to the sewer and has a low spot in it that water covers the inside of the pipe to make another P-Trap. 2 P-Traps will make a plug during high water levels being drained out of the system. Yes I did say 2 --- P-Traps makes a plug.
TURTLE
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J wrote:

buildup, in mine it looks like oysters shooting out when i put the hose into it to flush it out... the pills he puts in are nothing but clorine.. it kills the algea just like the stuff they put in swimming pools and this stops the algea from building up.... dont know what to say other than getting another person to do it and he might do it correctly or do it longer and get rid of more stuff, you might have a blockage at the end and might be able to clear it and put some screen on the end to stop bugs etc from getting in???? but gotta be careful as not to block it with the screen....
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One thing not addressed in the replies so far is to be sure there is proper pitch from the air handler down to the drain. If it's lying flat (and since Florida is both warm and humid) you may have some constant build up if the water is not able to continuously flow.
One thing I'm curious about - do you not have an overflow pan beneath this unit? I'm taking from what you wrote in your original post that this is your primary drain. If this is your secondary (emergency overflow) drain your initial concern would be to clear the primary first. In either case - they should both be functioning properly and you should never have had a fried water heater.
Jim

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