Funky smell from AC drain pipe

We bought a new house three years ago and had a new AC unit installed in the bedroom. With a water drain going into the wall to get rid of the moisture that the AC takes out of the air. We have had occasional problems with a nasty smell in the bedroom since about two years and the AC install guys came here to check at least 6 times and they clean the AC but can't find anything wrong. We even had Mitsubishi come over to check if there was a problem with the AC itself, but they can't find anything wrong.
So I just tried to find the problem myself and found that the pipe going into the sewer looks like it has some growth on it. (see picture). This is what I believe the smell comes from. Unfortunately it is situated in a position I can't reach to take out the pipe and clean it without detaching the entire AC from the wall.
Can this growth be caused because there is no trap in the pipe somewhere? The pipe goes into the wall and I can't check, but when the AC is on for a while I hear the water drip in the shower drain which is on the other side of the wall where the AC is installed. It is not sewer guess though, more like fungus...
Can I seal the pipe from the top with something so the smell won't come up? Or should I just call a plumber and let them break the wall to put a trap in there? Any ideas would be highly appreciated.
Kind regards,
Ilan
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Most times the condensate-water removed from the air is blown into a fine mist by a fan in the AC unit. The mist is used to help cool the compressor. Any left-over water usually goes to an outside drain or just runs / drips out of the unit. If you have a drain hose, it must be a very large window unit. Sealing the hose to drain conection is worth a try, but it may just bring the smell into the window unit instead of in the wall. If you can make a trap out of routing the hose, then if you sealed the hose-drain connection, you wouldn't/ shouldn't get the smell.
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wrote:

All condensate drains should have a trap.
But be careful. If you do not make the trap legs the correct length, the condensate pan will not drain properly and will back up. You'll have worse problems. I see this frequently at work.
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You want the trap as shallow as possible so it is easy for the water to flow over the trap and continue on downhill.
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On Monday, June 11, 2012 2:14:09 PM UTC-4, hr(bob) snipped-for-privacy@att.net wrote:

-+
No, you don't.
You have a negative pressure inside the air handler.
You need the trap to be DEEP enough so that the fall overcomes the negative pressure. Otherwise, the slight vaccuum will be enough to keep the condensate from flowing through the trap.
Shallow traps are the most common construction mistake I see. And if the housekeeping pad isn't high enough, sometimes we have to core drill the floor to get a deep enough trap.
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"Usually" the "trap" for the condensate it just deep enough to make up for the pressure drop across the filter. If you change your filter in a timely matter, the pressure drop should be less than 1" of water. N.B.: it routinely dries out in winter.
It drains (either directly or through a condensate pump) to a plumbing connection with its own trap. That trap, if only for the HVAC is equivalent to that used for, say, the washing machine. Often, a condensate pump discharges through the washing machine drain.
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