P-Trap On AC Evap Condensate Outlet??? - HVAC - DIY Chatroom Home Improvement Forum

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Old 07-16-2012, 11:09 PM   #1
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P-Trap on AC Evap Condensate Outlet???

My blower motor on my AC evaporator unit in the attic crapped out today (see thread "Heil A/C Evaporator Blower Noisy").

Anyway the HVAC ktr I called this evening (who I trust so far...) pointed out there is no P-Trap on the condensate drain line and he says its lack is causing water to remain in the pan under the evaporator unit.

I don't get this. When I questioned him on why I need a P-trap he said with out it the pan is loosing suction. Really???

I regularly check for condensate output as the outside drain is near my pool equipment. Once previously, I let the air filter go too long and had the A-coil freeze up. Lately, there has been no shortage of water dripping outside and the pan in the attic is not overflowing. There is water in the pan at all times however.

So; is a P-trap on the outlet from the drip pan under the evaporator needed? - or just a bill-padder? How would a P-trap pull more water out of the pan than just gravity?


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Old 07-17-2012, 12:09 AM   #2
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if you go so many years w/o it, why you care? mine one has no p-trap. I don't think it works anyway.
1. suppose no trap, as far as water level higher then output hole it drains
2. suppose you have the trap, as far as the water level lower then the output hole, air goes it and no suck any more
so no difference
3. for your blower, it may just be the bearings bad, get same bearings woudl be possible.


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Old 07-17-2012, 12:12 AM   #3
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Fancoils should have a ptrap for proper drainage.

If the drainline slope is bad enough the dips can also form a trap

Last edited by Houston204; 07-17-2012 at 12:18 AM.
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Old 07-17-2012, 06:44 AM   #4
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as long as there is one on the air handler itself the emergency pan doesn't need one but should be piped poout seperate to the front stop or back yard patio..so if you see water you know it is overflowing.there should be no water in the secondary pan under the evaporator.air handler..
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Old 07-17-2012, 09:56 AM   #5
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The p trap is installed so that no air can enter your system from the outside. some drains are conntected to sewer stacks and some just run on the ground. I can tell you from experiance that no ptrap (or one that is too shallow or installed wrong) could allow sewer gas, dust and or hot/cold air into the building. Also note that in parts of the country where this is very low humidity and a very shallow ptrap, the water held in the trap may not be enough to ride out the dry spells. a deep ptrap is better then a shallow one. Also, there must be a vent of some sort after the trap, water must have air behind it to move.....
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Old 07-17-2012, 10:51 AM   #6
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If you have a coil/drain that is AFTER the blower then you often don't need a trap, and in fact trapping coils like that can sometimes cause troubles.

If you have a coil/drain that is BEFORE the blower then you need a trap, otherwise you may find that the drain won't work at time (the blower will try to suck air back up the drain pipe, perhaps, along with odors or such).


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