# need advice on condensate P-trap design



## ClimateCreator (Nov 30, 2017)

Just use this https://amzn.to/2xyIEnU it's cheaper in the long run, easier, and MOST important, it's SERVICEABLE! 

They need cleaning periodically and this guy let's you into the right spots to do so, and it's CLEAR so you can see if it's in need of cleaning as well.

CC


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## paul_k (Jul 28, 2017)

ClimateCreator said:


> Just use this https://amzn.to/2xyIEnU it's cheaper in the long run, easier, and MOST important, it's SERVICEABLE!
> 
> They need cleaning periodically and this guy let's you into the right spots to do so, and it's CLEAR so you can see if it's in need of cleaning as well.
> 
> CC


Thanks for the reply. Are you saying that there is no reason to account for the *amount* of negative pressure at the drain? This Rectorseal seems to be a "one size fits all" P trap but would it still work in extreme cases, i.e. *very small* and *very large* negative pressures?


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## beenthere (Oct 11, 2008)

No, they don't work in high negative systems. So if you have a system with more than 1" pressure, it won't work.


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## ClimateCreator (Nov 30, 2017)

Super high pressures no, but it's uncommon.....I have yet to find a system it didn't work on. We even use them on large commercial rooftop units that move more air than a regular residential unit. 

Do you have some reason to think your system is different than everyone else's? 

CC


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## paul_k (Jul 28, 2017)

ClimateCreator said:


> Super high pressures no, but it's uncommon.....I have yet to find a system it didn't work on. We even use them on large commercial rooftop units that move more air than a regular residential unit.
> 
> Do you have some reason to think your system is different than everyone else's?
> 
> CC


not sure ... it is only a 1.5 ton air handler so possibly very small negative pressure, so if I understand correctly that wouldn´t *pull* the water out of the trap, which is good. 

But can a trap be *too deep* to the point that the condensate can´t overcome the weight of the trapped water to flow out? If so, how do you determine that?


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## paul_k (Jul 28, 2017)

beenthere said:


> No, they don't work in high negative systems. So if you have a system with more than 1" pressure, it won't work.


Ok, thanks. I don´t know how to measure that but would you expect a 1.5 ton air handler to have less than 1" of negative pressure? Or are there other factors beside tonnage?


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## Gregsoldtruck79 (Dec 21, 2017)

Unless the AHU units blower is set on manual or is one of those energy recovery units where the blower is running full time, I cannot see a trap having too much negative pressure on it full time. 

To me, the evap coils condensate water drains from its pan with the most volume, whenever the blower shuts down after the zone being cooled has reached its set point .....and it shuts down. JMO


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## beenthere (Oct 11, 2008)

paul_k said:


> not sure ... it is only a 1.5 ton air handler so possibly very small negative pressure, so if I understand correctly that wouldn´t *pull* the water out of the trap, which is good.
> 
> But can a trap be *too deep* to the point that the condensate can´t overcome the weight of the trapped water to flow out? If so, how do you determine that?



Liquid always searches for/seeks its own level. So not possible to be too deep that it prevents water from flowing out.


The difference in height of the bottom of the trap inlet and the trap outlet is the max amount of negative pressure you can have, and water still flow out.


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## beenthere (Oct 11, 2008)

paul_k said:


> Ok, thanks. I don´t know how to measure that but would you expect a 1.5 ton air handler to have less than 1" of negative pressure? Or are there other factors beside tonnage?



A residential unit should always have less then 1" negative pressure. However, a slow drain can cause the water to rise higher then liked in the trap, and a dirty air filter can give a negative pressure in excess of .5". Causing a drain pan over flow. When the filter is cleaned/changed on a good regular basis, and the trap cleaned out every year before use. There is generally no problem.


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## paul_k (Jul 28, 2017)

beenthere said:


> Liquid always searches for/seeks its own level. So not possible to be too deep that it prevents water from flowing out.
> 
> 
> The difference in height of the bottom of the trap inlet and the trap outlet is the max amount of negative pressure you can have, and water still flow out.


My AHU is built into a wall and the only access to the drain pan is from the bottom of the cabinet, through the air return slot. So, I am forced to pipe the condensate drain from the bottom and probably will need to extend the vertical pipe as shown in fig C below. Will this still work with the increased H1 height? Can I also assume that after the condensate stops flowing the water in the trap will remain at the same "J" level?


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## beenthere (Oct 11, 2008)

That will work fine. Use a tee on the outlet side of the trap instead of an ell so you can clean it periodically.


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## paul_k (Jul 28, 2017)

beenthere said:


> That will work fine. Use a tee on the outlet side of the trap instead of an ell so you can clean it periodically.


should I duplicate the trap for the *secondary drain*, or is there a better way to deal with that?


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## beenthere (Oct 11, 2008)

Secondary can be the same, or shallower. As it shouldn't have water in it. Unless the primary clogs/restricts.


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