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Old 03-08-2020, 08:14 PM   #1
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P-trap questions on HVAC systems. Steps to take prior to Cooling Season


OK.... I'm fairly certain I unserstand the function and need for a p-trap in my primary condensate drain line on my heat-pump home HVAC systtem. Apparently my unit is a negative-pressure draw-through design... and a p-trap in the condensate drain allows draining the forming condensate during the summer A/C operation months.... yet the water in the trap avoids atmosphere from the outside to be sucked-into the operating plenum of the A/C and air handling unit.

So the questions.

Right?.... condensate is produced only during the summer-months operation (A/C mode), where air is cooled and water is produced...right? Meaning... during the heating-duty time period in the winter months.... there is no condensate formed, and the p-trap can then evaporate over time, and then run dry over this operating time.... right?

So... as you transition from winter operation to summer operation, and you now intend to switch over to "Cooling-mode"... won't the p-trap (initially) be dry?... and if so...won't this be a problem on the first running of the A/C unit? There will be no water in the p-trap and inrush of air via the condensate drain line, into the plenum can splash the forming water onto all of the interior air-handling parts of the A/C system.... something a functionng P-trap avoids. So then this question...should the p-trap be "pre-charged" with water prior to entering the summer operating phase as a A/C unit?

This is a new home and new unit we're talking about...less than 3 years old, and I wonder if I should have done a "pre-charge" the prior 2 years as I enter the summer A/C season?... am I in danger of mold or other problems to have formed due to this inattentiveness?

?????

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Old 03-08-2020, 10:04 PM   #2
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


The drain should be cleaned prior to the start of the cooling season. It can be charged with water then.
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Old 03-08-2020, 10:36 PM   #3
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


If you have a modern natural gas condensing high efficiency furnace (most today are) it will produce condensate during heating, too.
We service ours annually. At that time we vacuum out the condensate line to clean it. Then it primes itself when it runs.
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Old 03-09-2020, 12:00 AM   #4
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


There won't be enough air volume and velocity to prevent the condensate from flowing out and filling the trap. Normally you will have no more than a .5" w.c. static and a 2" +- deep trap so not to worry about it getting sucked dry or staying dry at first of the season.
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Old 03-09-2020, 07:05 AM   #5
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


For record....this is an all-electric system. Southern home. To follow-on with surferdude2's comments...my core question was whether the unit even produces water during the winter months when it's in "heat" mode. It seems "no" it doesn't. Therefore seems to me by simple evaporation alone the trap will be dry after 4-6 months of running "heat" mode. From all the other responses....yep...seems best bet is to pull the trap, clean the trap, and vacuum out the line prior to cooling season, and then pre-charge the trap to avoid first runs "dry" during A/C season.... thanks !!!
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Old 03-09-2020, 07:33 AM   #6
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


My furnace produces much more condensate during heating than it does cooling.
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Old 03-09-2020, 01:33 PM   #7
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


@ cheeno.... for your furnace to produce a lot more water during the heating cycle compared to cooling...the only way this makes sense is if your furnace is a high-efficiency gas burning furnace, that has a secondary condensing heat exchanger in the design that draws out moisture to reduce flue temps.
Otherwise, that would make no sense ....... cold air cannot hold moisture as well as hot air, and the A/C cycle will be the only one that produces condensate...as is the case for an electric heating/Aircon system that I mention is what I have. Just trying to clarify....
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Old 03-09-2020, 03:09 PM   #8
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


I would vacuum out the line to clean it just before the cooling Season and forget about it.
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Old 03-09-2020, 03:15 PM   #9
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


That seems to be the consensus.... thanks all.
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Old 03-10-2020, 04:10 AM   #10
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


A small amount of condensate is produced during defrost cycles of your heat pump. Often not enough to keep the trap full.

Priming it before for cooling use is always a good idea.
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Old 03-10-2020, 08:21 AM   #11
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


Thanks for the added info.... I think we've got this..... cheers.
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Old 03-10-2020, 10:22 AM   #12
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


You don't need to vacuum it out. Just pour some water with bleach in it and run that thru the line to flush out any bacteria. Then run some fresh water thru it. That will prime the trap also.

If you forget to do it then it will prime itself when it stops after the first cycle. The water that is on the cooling coil will run down and fill the trap.

High efficiency furnaces can do that also and prime their own trap.

I would run the bleach thru it for bacteria reasons. If that is not a issue then some people may just want to vacuum it it/suck it out but in my experience bacteria builds in the line and within 5 years can plug it.

Rector Seal makes the best user friendly trap (EZ trap available on Amazon etc) plus Jennifer is cute in this video.

cheeno likes this.
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Last edited by yuri; 03-10-2020 at 10:26 AM.
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Old 03-10-2020, 10:44 AM   #13
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


Yuri:

Thank you for the added detail....but let me reply to your point as this discussion may help others besides helping me. Let me be clear....I have an all-electric heat pump system. No doubt... if the trap is dry as I start my cooling system (transitioning from heating season to cooling season), the first "run" under A/C mode will probably be enough to create water and fill the trap. However, if you do it this way, the very first run as an A/C unit, is still with a dry trap and you suck in unwanted air and possibly also splash forming water onto the internal components....something that can be avoided if you pre-fill the trap with water prior.

Then this....it is probably "good practice" to vacuum out the line (sucking from the house exit point) before the cooling season, regardless if there is a plugged obstruction or not. Doing this assures an open path as you enter the season...so "good deal" if you do this.

Now this point--> you suggest pouring a cleaner or bleach (followed by a water rinse) through the system. For this to be truly effective....you need to introduce this fluid "at the source" of the condensate piping system....which seems to me....to be at the insides of the unit where the condensate drain piping starts. There is an internal plenum and internal catch basin in the heat pump body.... at which this piping starts and is connected at its "origin". So the question....how do you get there to add cleaning fluid? Seems you need to take the furnace/cooling unit apart at this area to gain access. If you do this.... how is this done? Thanks.

Last edited by IPSC; 03-10-2020 at 10:48 AM.
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Old 03-10-2020, 10:58 AM   #14
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


You get the EZ trap installed.

You don't need to worry about the drain pan on the internal coil as 90% of the time it is the outlet that plugs right where it enters the trap that is the problem.

Followed by a plugged trap or line downstream. Mold/bacteria only grows in stagnant water and it won't sit and be stagnant in the pan drain as it evaporates there in off season or runs thru it fast in cooling season.

It is the off season with no water flow that is the problem.

Push a brush thru it and keep the trap clean and the drain line and yer golden.

With the EZ trap ( I need a sellers commission LOL) you can pour bleach and water thru. Plus it is clear and you can see if bacteria is growing in it.

You could also make a home made setup with a tee and cap which you remove to pour bleach in.

If you can make it fit then the EZ trap is very user friendly and convenient.

The one in the video has a optional plugged drain float switch tied into the circuit for the outdoor unit in case the line gets plugged. Will shut it off and prevent ceiling damage from overflowing.

An attic unit should have a secondary drain pan and line but it could also plug so the overflow switch or a wet switch ( water detector ) in the secondary pan is a good idea.

https://www.diversitech.com/en-US/category/cat_id/230

A great idea if you have a attic unit.

I doubt there is enough suction to suck water out off the drain pan and onto the coil. That is VERY rare and if you had that much suction then you have such poor airlfow that your coil would freeze when running and you would have another problem.
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Last edited by yuri; 03-10-2020 at 11:10 AM.
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Old 03-10-2020, 11:07 AM   #15
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Re: P-trap questions on HVAC systems. Steps to take prior to Cooling Season


Yuri.... you miss my point. I have a removable trap at the base of my house foundation outside the house. That is the "end" of the condensate line. I can likewise clean that trap or replace it with the EZ trap at that location. In either case...I would be cleaning at the far-end (exit) of the condensate line. To be effective....instead....you need to add cleaner at the ORIGIN of the line....at the heating/AC unit itself which sits in my attic. Therefore my question I posted above your latest response. I thought my excessive text made that clear?...how do you add bleach to clean the entire line?....I'm sure the material that causes the gunk and clog ...would exist throughout the line... not just at the end where my original trap (or replacement EX trap) would exist. I hope this is more clear.... thanks.
For added detail....see the many Youtube vids that speak to vacuuming the end of the line...here's one although the guy mis-spoke and calls the evaporator a condensor.....but the idea is still valid--->
It seems you may be saying add a EZ trap real close to my attic unit where the line originates.... I don't think many people will willfully cut into the line to add a trap there when there is one down-below at the point it exits the house.

Last edited by IPSC; 03-10-2020 at 11:12 AM.
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