I have a rental with an air conditioning condensate drain issue.

Here is a picture of how the 3/4" PVC drain is configured to run to the condensate pump. As it exits the air handler near the bottom it turns to the right (it cannot slope down too much because the filter change compartment is right below it}, then it does a horizontal 90 towards the wall, then down to a home made P-trap before draining into the pump. This has worked fine for years with no issue.







I recently had a new tenant moved in and she decided to put a few hangers on the horizontal 90 elbow that turns towards the wall, this put weight on the elbow and pulled it down a little. Then the condensate line backs up and caused a spill in the drain pan and water leaked to the floor.

I went to check it out and initially assumed the condensate line was clogged from hair/dust accumulation even though I pour bleach down the line every 30 days. But when I removed the hangers from the elbow and noticed that corner has an adversed slope, I gently nudged the elbow upward by a 1/4" and suddenly I heard some gurgling and all the water in the pipe drained out and the pump activated to clear the pump.

So there is no debris, just some kind of air trapped somewhere in that elbow that sagged? This is kind of like having two traps in series?

I put a two hole strap to hold the elbow in place for now, but am thinking may be change the trap to a running trap and drop straight into the pump instead of the convoluted p-trap that is there now.

you have eliminated the double trap by securing the pipe with the clamp

you are done



a 'running trap' offers no advantage over the existing 'p' trap

possibly a disadvantage as the water seal 'may' be too shallow

the trap seal needs to be 2" PLUS the maximum 'suction head' of the air handler's fan with a clogged filter

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

I used to have the strap much further back, and when my tenant used the PVC elbow to hang clothes it shifted down while the strapped point became a "hinge". I need to put another strap in the back side to stop the rear section from being lifted higher.

I am just surprised the 1/4" shift would totally stop the draining.

formed an air pocket which the head pressure of the condensate could not 'move' downwards


IF the pipe, which can't because of the filter rack, went straight down then it would have enough 'head pressure' to move the air pocket along



you are good to go as long as you secure the piping maintaining sliiiiight pitch downwards

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



Edited 1 times.

Get rid of the trap at the pump. It does NOTHING and serves no purpose. Drop the line directly into the pump.

do NOT, repeat NOT, get rid of the trap

it DOES, repeat DOES, serve an IMPORTANT purpose


without the trap the air handler will suck in air PREVENTING the condensate pan from draining


then you will have PERPETUAL flooding from the GUARANTTEED condensate leakage

see:
[www.csemag.com]

and (as per Trane):



in my own home:






the purpose of a condensate trap on a condensate pan drain is to stop AIR FLOW but permit water flow

ps. (if, however, the pan/coil is on the DISCHARGE side of the blower with no trap it will not 'leak', but WASTE a LOT of cooled of air blowing out)

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



Edited 2 times.

I agree with a lot of what you said, bern, but the suction in the air handler will prevent,or at least hinder, drainage, even with the trap. The trap makes no difference. Any accumulated water in the pan will drain out once the circulation fan stops.
I agree trap prevents wasted cooled air.

wrong wrong wrong

the pan BECAUSE of the trap drains CONTINUOUSLY while the fan is running

else how could the system drain on a continuous operating hot day ?? !!!!!!!!!



i will bet my home that what i have stated is correct


i have multiple decades of HVAC piping experience including commercial work


(ex. fitter 638 HVAC member)



The trap MUST, repeat MUST, start 2" + the static head suction" BELOW the outlet of the pan allowing for the static head of the condensate to overcome the 'pull' of the fan.

That is why sum's trap placement is actually ideal PROVIDING the horizontal run under the filter housing pitches downward (even sliiiiightly)


The longer the vertical drop before/into the trap the better.


All the above assumes the coil is located on the suction side of the fan, which in residential units is virtually always the case.



This case is NOT subject to argument or disagreement but is the ONLY proper way to pipe the condensate drain from a cooling coil inside an air handler.




==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



Edited 1 times.

Let me put it another way - the effect of suction is the same whether there is a trap or not. Suction is acting against the head pressure causing drainage - the difference in height between the inlet to the drain (which is the pan fitting) and the discharge of the drain, and this pressure (measured in inches of water column) is the same whether there is a trap or not.

As an example, let us suppose the difference between inlet and discharge is 6 ins and there is 3 ins WC suction. There is only 3 ins WC propelling the movement of water in the drain pipe.

All this is assuming there are no air pockets in the drain.

you miss the key point

assuming the pan is on the suction side of the fan:

without the water seal of the trap to prevent the rush of air being 'pulled' into the unit there would be NO, repeat NO, flow of condensate out of the pan

if you were to remove all drainage piping from the condensate pan 'tapping' there would be NO condensate outflow with the fan running - if enough condensate is formed to fill the pan before an 'off cycle' occurs the pan would simply overflow ALL AROUND (if level) - the incoming 'rush' of air would simply prevent outflow - unless the pan itself was 2-4" deep - residential pans are approx. 1/2" - 3/4" deep relative to the internal invert of the attached drain piping

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

{quote}

One afternoon a mechanical contractor reported a situation on a project. He said the cooling coil drain pan inside the air handler was flooding. He also advised that the condensate pipe was not draining any water. We were surprised and couldn’t think about what could have gone wrong. We went to the site and found that drain pan and the air handling unit floor was flooded and the fan inside the air handler was wet. When the coil section door was opened, however, the water in the pan drained very quickly.

We had studied the design of the condensate trap. We inspected the trap to figure that the difference in elevation between the AHU drain pan outlet and the exiting end of the U trap (termed here “H”) was zero. The reason this was happening was the coil section was at negative pressure since this was a draw through AHU. Since the “H” of the trap was zero, the suction pressure through the U trap inhibited the draining of any water. This leads to the accumulation of water in the drain pan and resulting flooding inside the AHU (see Figure 1).

Once we fixed the trap, the problem went away. It came at the cost of $2,000, which included the cost to raise the AHU by 4 inches. The client was not happy about the change. There have been other cases where the AHU had to be raised, or condensate pumps had to be installed because the details were not thought out during design. The cost of these changes could be in the tens of thousands of dollars.

Ignoring coil condensate design makes engineers look incompetent. It can cause health issues because of mold and algae growth when not noticed early. Sizing the condensate trap is commonly overlooked, and there is no good literature that covers all aspects of design.

{end quote}

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

OK I am still trying to understand this.

I am the one who piped the original P-trap. All of my rental HVAC has P-traps on the condensate lines.

However, some of my condensate lines are gravity piped outside, and some are piped to a condensate pump that pumps up and out if the location is far from an exterior wall.

On the condensate lines that goes out, most of them have a P-trap very close to the handler unit. A few of them I placed the P-trap where the condensate pipe exits the exterior wall, on the outside. That means those P-traps can be 10+ feet away from the air handler. Does this distance matter?

On the ones with condensate pumps, I have always put in a P-trap but I thought it was unnecessary, I thought the reason for a P-trap is to keep out sewer gas and to prevent insect from crawling from the outside in. With a condensate pump there is no gas and no insect, so I thought by adding a P-trap it was kind of redundant.

But now Bernabeu shared this information which I have read but still need to digest.

My air handlers suck the air through the filter, then the coil. There is always a drain pan around the perimeter of the unit, the water comes out of one of the 3/4" outlets and that water gets carried away. On newer units they have a TEE pointing up with a float switch, once that pipe is full and the float switch activates the unit shuts off to prevent more water being leaked. I am still trying to understand how the suction of the air handler relates to the P-trap, is the suction happening at the air handler creating a negative pressure in the condensate piping?

yes, the fan is creating a negative pressure in the condensate drain piping

said neg. press. will pull air AGAINST and PREVENT the (exit) flow of condensate


the trap stops said air flow

the trap must be designed in such a manner that the head pressure of the condensate overcomes the 'suction lift' of the fan



ps. the tee and float assembly you mention is there for a 'broken system' to prevent flooding caused by blocked or improperly installed condensate drain - it should NOT be 'controlling' the AC but merely act as an emergency stop




for the typical residential air handler:

[th.bing.com]




ps. @sum, your install will work very well

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



Edited 1 times.

to further clarify the overflow switch connection:




(the overflow drain tapping has a built-in dam)

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

I understand what the float switch does and how it works, and I had a few occasions where I had to bypass a float switch to get the AC unit working. It seems some of the newer float switches are "disposable" in that they cannot be reset. So if a tenant is curious and removed the float switch to look at it, like turn it upside down so the contact is made, well, that's it, the unit is shut off. The entire float switch is one piece that cannot be reset and it has to be replaced. So the only thing I can do is to bypass the float switch while I go get a replacement float switch.

Thinking back on the air handling unit creating a negative pressure, I get that but in my mind I cannot imagine it would generate enough negative pressure against the condensate pipe because the much larger area where the air passing through the filter (a 20x20, 20X16, 24X20 etc area) and it seems whatever pressure acting on the water in the pipe would be negligible in comparison. But I guess if the filter becomes dirty and clogged, the pressure will be higher on the pipes.

So the other question is the location of the P-trap. Does it matter if that P-trap is near the handler unit like the set up I have in this thread vs much further away where the condensate pipe exits the house say 15' downstream and the P-trap is after the pipe exits the house?

4' is the manufacturer's recommendation

IN THE SAME ROOM




the static head is just like brake fluid - the pressure per square unit (albeit measured in terms of water column) does not vary because of the piping size or the 'size' of the filter

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638