# Furnace / AC condensate drain p-trap



## PBH (Nov 7, 2007)

Who's the resident expert on plumbing / HVAC?

I've got a couple units in my attic -- unconditioned space.
I had to go into the attic the other day to run some wire for my sprinkler controller. I noticed water on the platform under one of my units. Upon closer inspection, I could see that the p-trap on the condensate line was broken:



















Luckily, we haven't had a lot of humidity until recently, and the water hadn't made it past the osb board. I cut the section of pipe out, and replaced it with a new p-trap.

I figure that the break probably happened last winter -- the water most likely froze in the trap and broke the pipe.
So, my question is: what to do to prevent this from happening in the future?

Options:
A. do nothing. check it periodically. One unit is easy to check -- just open the man-door and look in. The other unit, on the other side of the house, requires the extension ladder, climbing into the attic, and checking the other unit. And I have to remember to do it.

B. Heat tape. This comes with additional expenses (electricity) and risks (potential fire).

C. replace p-trap with pex pipe. This might help. Pex doesn't break as easily as pvc. But it doesn't really prevent the water from potentially freezing.

D. Remove the p-trap. My only concern here is what I've read on the interwebs. searching for "condensate drain trap", you can find numerous articles explaining negative air pressure sucking air back through the drain and causing more problems with the system / water.







Recommendations? Let me have it...


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## wyogoob (Sep 7, 2007)

I recommend just letting the condensate drip onto a cookie sheet, remove the P-trap.


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## DallanC (Jan 13, 2009)

Or you could drill and tap in a nipple at the bottom of the P, put on cheap 1/4" line and run it down into a more accessible spot, then put a twist valve on that to just drain it as we get into colder water.

-DallanC


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## backcountry (May 19, 2016)

So it's serving a dual purpose of AC condensate & furnace condensate?


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## PBH (Nov 7, 2007)

BC -- I guess...
It's coming out of the furnace.


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## KineKilla (Jan 28, 2011)

I'd see if I could get rid of the trap and just run a sloped pipe to the drain or wherever it goes. 

P traps serve to keep sewer gasses from coming out of your drains. Unless this condensate line is tied directly into a sewer line or area drain without a trap of its own it isn't doing anything but creating a valley where water sits and then freezes.

I guess they could serve to keep some air from blowing out of the pipe instead of into the house where it's wanted. I think the air loss would be minimal.

Another option is flexible hose in lieu of rigid. This won't stop the water from freezing and becoming an obstruction.

My $.02

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## PBH (Nov 7, 2007)

KineKilla said:


> I'd see if I could get rid of the trap and just run a sloped pipe to the drain or wherever it goes.
> 
> P traps serve to keep sewer gasses from coming out of your drains. Unless this condensate line is tied directly into a sewer line or area drain without a trap of its own it isn't doing anything but creating a valley where water sits and then freezes.
> 
> I guess they could serve to keep some air from blowing out of the pipe instead of into the house where it's wanted. I think the air loss would be minimal.



so, in your opinion, the ptrap isn't necessary. What about the video I posted? Any validity to that?



KineKilla said:


> Another option is flexible hose in lieu of rigid. This won't stop the water from freezing and becoming an obstruction.


I know pex isn't flexible, but it certainly is better with freezing (won't break, has memory, etc.). So maybe convert the p-trap to pex?

maybe the flexible hose (rubber) is the best option. A rubber would be cheap, and easily replaceable....


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## hunting777 (May 3, 2009)

I agree with removing it completely. At my place of work, we had those as well. We always had problems with them building up algae. It is always stagnate water. They would plug up and then the condensate would overflow and leak through the ceilings all the time. We replumbed with a slight slope and haven't had problems since.


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## backcountry (May 19, 2016)

PBH said:


> BC -- I guess...
> It's coming out of the furnace.


That will greatly affect freezing, is the primary reason I ask. 

I would recommend finding the manual and/or installation instructions for your unit. It's new enough that the decision could greatly affect performance. 

Problem is people are all over the place on condensate line design in S. Utah. I couldn't find anyone willing to problem solve mine, but it was a furnace line only. If you know the install team and trust their design then I would not remove it. Otherwise read the instructions to see if it even needs one in the first place; some systems have an internal trap as part of the design. 

If it is removable I would expect to still get some freezing during the winter, but you can probably prevent bursting via slope. I had the opposite problem as my furnace is below grade and I had to pump the condensate up into first floor for drainage. Whoever installed it just let condensate drip into crawlspace.


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## DallanC (Jan 13, 2009)

You could always leave the trap, and run one of those heat cords back up through the pipe. They have a thermostat so they only turn on when its below 34F or whatever. That solves all the issues... assuming you have a place to plug it in.

-DallanC


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## backcountry (May 19, 2016)

I just read the install instructions on my furnace and it recommends self regulating heat tape on the condensate line if it's in an unconditioned space. Could be a cheap & easy solution.

I've never used the stuff, might be the same as what Dallan recommends, though it sounds like it runs on the outside of pipe.


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## PBH (Nov 7, 2007)

the problem with heat tape is just the increased risk of fire if they fail --ie: if the thermostat fails, or if the pipe dries out, etc. We use heat tape all over here at work -- and I'm just not too excited about adding it.

It's kind of similar to IT security -- when trying to find a solution to one risk we introduce another risk. So, no heat tape = potential water damage due to frozen pipe. Add heat tape = potential fire danger. 🤷‍♂️


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## Critter (Mar 20, 2010)

After doing a Google search for information on this I came to the conclusion that I would contact a contractor or whoever installed the HVAC system in your home and talk to them about it. 

Sometimes talking to a professional is the best way to go about fixing a problem.


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## DallanC (Jan 13, 2009)

Wrap the pipe in thick foam pipe insulation then?

-DallanC


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## DreadedBowHunter (Sep 22, 2021)

It’s not enough condensation to need the trap. A Tupperware plastic is what I use and all I ever see in it is scale 🤷🏼‍♂️


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## DallanC (Jan 13, 2009)

The fact it froze and shattered like that shows there is quite a bit of water pooling up.

-DallanC


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## KineKilla (Jan 28, 2011)

This thread got me thinking so I looked at mine. Almost brand new furnace and a/c, installed in April or May.

You will notice that there is no P trap. When you put your hand over the upper pipe you feel air coming out, not being sucked in.

Get rid of the trap, maintain adequate slope and be done with it. That's my opinion anyways.
















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## MrShane (Jul 21, 2019)

Just be grateful your electrician screwed up so badly on the irrigation controller power location that it forced you to go in to attic to run direct bury wire.
It helped you find a problem that could of ended up being extremely costly.
You are actually very blessed!


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## PBH (Nov 7, 2007)

So, I checked the other unit on the other end of the house (in attic). That unit has no trap.

The user manuals on the furnace show "non condensing furnace".

I think what I might do is just move the trap so that it's sitting inside the tray -- that way if it ever breaks again and I don't notice it, it will at least leak into the tray, then still flow out the overflow.

I could do what Critter mentioned, and actually talk to a professional and ask them the what's and why's....


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## backcountry (May 19, 2016)

Sounds like you found a solution.

Out of curiosity, why do you think the p trap broke if its actually a non condensing furnace? Did the installers prime the trap and that water
froze?


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## DallanC (Jan 13, 2009)

PBH said:


> The user manuals on the furnace show "non condensing furnace".


So you might have another issue, if you had enough water in the trap to cause it to freeze and break. It doesn't sound "non condensing" to me. Thats a pretty big volume within the elbow to have been full of water.

-DallanC


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## PBH (Nov 7, 2007)

trying to learn more....
...I'm no HVAC expert.


So, here's what I'm figuring out: I have a Bryant furnace (800SA series non-condensing) with an attached 3.5 ton evaporator coil (CNPH series). It's the evaporator coil that has the drain lines. I found the manual for this evaporator. It states under the Condensate Drain Line Connection section:


Condensate Drain Line Connection said:


> A trap is not necessary on the condensate line. Consult local codes for additional restrictions or precautions...
> 
> ...If unit is located in or above a living space where damage may result from condensate overflow, a field−supplied, external condensate pan should be installed underneath the entire unit ...
> 
> ...


My unit is in the attic, with two drain lines (primary and secondary) which are both plumbed to the outside of the house. The secondary is located where it would be noticeable.
The trap has nothing to do with air-flow as suggested by the youtube video in my opening post. It is only necessary if draining to sewer to prevent gasses -- like any other p-trap in your house. If I'm draining to the outside, no need for trap.

So, from this doc -- I don't need a trap.
I'm going to remove it.


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## tuffluckdriller (May 27, 2009)

The purpose of the p-trap on both the furnace and the A/C coil is to keep it from sucking in dust. If it were me, I'd use vinyl tubing that can freeze and not break. I'd also do the vent after the trap, so siphoning doesn't suck the trap dry. It's not just for sewer gas... But whichever you want. I'd also make sure there's enough vertical drop from the entrance to the trap to the exit so that it doesn't get sucked back into the unit.


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## mtnwldman (Mar 23, 2014)

I have been doing commercial HVAC for 28 years and have seen many units on roof tops with no drain attached to the unit at all and they have no problems at all. So I would just run a straight drain line out to avoid any drips that might ruin your ceiling


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## wyogoob (Sep 7, 2007)

KineKilla said:


> This thread got me thinking so I looked at mine. Almost brand new furnace and a/c, installed in April or May.
> 
> You will notice that there is no P trap. When you put your hand over the upper pipe you feel air coming out, not being sucked in.
> 
> ...


Yeah. Some of this stuff is Building Code regs. 

In high-humidity Illinois condensate traps n drains for air conditioners were a big deal, essential. Here in the low-humidity southwest Wyoming part of Utah they're not needed.


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