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Locating an HVAC condensate drain in a Passive House

What is the best approach for dealing with condensate drains in a Passive House?

We will have 3 mini split cassettes on the main floor and one for the basement for the Passive House we are building in Maine. See our blog at www.EdgewaterHaus.com.

Options we have considered so far:
- Standard local practice is to bundle the condensate drain with the refrigerant coil and control wires directly outside. Unit drains in summer A/C, not in winter heating so no freezing concern. We don't like this approach because the condensate tube is an air pathway into the house.
- Tie the condensate drain into the interior perimeter drain in the basement. The perimeter drain will also serve as a radon vent if needed after construction is complete. We don't like this approach because it again leaves an air pathway, albeit underground to a storm water main, into the house. Unclear if code would even allow this.
- Tie the condensate drain into the waste line in the house. This is our preferred approach as it eliminates an outside penetration, and would use the plumbing trap to air seal the path back to the wall cassette. The downside is the need to snake a 3/4" line up to 30' with proper slope through interior walls to reach a plumbing waste line.

Are there other options? Are air admittance valves possible for use in the first two options?

Asked by Roger Normand
Posted Aug 23, 2012 10:33 PM ET


15 Answers

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I would go with #1. Your house is going to have some holes in it, this one makes sense, and it's very small. Besides, you'll have a trap in the line that should usually have water in it, forming a seal.

#2 sounds sorta OK but how do you know there won't be some funky air in that drain? #3, same issue but the air is guaranteed funky.

Answered by David Meiland
Posted Aug 24, 2012 12:02 AM ET


I agree with David. The air leakage through such a small drain line will do your house no harm. I think you are overthinking.

Answered by Martin Holladay
Posted Aug 24, 2012 6:04 AM ET
Edited Aug 24, 2012 6:06 AM ET.


Thanks David and Martin. I had not considered the possibility of putting a trap in a 3/4" condensate line before it exits the house. I expected it to be simply a direct line outside. I'll check with our plumber on doing so.

Answered by Roger Normand
Posted Aug 26, 2012 10:13 PM ET


a hole is hole, if it can be prevented do so (especially in a PH) - the trap will be dry in the winter too, so then leak air anyways, unless you fill it every few days.

so either option C, or save yourself the plumbing cost and drain your AC condensate in a nice big plant under the AC.

Answered by floris keverling buisman
Posted Aug 27, 2012 12:49 AM ET


Maybe there is something different about Mini-splits ???
Around here (N. Texas) we are required by code to drain condensate lines into the Sanitary Sewer...
The only condensate lines that I see draining to the outdoors are Overflow condensate

Answered by John Brooks
Posted Aug 27, 2012 8:51 AM ET


David, why would option 3 have funky air? There's a trap in the waste line.

I'm asking because that's how we plumbed the mini-split in our brand-new Passive House, and we've noticed an intermittent funky smell in parts of the house. My husband realized this morning that we haven't hooked up the washing machine yet and that the empty drainpipe is sticking out of our laundry room floor. I poured water in to seal the trap, so hopefully that will solve the problem. (The washing machine will get hooked up next week.)

Is there really a chance that funky air is coming in through the mini-split condensate drain? If so, wouldn't we have the same problem from our sinks, etc.?

Answered by Andrea Lemon
Posted Sep 1, 2012 2:39 PM ET


Mini splits may or may not like traps. The installation manuals seem to come out against them. My first install I put one in and it was fine. Two of the 3 in my current house cannot deal with them. My thought is that the units are so tightly packaged, and the drain pans so shallow that any restriction causes the fan to pick up water. Only a guess.

My experience makes me guess that if you can run straight down vertical as soon as the piping leaves the unit, a trap at the bottom will work. Much in the way of horizontal run may give you trouble.

It is a pretty small hole.....

Answered by Keith Gustafson
Posted Sep 2, 2012 4:05 PM ET


Andrea, my concern would be that the trap may dry out at some point.

Keith, do your units drain into the waste line?

Answered by David Meiland
Posted Sep 2, 2012 10:18 PM ET


Re David

no, outside

Answered by Keith Gustafson
Posted Sep 3, 2012 7:30 AM ET


Indeed, a dedicated trap could certainly dry out. That's why our mini-split shares a trap with the kitchen sink, which is not likely to dry out. Likewise, our HRV (which has its own trap) also uses the same trap as the washing machine.

(BTW, filling the trap for the not-yet-hooked-up washing machine seems to have eliminated the funky smell in our house, but I'll give it another week before declaring victory.)

Answered by Andrea Lemon
Posted Sep 3, 2012 8:42 AM ET


Andrea, sounds like you used a garbage disposal tailpiece at the sink but connected the condensate drain instead. Seems like a good strategy, assuming the outlet opening doesn't gunk up over time.

Answered by David Meiland
Posted Sep 3, 2012 10:18 AM ET


Keith has first advice: check mfgr instructions on traps (recommended methods/ already internal to unit). If you really can't live with the small holes: #1 Tie to closest lavatory or sink trap arm (a common detail); indirect (air gap to a laundry sink if allowed locally), or just combine all 3 units at a single condensate pump and run to a convenient/trapped location. Sure the pump uses energy, but how much run time will you be having in Maine?. If you want to trap it, advice to do so after vertical run is good.

Answered by Mark Heizer
Posted Sep 5, 2012 6:04 PM ET


I am struggling with the same question for our mini splits. It has been suggested to me that I install a check valve in the condensation line before we terminate it in our sealed sump pump basin. Would this be an option?

Answered by john bell
Posted Dec 22, 2012 7:55 AM ET


I am betting it will not drain correctly. If it is in a sump it is not outside so no problem. right?

Answered by Keith Gustafson
Posted Dec 22, 2012 8:43 AM ET


If the exit side of a U-trap located inside is lower than the
coil tray of the minisplit, is there any reason to believe
that it wouldn't drain properly? For heating season you could
simply wet-vac out the whole drain and tray, and plug the tube
to the outdoors to eliminate the air leak.


Answered by Hobbit _
Posted Dec 23, 2012 1:00 PM ET

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