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Water Heater Drain for Pier and Beam Foundation

user-322250 | Posted in General Questions on

Preventing Air Leaks Thru Hot Water Heater Drain Pan Connection To Exterior

My house currently does not have a water heater drain pan. I would like to add one. I also plan to install a heat pump water heater in the future so I will also need a condensate drain at some point.

The house is on a raised pier and beam foundation and is open underneath (no skirt) with plenty of clearance to ground. By far the easiest way to implement a drain would be to run a line thru the floor to the exterior.

I would like to prevent a draft (air leak to conditioned space) through the drain and I am unclear how this type of situation is typically handled. It seems like the options are either a conventional trap (e.g. P trap) or waterless trap of some sort.

The conventional trap would need to be located inside the building envelope (to prevent freezing) and this is pretty inconvenient because it means that the water heater pan need to be elevated to allow height for the trap above the floor. The trap would likely need to be refilled periodically to deal with evaporation.

Googling around I see some waterless trap designs that appear to be intended for use in drains. Many of the products are from companies that I have never heard of and (as far as I can tell) don’t have relevant certifications. The less sketchy looking products are pricey. It would be a bummer to install one of these guys and have it fail closed 5 years from now when the water heater finally starts leaking and the drain pan is called to do it duty for the first time.

My questions:

1. What is typically done in this situation to prevent air leakage thru the drain ? Is there a solution I am missing ?
2. Is a waterless trap a reliable solution here ? Anyone have experience with products (hopefully not crazy expensive) they could recommend ?
3. Is the calculus any different for a condensate drain (which I will also eventually need) vs drain pan drain ? Airless traps seem to be somewhat more common for condensate lines. Any experience positive or negative with airless traps in the context of condensate lines specifically ?

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  1. Expert Member
    DCcontrarian | | #1

    Fill the trap with mineral oil. It won't evaporate or freeze. You can put it wherever you want. If the drain actually gets used refilling the trap will be the least of your worries.

  2. nynick | | #2

    My plumber ran a 1" PVC pipe through the wall with an elbow outside point it downwards. Inside the house, he took the 1/2'" rubber condensation line from the Heat Pump Water Heater and stuck it into this pipe. This left room around the condensation line for me to stuff mineral wool to block the cold air from coming in. I suppose a dab of spray foam would work as well.

    He did this because condensation lines can freeze here if just run to the outside. This has happened to me before in another house. I thought this was a neat solution but also gave me the opportunity to block an air leak.

  3. user-322250 | | #3

    Thanks for the responses.

    @DC Contrarian: The oil trick is clever. Poking around online, I see companies selling jugs of some sort of non volatile freezeproof liquid that you can pour in your drain to similar effect. With this type of approach, I would be worried about normal differentials in air pressure causing dribs and drabs of oil to slosh out of the trap from time to time making a mess outside where the drain empties (think about toilet water sloshing around in the bowl when there is a strong wind). Also, I can't use this trick for the condensate drain.

    @nynick: Not sure but I think your air leak may be different than what I am worried about. I am concerned about an air leak between the condensate pan and outside (or in the case of the water heater drain pan, the drain pan and outside). I am looking for a solution that prevents or limits air movement thru the the drain line but allows water to move thru the line.

    1. Expert Member
      DCcontrarian | | #5

      The condensate line should have a condensate line trap. Google it.

  4. Expert Member
    DCcontrarian | | #4

    I've lived through several hurricanes and never seen toilet water sloshing in a strong wind.

  5. Hockey | | #6

    I solved a similar situation with RV anti-freeze, which is non-toxic since it is used in RV water lines. Unlike DC’s mineral oil, there will be evaporation so the trap will need to be primed occasionally. My situation, I have 3 air handlers in a vented attic (bad, I know), the water in the condensate line traps evaporates in the Fall so I don’t need to worry about freezing temps (I’m in NY) and cracked lines. However, I have hot water coils attached to the air handlers so I was pulling in outside air through the condensate lines every time they ran (draw through air handlers). I also have a 2nd floor floor drain in a laundry room that drains directly outside the house. I prime this with water since the trap itself is inside the body of the house.

  6. Expert Member
    Akos | | #7

    You can also look at something like this:

    There are other waterless trap seal options as well but a standard trap with glycol as suggested above is really your best option.

  7. user-322250 | | #8


    @Hockey: How often do you need to add glycol. Is once per off season enough to keep the traps working until there is condensate again ? Have you ever looked into airless traps along the lines of the rectorseal product Akos has linked to ?

    @Akos: I have seen a bunch of similar waterless trap products for condensate lines in my web browsing and I wondered how reliable they were. When you say that a conventional trap with glycol is the best option, do you mean for the drain pan trap ? Would you consider an airless trap design for the condensate trap ?

    1. Expert Member
      DCcontrarian | | #9

      There's two problems with glycol in this usage. One is that it evaporates over time. The rate varies with conditions but I'd say it's about the same as water. The other is that it supports mold growth, it can get a thick layer of mold which in time can clog the trap.

      Mineral oil doesn't have either of those issues.

  8. gusfhb | | #10

    if this is going to double as a condensate drain, I don't think oil is the answer. My hot water tank is up on a couple solid concrete blocks, probably to protect it in case of basement flooding, and no one notices. This would allow a standard trap

    1. Expert Member
      DCcontrarian | | #12

      Code doesn't allow the pan drain to be used for other purposes, the condensate drain has to be separate.

  9. Hockey | | #11

    user-322250: I’ve only had one winter with the glycol in the condensate traps and it did need to be reprimed mid winter. My floor drain is primed 2x year at the start of heating and cooling season and that seems to be enough. I do have a waterless trap in hand but I haven’t had a chance to try it out in the condensate line. I highly recommend a clear condensate trap, it’s easy to inspect. The ideal solution would be one that requires no user interaction but I’m not there yet.

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