We have a condo in Northern WI that gets used sporadically over the winter, we keep the furnace running at a low temp while we are not there. Last winter the furnace condensate draining into the septic ended up freezing and blocking the pipe where it lets out into the septic system well outside of the house, we believe due to the constant very slow drip and small flow of furnace condensate.

Is there some kind of system that we could set up to collect water coming from the furnace until it reaches a larger volume (i.e. a gallon etc.) and then drain it all at the same time, to avoid the slow drip of water into the below-freezing septic pipe? I see furnace condensate pumps, we could set one of these up, but we don't really need the pump part, we just need something that will build up water and then release it all when it hits a set volume.

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    Wait... did the septic system and plumbing freeze, or is the just the condensate drain? Like... the condensate doesn't go into just a normal drain to the septic tank but has its own special line that runs outdoors, uninsulated? Normally you would just route the condensate to the floor drain and that's the end of it. This doesn't fail unless your plumbing drain freezes, and that should never happen (and is a bigger problem altogether).
    – J...
    Commented Nov 15, 2021 at 16:41

5 Answers 5


Use a 5-gallon bucket and a standard sump pump.

Put the sump pump into the bucket. Plumb it to drain into a convenient nearby drain, then run the condensate drain into the bucket. Once you've finally got enough condensate to trip the sump pump's float, it will push a gallon or so (the float is adjustable) down the drain all in one big shot instead of the slow steady dribble you've currently got.

As mentioned by isherwood (comment below), make sure you get a sump pump with the sliding float that will go straight up and down. There are some with the float on an arm that pivots, and that may take more room than you have available in a 5-gallon bucket, and it will end up jamming and not pumping.

You'd want to test by filling the bucket with a garden hose or a pitcher from the sink, just to make sure it's kicking in at an appropriate time. You might even be able to set the float high enough that you've got the best part of 4 gallons going down at once - this will really help ensure you don't run into freezing issues, and it will minimize the pump run frequency to minimize power usage when you're not there.

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    "... minimize the pump run frequency" - if the pump has a near-constant flow rate, why does the frequency matter? It's going to run for the same amount of time to push a particular volume of water, so how much difference does it make whether it runs continuously for 4 gallons or in four 1-gallon increments?
    – Moshe Katz
    Commented Nov 15, 2021 at 14:49
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    @MosheKatz: That assumes the pump always uses the same amount of power for the same flow rate. In practice that may not be the case: if it's pumping the water to somewhere lower than the source, it just has to overcome gravity to get the first water over the side of the bucket, then the water will flow more or less by itself (and the pump motor won't have to work hard at all). So it can be advantageous to do fewer, bigger emptying cycles.
    – psmears
    Commented Nov 15, 2021 at 15:29
  • @MosheKatz because it sounds "environmentally friendly" and "green". Doesn't have to mean anything, it just has to sound good.
    – FreeMan
    Commented Nov 15, 2021 at 18:44
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    Slight downside is if the condensate volume is low, it may take a while for the bucket to fill and the pump to run. This can allow the water to go a bit stank and start growing things, which could clog the pump. I'd add a dose of bleach or similar to the initial bucket, which will dilute over time but help reduce any algae etc.
    – Criggie
    Commented Nov 15, 2021 at 20:50
  • @MosheKatz: In theory, an ideal pump would consume the same amount of power to lift the same amount of water the same distance, regardless of whether this was done in many small portions or in a few large ones. In practice, any real pump will waste some power every time it is started, for instance due to moving parts that will need to be brought up to speed (and their kinetic energy is not recovered when the pump stops). Also, frequent starting and stopping will generally wear down the pump itself faster, too. Commented Nov 16, 2021 at 0:01

If you want something that you can purchase off the shelf and install easily, a condensate pump like you have mentioned is probably the closest thing to what you want.

If you want to try to build something yourself, you should probably consider a "Greedy Cup" Siphon a.k.a. Pythagorean Cup.

Here is a great demonstration of how it works: https://youtu.be/Cg8KQfaT9xY

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    I wouldn't use a home-made device like that in an unattended property. You need to use something that has been thoroughly tested and proven to work reliably.
    – jay613
    Commented Nov 15, 2021 at 17:06
  • I think condensate pump is the right answer and should be easy to install. I'm just not sure how much water they hold before running. It may need to be modified to collect more water depending on OPs needs.
    – JPhi1618
    Commented Nov 15, 2021 at 21:13

Although many of the suggestions are good, my concern with some of them using a regular sump pump type solution is that the water from a high efficiency furnace is somewhat acidic, and may cause issues long-term with a pump not designed for a mildly acidic environment. (see https://buildingadvisor.com/will-furnace-condensate-corrode-cast-iron-pipes/). If your electricity is relatively cheap, I might suggest going with some sort of heat tape, combined with good insulation around your drain line to keep it above freezing at all times.

  • slight correction - the condensate from a high efficiency furnace is not "somewhat acidic", it can be highly acidic. My boiler condensate drain was routed over a floor drain with a brass grate. It took only a few years for the condensate to eat a hole in it. Any installation instructions I've ever seen make clear that you can use only non-metallic pipe/drains for condensate.
    – Llaves
    Commented Nov 16, 2021 at 3:59
  • This is good info, and I wasn't aware of the acidity issue. Instead of putting it in your answer, though, it would be better as a comment on mine (since I'm the one who suggested the sump pump) to make it more obvious for the OP or anyone else considering that option in the future. Maybe the OP might find and install an all plastic sump pump, or discount my suggestion if he cannot find such a beast. Then edit your own answer to be just your answer.
    – FreeMan
    Commented Nov 16, 2021 at 16:38

There's a type of toilet cistern known as an "auto-cistern" that used to be commonly used for flushing urinals on a regular schedule (before better water saving alternatives came into common use). Essentially this is a regular toilet cistern that slowly fills and then, once full, empties all of its contents at once. In your case you'd fill the cistern from your condensate drain and connect the outlet to your septic system.

Something like this: https://www.screwfix.com/p/thomas-dudley-ltd-automatic-urinal-cistern-13-5ltr/9357r

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    This uses a Pythagorean Cup siphon, as described in my answer and mentioned in the linked YouTube video at 4:34.
    – Moshe Katz
    Commented Nov 15, 2021 at 19:10

I remember an evaporator system used on an AC system at a former workplace. It had an electrically-heated block in the bottom of a pan, and slowly dribbled condensate water from an AC unit into this. The water was supposed to evaporate and humidify the server room's air to minimize electric shocks from static, and it worked well for this.

Downside is that the bowl got filthy over time - a warm, damp space tended to grow algae which cooked off when the water was completely evaporated. This lead to a crusty mess of plant-like material filling the bowl, and causing overflows in the end.

However it never froze, so perhaps a gently warmed drainage line is your answer. The line doesn't have to be hot or even warm, just not-frozen. You'll just need to clean it when on-site.

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