furnace condensate - system to collect and then drain periodically

Question

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.

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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

Answer 5

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.