Short-Term Solution for Furnace Condensate Freezing Problem

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Figure 1: My Short-Term Solution to Stop Condensate from Freezing in My Septic System.

Figure 1: My Short-Term Solution to Stop Condensate from Freezing in My Septic System.

Minnesotans have endured a cold winter with relatively little snow, a situation that causes the ground to freeze deeper than expected. In northern Minnesota, we plan for 60 inches of frost depth, but this year the frost has gone much deeper. For those cabins with condensing furnaces, this extra frost depth has resulted in many frozen septic lines. This winter, I have frozen both my cabin and garage septic lines. The problem has been pervasive enough that the local newspapers have covered it (example).

As I have discussed in a previous post, these furnaces generate ~1 gallon of condensate for every 100,000 BTUs of propane burned. On the coldest days, my garage furnace produces 5 gallons of condensate per day, which means that I have a constant trickle of water flowing into my garage septic line. Unfortunately, this flow rate is so small that the water only slowly moves through the pipes. This slow movement of the condensate means that it can freeze, resulting in a clogged septic line.

I first tried a 1-liter condensate pump, which stores 1-liter of condensate before putting it out in a surge. The idea is that 1-liter slug of water will be less likely to freeze than a trickle. Unfortunately, a 1-liter slug of water was not large enough, and it froze in the pipe.

Figure 1 shows the approach that worked. I have the 1-liter condensate pump fill a 110-litter tub. This tub contains a sump pump that puts out a surge of about 40-liters. This surge was sufficient that it did not freeze. Both my garage and my home have the same setup. I should mention that I chose to use a vertical-float switch rather than a tethered-float switch to eliminate the possibility of the tether catching on an obstruction.

Long-term in the garage, I will pound out the concrete floor and put in a sump basket. The sump will be lower than my furnace, which will allow me to use gravity to feed the sump with condensate, eliminating the need for the 1-liter condensate pump. I will put the sump pump in the basket, which will then pump 40-liter surges into my floor drain. Unfortunately, I cannot install a sump pump in my house because its concrete floor contains in-floor heating tubes. For the house, my short-term solution is also my long-term solution.

The following video shows how to install a sump pump in a concrete floor.

Figure 2: Good Video On Installing a Sump Pump in a Concrete Floor.
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4 Responses to Short-Term Solution for Furnace Condensate Freezing Problem

  1. GZ says:

    Hi Mark,
    Too funny... I'm in Minnesota too and was struggling with septic freeze ups at our cabin (2018 was brutal) and built something very similar: https://cabindiy.com/how-to/condensate-pump-to-prevent-septic-freezeups/

    • mathscinotes says:

      Yup! Very similar. It is interesting that the building contractors have not come up with a standard way to handle this. I like your website and have bookmarked it.

      mark

  2. Dana Smith says:

    This happened to us in the Twin Cities area this winter. We were on vacation for two weeks. What was really odd was that we were back and everything functioned just fine for a week before it froze solid. We think most of the freeze up happened while we were on vacation where there was nothing flowing except the furnace condensate. During the week that we were back it must have been cold enough (and maybe for 24 hours there was little enough other water flow) that it froze solid. Have been looking for a solution. The people who pumped the septic tank said that their continuing ed people said to recommend that you have someone come into the house everyday that you're gone and run the dishwasher. I like your solution better.

    • mathscinotes says:

      It certainly has worked. I really wished my contractor had listened to the HVAC guy who told him to put in a sump basket (he made the same mistake on the house he built for his parents). The main issue with my solution is that the small lift pump will eventually fail and need to be replaced.

      Thanks for commenting.

      mark

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