Short-Term Solution for Furnace Condensate Freezing Problem

Quote of the Day

Politicians complaining about the press are like sailors complaining about the sea.

— Winston Churchill

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.