Dealing with Furnace Condensate At Low Temperatures

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Figure 1: A Typical Condensate Pump. (Source)

Figure 1: A Typical Condensate Pump. (Source)

While working on my retirement home and workshop in northern Minnesota, I have noticed that my furnace is generating between five and seven gallons of condensate per day. The furnace is on quite often this time of year because the outside temperature is running about -30°F (-35°C). I currently pipe the condensate over to a floor drain, which is connected to my septic system.

I mentioned the amount of condensate to my General Contractor (GC), and he said that this condensate can be an issue with a septic system in a cold climate because septic systems work best when they receive significant amounts of water flow. He said that he trickle of water can create a blockage if the flow is so low that it can freeze. Frozen pipes mean broken/blocked pipes and condensate water backing up into the house. If you are a homeowner that is gone for long periods of time during the winter – like vacationing in a warmer climate – you could return to a house with water damage.

My GC  said that condensate pumps (Figure 1) resolve this issue by collecting the condensate water and releasing it in surges, which ensures that a significant amount of water is sent down the drain. These surges are very similar in size to that produced by a toilet and are very unlikely to freeze. I went online and confirmed that others use this solution to resolve their issues with a condensate pump (example).

This discussion generated a few questions that a bit of math can help me answer.

  • How much water is generated per BTU of furnace heat?
  • How much propane is consumed per BTU of furnace heat?


Propane Characteristics

I obtained my information on propane from the Wikipedia:

Where is the Water Coming From?

Burning hydrocarbons generates water. We don't think about this water much because it often floats away in the form of steam. In the case of a furnace like mine, this steam is condensed so that its heat of vaporization can be captured and used to heat the building. In the case of a propane system like mine, the amount of water produced by the propane combustion can be computed by looking at the chemical formula for propane combustion (Equation 1).

Eq. 1 \displaystyle {{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}\text{+5}{{\text{O}}_{\text{2}}}\to \text{3C}{{\text{O}}_{\text{2}}}\text{+4}{{\text{H}}_{\text{2}}}\text{O}

where the chemical symbols are:

    • Propane, C3H8
    • Oxygen, O2
    • Carbon Dioxide, CO2
    • Water, H2O

Equation 1 tells us that for every mole of propane burned, we generate four moles of water.


My Furnace Characteristics

To answer my questions, we need to discuss my furnace and how it is running during this cold snap. Here are the critical parameters:

  • Furnace heat output: H = 100,000 BTU/hour (hr)
  • Furnace efficiency: ϵ = 96.3%
  • Furnace duty cycle: dc = 30%


Figure 2 shows my calculations. The key results are:

  • A propane-powered furnace will generate about 1 gallon of condensate per every 100,000 BTU of heat generated.
  • My furnace under these cold conditions is generating between 6 and 7 gallons of water condensate every day.
  • During the cold spell, I am burning about 34 pounds of propane each day.
Figure 2: Analysis of Condensate Generation and Propane Consumption.

Figure 2: Analysis of Condensate Generation and Propane Consumption.

Propane Heat of Combustion Propane Molar Mass


I now understand why I am seeing so much condensate water now. I will be installing a condensate pump this weekend.

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10 Responses to Dealing with Furnace Condensate At Low Temperatures

  1. I wonder if it's worth thinking about the chemistry at all? Furnace condensate is pretty acidic; if you're pouring that much low pH water into the septic system, you might give the bugs in the tank some adverse conditions.

    • mathscinotes says:

      You are correct that the water is acidic. In fact, I am checking into this very question right now.

      Thanks for the comment. I will amend my post when I get more data.


  2. Randall says:

    Just a thought: have you considered an automatic intermittent syphon:-

    • mathscinotes says:

      I have never seen that device! How cool! I will be looking into it further. For those who are interested in a video of this device, see this link.


  3. Paul says:

    Hey great post as usual. I am a neighbor of yours, per se, in Ham Lake Mn and had a similar problem with a boiler installed for floor heat when I added on an addition. The contractor did not tell me that I needed a water drain so the boiler installer put a 3/4" pipe through an outside wall (I was not happy about this but it needed to go somewhere). The first day the temperature went below 20 degrees F, the pipe was plugged with ice. I called the installer and they said they could install a pump like yours for several hundred dollars. This was too much and I was not happy with what they had done in the first place so I took a DIY approach using an old fish tank and a cheap toilet flusher and a re-route of the plumbing into a toilet like inverted siphon. The following year I routed the outside plumbing into a french drain below the frosty ground. I also used an old solenoid to flush the tank. However I have manually had to flush it (by hitting a button) every day - which has not been a problem. I am now playing with an arduino to automatically flush it using a water level sensor. In any case my frustration turned into a fun project to play with and ironically, when I sell this house I will most likely have to replace the tank with a real pump (as you spec'ed out) because I don't think any home owner would want this contraption.

    • mathscinotes says:

      Hi Paul,

      I occasionally pass through Ham Lake, but lately I am pretty focused on my cabin north of Grand Rapids. I expect to have my closing on my retirement home in 2 weeks. My garage is going well, but I am doing that myself and it will not be done until April. It is far enough along that I will have my final electrical inspection in 2 weeks.

      That sounds like quite a setup you have for your condensate! I like the integration of a fish tank and toilet flusher. Adding an Arduino would be nice touch.


  4. Edward Celarier says:

    I'm trying to figure out why you're condensing water inside the house at all.

    Your combustion products should be ducted out of the house, and I think should even be combined with some house air drawn into the duct (I presume that is so that there is a draw of the CO/CO2/H2O after the burner shuts off. CO in the house is bad.)

    Do you have a humidifier attached to the furnace? I've been battling ours this month--It's been discharging huge amounts of water that flows through the humidifier, but not evaporated--and the discharge is handled by a pump nearly identical to the one you've got in the photo. I've gotten it down from about 20 gallons/day spillage to nearly none at all, just by partially closing the humidifier's water supply valve.

    Where the combustion products (listed above) are should be (1) completely separated from the house air at the heat exchanger, and (2) very hot, so not condensing water. How long is your chimney? And how much of that is exposed outside the house? I suppose that if there's part of the chimney that stays cold (why?), it could act as a condenser, but the water would have to reflux back into the burner (through a hot pipe) to get into your house as a liquid.

    • mathscinotes says:

      Hi Edward,

      The water condensate comes from the combustion itself. My furnace burns propane, but similar logic holds for natural gas (i.e. methane). A furnace that burns very coal would only produce CO2 because that coal would be carbon (C). No water would be generated. If you burn methane, the combustion reaction is CH4 + 2 O2 → CO2 + 2 H2O. Notice that water is created and released in the form of steam. This water is a by-product of burning any hydrogen-bearing fuel. I have a condensing furnace, which recovers the heat from the steam. When the steam loses heat, it turns to water. That is the condensate. I have confirmed that my rate of generating condensate matches my rate of fuel burning.

      Good question. My wife asked it, too.


  5. Edward Celarier says:

    Ah. Condensing furnace. That would do it.

    Just a note, since you mentioned coal. Unless you are force-feeding oxygen to the burner, you generally produce some CO as well as CO2. Good gas burners (as you seem to have, judging by your efficiency number) will produce almost no CO--which is good. But coal- and wood-burning produces quite a lot of CO. There is somewhat less heat released in the reaction C+1/2 O2 -> CO.

    I'm really enjoying reading your articles. Thanks.

    • mathscinotes says:

      Your mentioning CO generation is very important. Incomplete combustion is an issue. One of my favorite episodes of Car Talk discussed how cars in some places are driven using wood gas, which gets much of it energy from the carbon monoxide generated by the incomplete burning of wood. Thanks for mentioning that.



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