Quote of the Day
If all printers were determined not to print anything till they were sure it would offend nobody, there would be very little printed.
— Benjamin Franklin
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?
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).
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 = 96.75%
- 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.
I now understand why I am seeing so much condensate water now. I will be installing a condensate pump this weekend.