I inspected a home today that had a condensate pump for the furnace. There was no AC. I didn’t think condensate was produced with only a furnace. Can anyone tell my why they would have this?
A high efficiency furnace takes so much heat out of the fuel that most all that is left is water, Jesse. 
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Simply put…one of the by-products of combustion is water. Combine that with humidity in the air, there can be a lot of water in the combustion gases going out the flue. In a high-efficiency furnace (also called a condensing furnace–hint-hint) the flue gases are too cool to hold the water vapor in suspension and water condenses in the flue. Also, this condensate is slightly acidic, for reasons that are another discussion. That is why high-efficiency furnaces have plastic or stainless steel flues. This condensate has to be managed, because it can be a few gallons of water per day.
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Now ‘un-simply put’; Lon explained the ‘condensation’ effect of combustion gas being cooled below it’s dew point. Old furnaces are not as efficient because they must keep the flue temperature above the dew point to prevent the condensation and corrosive effect on the flue. Any furnace that you turn down the gas pressure will cause condensation. But the heat from the condensation is not efficiently put into the house. It goes up the flue.
What makes a furnace a high efficient furnace is that it removes Latent Heat with a second coil in the indoor air stream which puts the heat into the house. Latent heat of water is about 970 btu/lb. That’s 7,760 btu/gal. Latent heat of condensation is the amount of heat added or removed during a change of state from a vapor to a liquid without a change in temperature. Thus ‘Latent’. It can not be measured with a thermometer.
So we are not just removing condensation, we are extracting heat and putting it in the house. Condensate is a byproduct of this process. We are not just turning down the heat going up the flue, we are extracting every Btu we can from the fuel used. Thus a 95% furnace extracts that amount of Btu produced during the combustion process. Not just by extracting latent heat but also producing a more efficient combustion (end shot burners vs. slotted), controlling flue draft, obtaining combustion air from outside the house etc… ie. a 95% furnace is not as efficient if you don’t hook up the combustion air intake pipe to the exterior when the furnace is inside the house. The air used by the furnace must be replaced by outdoor air being pulled through the building envelope causing the furnace to run longer to overcome the cold infiltrating air entering the building.
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This was all very helpful. Thank You!
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