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Does the furnace dry the air

After reading Dans’ last letter, I feel it deserves a little clarification, for now its time for the rest of the story…While the furnace it self does not dry the air, operating it definitely does. A furnace requires air for combustion, excess air, and dilution air. To make it simple, each cubic foot of gas requires:

10 cubic feet for combustion

5 cubic feet of excess air

15 cubic feet of dilution air if equipped with a draft hood.

In total 30 cf are required for each cubic foot of natural gas on a draft hood equipped appliance.

Take into account that natural gas has an average heating value of 1000 btu/cf, then a 100,000 btuh furnace would requires 100 cf of gas for each hour of operation That means that 3000 cf of air are required as combustion dilution, and excess air. (100 x 30). If the furnace was sized properly, on a 0 degree day, it would operate continuously and require 72,000 cubic feet of air for proper operation per 24 hour period. (3000 cf x 24 hours)

Now take an average 2500 square foot home with 8’ ceilings and it has a volume of 20,000 cubic feet. (2500 x 8) / 72000 x 24hr = 6.666 hours.

6.666 hours is the amount of time it would take to completely put all the conditioned air up the chimney as it is used for combustion dilution and excess air.

Next where does that air come from??? Outside. It gets in by ventilation or infiltration.

Now to the psychrometric chart… If the out door air was 100% rh at 0 degrees, and heated to 70°F, the resulting RH of the air would be 5.1%

That is why you need humidity in the winter and nothing more..

Now take that one step further and it takes 1.25 btu to heat 1 cubic foot of air from 0 to 70° take that 72,000 x 1.25 btu, and you are looking at 90,000 btus of heat energy to warm the air you are bring in for combustion.

So for every 24 hours of operation, 1 hour is used solely to heat the air required for proper operation. It doesn’t matter if it is a furnace or a boiler, the same holds true.

Now consider this… Use a two pipe venting system, and none of these losses are realized. For every 24 hours, you get a free hour of operation, and your house will not dry out.

And now you know the rest of the story………….


  • Barbarossa
    Barbarossa Member Posts: 89

    I understand the train of logic for a non gun applacation you present but please explain the 5cf for excess air.
  • Mitch_4
    Mitch_4 Member Posts: 955
    The 5cf

    10 cf is stoiciometric combustion only. If you design equipment strictly on that basis, then ANY dirt / obstruction etc.. in the combustion process will result in incomplete combustion.

    50% (5cf) is required to ensure complete combustion.
  • Jim Bergmann
    Jim Bergmann Member Posts: 24
    5 cubic feet

    The post above is correct, because the fuel and the air never mix perfectly, and the heat content of the fuel can change, excess air is required to assure complete combustion.

    In the case of an 80+, the excess air is also the dilution air. Dilution air minimizes the chance of condensing
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