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Stop and consider

I was working through this with my students earlier this year (high school) and it came up down below, but I thought it was good food for thought and appropriate for up here as well.

Some cool things to consider.

When it is wintertime, we are using gas appliances that require air for combustion and flue gas dilution. This air comes from inside for the most part (older appliances) and it must be replaced with outdoor air. (infiltration air) When you start to consider, to burn 1 cf of gas requires 15 cf of air, (10 cf to burn and 5 cf excess air) and if you have a draft hood an additional 15 cf is required (dilution air) you are talking a lot of air.

Consider 100,000-btuh boilers at 75% AFUE efficient

1 cf gas has about 1000 btu of heat energy

So the furnace requires 100 cf/hr of gas per hour

30cf x 100 cf/hr = 3000 cf/hr of air to burn and for dilution

In a 24 hr continuous on period this would mean 72,000 cf of air just to burn and for dilution.

This would mean all of the air in a 20x40 house with 8’ ceilings would be changed 15 times every 24 hours with new air. (This air would have to come from outside.)

If the outdoor air was 20 degrees at 95% RH and you warmed it to 75 degrees the RH would be 11% RH. (See attached chart) That’s why the house gets dry!

If you consider the density is .0824 lbs/cf, for air at 20 degrees and 95%RH and the specific heat of air is .24 btu/lb on average:

The heat required is Q=Weight x Specific Heat x Temperature Difference)

72,000cf x .0824cf/lb x .24Btu/lb/°F x (75-20) = 78,312 btu/24 hour period just to heat the infiltration air that is required by the old boiler. (74,530.09 exactly if you are anal enough to consider the mass flow rate)

1 hour of run time is required just to heat the infiltration air require for combustion!

100,000 btuh x 75%

Talk about a good reason for sealed combustion!

Now consider the dryer vents, bathroom exhausts vent, kitchen hood vents, and other ways air escapes the building and you have a lot of wasted energy.

But you cannot seal the house without provisions for combustion and dilution air. Wonder if they consider that in the AFUE calcualtion?


  • Ken_40
    Ken_40 Member Posts: 1,320

    The AFUE of the boiler is technically irrelevant to the house's design and construction.

    Apples and bananas lad.
  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79

    I was talking specifically of the boiler losses for combustion and dilution air. Also consider the draft hood is drafting 24/7

    I really thought just seeing the numbers was cool, you don’t think about those things all of the time.



    Your numbers appear to be exact and you've asked the perfect question. Combustion air must come from somewhere...
  • John L
    John L Member Posts: 118

    that all flame/fire needs air for combustion and that combustion air has to come from somewhere, connecting hte dots for the homeowner will present greater oppotunities for sealed combustion sales wherever applicable.
    In situations where direct venting is possible I tell the HO, that they will pay for a direct venting whether they buy it or not. This usually leads to a lot of questions and a most of the time the HO will go to direct venting.
  • Mark Hunt
    Mark Hunt Member Posts: 4,909
    100% correct

    and let us not forget the potential for depressurizing the combustion appliance zone. That 1000cfm exhaust hood over the stainless steel gas oven needs to get those cfm's from somewhere. That can turn a chimney into a straw in the blink of an eye.

    I agree Jim. Atmospheric equipment is on it's way out AND it should be. No good reason for it other than it's cheap.

    The Iroquois indians lived in long houses and had central fires in them for cooking and heat. They had no chimnies. They cut holes in the roof for ventilation. Every time I see an atmospheric appliance, all I can think of is "Long House Technology". We just dress up our holes so they don't let the rain and snow in.

    In the event that the hole becomes a straw, atmospheric appliances have that "draft diverter" that will "allow the appliance to vent should the chimney become blocked or suffer down draft". As the Guiness guys would say, "BRILLIANT!"

    "Long House Technology" was fine when people actually lived in long houses. A chimney is nothing more than a slight advancement on that old technology. Both designed LONG before health and safety were concerns much less energy conservation.

    Mark H

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