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**LazyPup** · 09-11-2005, 02:04 PM

In order to understand how draft works let us begin by examining a simple wood or coal stove. (See Illustration below)

A simple rule of physics states that Hot air is less dense than cold air, hence hot air rises and cold air falls. As heat is generated in the firebox it produces hot air (cumbustion exhaust gasses). The hot gasses then rise up the stack. Given that nature abhors a vacuum, as air is dispelled from the firebox in the form of exhaust gasses an equal volume of air is then drawn in through the dampers.

When the fire is first lit the stack is still cold so the ash pit damper is left open and the firebox damper is left closed. During the period of time the combustion remains low until the exhaust gasses have a chance to preheat the stack and begin the draft. The causes the incoming make up air to enter under the combustion grate and be drawn up through the fuel to produce maximum combustion. (This has the same effect as blowing on a campfire to get it started.)

AS the firebox and the stack heat up the amount of gsses rising in the stack increase proportionally, which in turn inceases the draft through the firebox.

The ash pit door is now closed and the combustion air is regulated by the draft door damper. By regulating the combustion air we can regulate the rate of combustion in the firebox.

In order to acheive complete combustion we must supply 7lbs of oxygen for each pound of carbon in the fuel. Once we have complete combustion in the firebox if we were to attempt to reduce combustion by closing down the draft door, the end result would be starving the fire of oxygen resulting in incomplete combustion and excessive smoke.

The alternative is to open the flue damper, which then allows cooler ambient air to be drawn into the stack, thus reducing the stack temperature and the stack draw while still maintaining the draft damper at the 7:1 ratio for complete combustion. Small applinces such as water heaters us a draft hood in place of a flue damper.

As you can see,in order to maintain fuel efficiency it requires a delicate balance of combustion air and stack temperatures.

Under ordinary circumstances is requires about 30% of the BTU energy of the fuel to maintain the stack hot enough to support complete combustion. That may have been acceptable back in the days of cheap coal and gas but in todays society we need to find more fuel effecient methods of combustion.

One method is to increase the height of the stack. By increasign stack height it increases the static differential between the exhaust and ambient air intake points, which increases the draft rate. increasing the stack height is commonly used on commercial production boilers such as used by the Electrical Generating facilities which often have stacks as high as 500ft.

Another method is to reduce the diameter of the stack which results in a higher velocity of flow as the exhaust gasses rise. (The Venturi Principle states that if you increase velocity you decrease pressure), therefore reducing stack diameter increases the differential pressure between exhaust and ambient make up air or combustion air.

It was Sir Isaac Newton who postulated that for every action their is an equal and opposite reaction, and such is true with inreasing stack height or reducing stack diameter. While decreasing diameter or increasing stack height do effect a better air flow, thus more efficient combustion, the opposite or adverse effect is colder emission temperatures at the upper reaches of the stack. As the stack internal temperatures decrease the water vapor in the flue gasses starts to condensate on the stack liner. In turn all fossil fuels have some minor content of sulphur which produces sulphur dioxide (SO2)when burned. The Suphur Dioxide (So2) in the flue gasses then begins to react with the water vapor (H2O) and produces Sulfuric Acid (H2SO4). The concentration of sulfuric acids have a very detrimental effect on mortar in masonary chimneys and the steel components in flue pipe.

In order to achieve more efficient combustion we need to have a method of insuring a proper draft without forgoing 30% of our BTU content to support the necessary stack temperature. Enter the "Induced Draft Fan". A powered fan is placed at the base of the stack to push the air up the stack. The combustion flue is then tee'd into the stack and the flue gasses are drawn out of the combustion chamber by venturi force thus permitting a much cooler discharge from the combustion chamber. We can then install larger heat exchangers in the combustion chamber to recover more of the heat energy as usable heat. The end result is combustion efficiencies as high as 89% rather than the maximum 70% with conventional stack draft.

**Rewired** · 09-12-2005, 05:53 PM

Thank you LazyPup!
I think I understand, those devices such as draft hoods and draft regulators are just simple devices that will allow you to obtain complete combustion of a fuel, while being able to control the combustion somewhat (by controlling the draft up the flue. the only "catch" is that by introducing cool air into a flue, you cool the flue reducing draft BUT you can have sulphur dioxide and sulphuric acid formed right inside the flue.. acid being formed when the sulphur dioxide reacts with water vapour.
also, If i read right "draft" can be altered by changing flue diameter as well as its height correct?

This all does make sence to me!
I did always wonder for example: girlfriends MID effiency furnace has NO draft hood or regulator built in or installed in the unit or flue, but the water heater that shares the flue does.
Also, I am in a lot of mechanical rooms of both newer and older buildings... Some boilers have draft hoods, some have a regulator, and some have none whatsoever.. I have even noticed in some of those old "primative" buildings where incinerators exist sometimes there are draft regulators mounted in the flue somewhere, sometimes there is nothing of the sort...
Hmm.. makes sence!

Lazypup: Thank you again!

A.D

**imeduc** · 09-13-2005, 08:46 AM

You will also run into the oil units that have a barometic damper in the flue pipe. This is to control the amount of draft that is over a oil burner when it is on.

ED

My mistakes dont define me they inform me.

**LazyPup** · 09-13-2005, 11:05 AM

As Imedec pointed out, the choice of draft controls is primarily determined by the type of fuel and combustion equipment.

For solid fuels such as coal or wood and some old gravity feed style oil burners they use the ash pit damper, firebox damper and a flue damper.

Heavier liquid fuels such as Heating Oil or Bunker Oil require an atomization process to mechanically mix the fuel with air to achieve proper combustion air fuel ratio. Given that the proper air fuel ratio is determined by the burner/blower assembly there is no need to provide either ashpit or firebox dampers. As Imedec pointed out, on oil burners the stack temperature is regulated by a barometric control that senses the differential pressure in the stack and regulates the flue damper releif air to maintain a preset limit.

While some modern gasseous fueled devices (Natural Gas or LP) do have a blower/fuel regulator similar to that used on oil burners, most gasseous fuel burners simply feed the fuel to the burner by the line pressure from the fuel source, either the Natural Gas supply line or the LP tank bottle pressure.

See the illustration below you can see that the gas is injected into the burner from the fuel nozzle. As the gas goes in it increases the velocity of flow in the burner premix chamber. As the velocity increases internal pressure decreases and air is then drawn in through the air intake ports where it mixes with the gas in the premix chamber.

On the end of the gas burner intake there is a movable sheet metal damper that can be open or closed to adjust the air fuel ratio. You will find this type of combustion air draft control on most water heaters, gas ranges and many gas furnaces.

If the stack of a water heater or some gas furnaces was to be connected tightly to the combustion chamber as the velocity of flow in the flue increases from the heat, the velocity of air entering the combustion area would need to increase proportionally. This could result in causing a low pressure condition in the combustion chamber that would literally suck excess gas through the fuel control and over heat both the water heater and the stack. That is potentially a very dangerous condition. By adding the open stack hood as the velocity of flue gasses increase the flue can now draw in relief air through the hood to limit the draft through the burner to the preset amount thereby preventing the water heater from overheating.

**Rewired** · 09-22-2005, 07:18 PM

I did also witness something in regards to a 'hood" and a "downdraft" situation at a local seniors residence in the pool room..
Seemed there was a powerful exhaust fan in the room pulling air DOWN the flue when it was running and the doors to the room were closed.. Exhaust gas was spilling out into the room via the draft hood but the heaters were operating "normally" or so it appeared.
( there ARE big grilles to let air into the room so the place wasnt a death trap but still, what I saw was wrong and I know it..

I guess a "hood" will allow for a 'downdraft" to happen whereas if the stack were connected directly to the appliance it could cause a really dangerous condition to occur should there be a downdraft present?

A.D

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