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Any thermosyphon direct vent fireplace does not have means for draft control. On vertical applications, an orifice plate may need to be installed to restrict the flow so high velocity gases do not disturb the secondary air feeding a standing or intermittent pilot.

A barometric damper is the classic draft control

On very large boilers, there are complex systems that measure stack gases, draft, etc. and meter the intake air (O2 trim). However, as with most large boilers, they use dampers, which are flow controls- not pressure controls.

On CAT IV condensing gas appliances, the intake air may exceed the requirements, resulting in lower efficiency. In such cases, a metering valve may have to be installed immediately upstream of the appliance balanced using combustion analysis. However, again this is a flow control as the unit is exhausting via a power venter. In the sense a power venter can modulate with the firing rate with some units, yes, it could be considered a 'draft control' albeit positive vent pressure. Natural draft is a negative pressure differential With Respect To the Combustion Appliance Zone. Technically, the lighter density of stack gases are forced up the chimney by the denser, cooler CAZ air but the stack pressure remains negative.

Gimmicks like the Thermiser affect the stack velocity by creating a buffer tank, which slows the gases exiting the combustion chamber resulting in a longer residence time (time, temp., turbulence). Then, the orifice plate (vena contracta) causes the effluent velocity to rapidly increase resulting in a quick evacuation of the secondary chamber. The problem with orifice plates is eddies immediately downstream. This creates a higher static pressure, which can negate the positive effect of higher exit velocity. ASHRAE calls for an optimum natural draft velocity of about 0.8 meters per second.

Regarding changing flue diameter as a means of control, consider your blood vessels. When you get scared, epinephrine (Adrenalin) and norepinephrine cause the smooth muscle of the vessels to constrict. This results in a flow restriction to the affected area while the back pressure between the area of peripheral vasoconstriction and the heart sees a rise in blood pressure. This is due both to the backpressure and to an increase in heart rate, force and automaticity by the catecholamines. If your feet get cold, put out the cigarette and put on a hat. As the blood in your head warms, the catecholamines with be stopped allowing vasodilation. Your skin warms and gets red again while the peripheral perfusion pressure drops. We don't have vents and chimneys that can modulate like the smooth muscle of arteries, arterioles and capillaries. If we did, THAT could be considered both a 'draft' and 'flow' control.

To 'control' draft, you must be able to regulate it up or down. Draft is a pressure differential- not flow. I always use the term 'draft pressure' to differentiate it from mass flow. A blower causes mass flow. On an inducer, the net effect is a negative pressure in the combustion chamber to make room for the expanding byproducts of combustion and excess air while drawing in combustion air. Once heat creates natural draft, its role is negligible but it does not 'control' the draft. Likewise, a draft hood allows dilution air, which inhibits draft pressure but it does not 'control' it. It kills the draft at standby, at low fire or high fire but at a relatively fixed rate. The only variable is how much air gets entrained through the fixed orifice of the net free area of the hood by the Bernoulli Effect, which shoots the flue gases up the stack where draft pressure carries them to the atmosphere--unless there is a negative pressure differential in the CAZ, in which case the hood can provide a pathway for spillage or backdrafting.

You have natural draft or gravity venting or atmospheric venting, then you have induced draft as with 80% furnaces then forced draft with power venters.