BAROMETRIC DRAFT CONTROLS
A barometric draft control (Figure 11-5) works on a very different principle. It contains a hinged and weighted flap that is normally closed when the appliance is not in use. During use, when the draft in the chimney exceeds a preselected value, adjusted by moving a small weight attached to the flap, the draft pulls the flap open. This lets room air into the chimney.
Barometric draft controls are commonly used with oil furnaces and boilers to improve energy efficiency. The controller maintains the right or optimum amount of air. Without the draft controller, when the draft exceeds the optimum value, more air than necessary is pulled into the combustion chamber, and excess air always decreases heat transfer efficiency.
In the case of wood-fueled heating appliances, there are both benefits and possible liabilities of using barometric draft controllers.
BENEFITS OF BAROMETRIC DRAFT CONTROLS
A significant benefit is a steadier burn and therefore steadier heating. In solid fuel heaters, combustion rates are usually limited by the air supply. (In oil appliances, the combustion rate is limited by the fuel supply.) Thus, by limiting the draft, barometric draft controls shave off the peaks and extend the duration of the burn (Figure 11-6).
Energy efficiencies do not seem to be affected dramatically. Based on careful measurements at SER of 1 typical radiant stove, it seems the main effect of a barometric draft control on energy efficiency is due to the cooler stack temperatures resulting from the air entering through the control. If the control is placed immediately after the appliance, the stovepipe will be cooler and give off less heat. However, if the control is placed at the upper end of the stovepipe just before the chimney, then only the chimney itself will be cooler; there will be no effect on heat transfer unless the chimney is interior and exposed.
Even though a barometric draft control cools the chimney, less creosote will accumulate, because dilution air in a chimney decreases creosote accumulation. Creosote reductions of up to 75 percent have been observed in experiments at SER.
This reduction in stack temperatures has another benefit. There is less thermal stress on the chimney and a decrease in possible fire hazards to surrounding combustibles. This can be of significant value, particularly in marginal chimneys, such as unlined masonry or masonry without adequate clearances. The benefit also is particularly relevant with large capacity heating systems, such as furnaces and boilers, where flue temperatures can be high.
A barometric draft control, by limiting draft, prevents dangerously hot fires in the appliance.
LIABILITIES OF BAROMETRIC DRAFT CONTROLS
There are 2 possible liabilities of barometric draft controls. Using a barometric draft control results in additional house air lost up the chimney. The net heat loss this represents depends on the excess draft in the chimney, the outdoor temperature, and on how much of the air flow represents extra house exfiltration caused by the damper (versus a rerouting of air through the damper--air which would have left by another route). Tests to determine how much new air infiltration is caused by using a barometric draft control are difficult to perform, since they must be done in actual homes. Since the amount of air flow into the control is typically small compared to house air exchange rates, it is likely that most of the air flow into the control is merely a rerouting of the air, not new infiltration, and, therefore, it does not represent a new heat loss.
In the worst case of all--the air flow being a new net loss--the net effect on the energy efficiency of the system is roughly a few percentage points. Thus, the actual effect cannot be large, but could be significant.
Do not duct outside air to a barometric draft control to eliminate the house air loss. This will defeat the primary draft-controlling function of the device, particularly during windy weather. Also, admitting cold air into the flue might not have the same effect on creosote as room temperature air.
The other possible significant liability of a barometric draft control is that during a chimney fire the control will be wide open, admitting considerable air. This could make the chimney fire more intense. But it is also possible that the extra air might have a neutral or even positive effect in terms of safety. Chimney fires are sufficiently complex that only testing can determine that net effect. It seems most likely that the effect is not good, but may not be very large. However, note that if the chimney is kept clean, this possible problem literally disappears.
NFPA standards (NFPA 90-B) require a barometric draft control on hand-fired thermostatically controlled wood furnaces. My feeling is that with large appliances, such as furnaces, or with any appliance hooked up to a chimney with excessive draft, a barometric draft control is of benefit, but only if the chimney is kept sufficiently clean that a large chimney fire is impossible.
