Motorized draft control may be necessary to achieve full turndown, especially during cold weather.
Getting full turndown can be hard if draft is excessive or erratic. And the large chimneys in New York buildings can create very high draft. Draft issues may be hidden when modulation is bypassed or minimized. But efficient operation requires modulation and full turndown, so motorized draft control may sometimes prove necessary. When set up properly, these controls reduce flameouts and maximize combustion efficiency in addition to making high turndown possible.
If flameouts occur when running at full turndown, follow this procedure to determine if you need motorized draft control.
1. Check the Design Furnace Pressure
First, determine what draft should be maintained. Most burner manufacturers call for slightly positive draft at the burner head. Check the burner literature to confirm the proper draft.
2. Make Sure the Existing Draft Control Is Set Up Right
If the boiler has a barometric draft control, make sure it swings freely, and does not have excessive weight holding it still.
If the boiler has a motorized draft controller, make sure it’s hooked up and running.
3. Measure the Draft
Ideally, check draft on a cold day, since that’s when it’s strongest.
Measure draft while the boiler is running at both low and at high fire, to make sure the control responds correctly.
If you’re taking your measurement through the burner plate, draft should match the burner’s design furnace pressure. If you’re taking your measurement at the smoke box, draft should be just slightly positive (~0.1” WC).
If you can’t achieve correct, stable draft using the barometric damper, the system needs a motorized draft control. If there’s already a motorized draft control in place, you need to adjust it.
If the existing control does maintain proper draft, try to return during different weather to make sure the draft is still correct.
If you decide to install a new motorized control, select a model with an information screen. Cheaper models are tuned using dials, which are difficult to adjust accurately. In fact it may be impossible to set them up in a reasonable amount of time.
It is possible to get motorized draft controls to work, especially the newer ones, which come with electronic controls.
Controls with information screens are the easiest to work with. The screens display pressure, pressure set point, and what the various adjustments do.
Run the boiler at high, medium and low fire to make sure the draft control works well at all firing rates.
Ideally, make a return visit during different weather to check draft and tweak the settings.
These are the adjustments that can be made to motorized draft controls:
Damping: This controls the tradeoff between quickness and stability. When deciding if it should open or close, the control calculates the average pressure over a period of time, then opens or closes. Lengthening the period of time keeps the control from hunting, but can cause it to miss out on rapid pressure changes. The best damping setting is the lowest one that doesn’t result in hunting.
Set Point: The pressure that the control will try to maintain. If using a basic model, you need to verify the pressure with a manometer while making adjustments. Make small adjustments and wait at least a minute to allow the control to stabilize before adjusting further.
Dead Band: The allowable pressure range around set point. As long as the pressure stays within the dead band, the control will sit tight. If the pressure moves outside the dead band, the control will open or close as needed. As with damping, the best dead band setting is the lowest one that doesn’t cause hunting.
Prop. Band: The proportional band sets how aggressively the control reacts when the pressure strays out of the dead band. If the proportional band is too low, the control won’t react quickly enough to reach setpoint. If the proportional band is too high, the control will hunt and overshoot.
The burner manufacturer calls out the pressure needed at the burner head, but the following excerpts from various boiler manuals show the importance of accurately maintaining positive pressure in the furnace.
Burnham
Burnham specifies .10” to .30” of positive pressure just before the slide damper, and neutral pressure just after it.
The boilers are optimally designed to run with a flue outlet pressure of .10-.30”wc pressure. The stack design, taking into account the draft effect and all friction losses, should arrive at 0.0"wc +/- .10"wc pressure at the inlet. Horizontal runs should be kept to a minimum. This will ensure the boiler will meet the maximum capacity, and will light off and have stable and consistent combustion.
http://www.burnhamcommercial.com/assets/pdf/newiomanual.pdf
Easco
Easco calls for a motorized draft control whenever the chimney is more than 30’ high.
DRAFT CONTROL
A1. Contractor should provide and install a barometric draft control for the boiler(s). The control shall be of the industrial type and shall have a free area equal to the cross sectional area of the boiler breeching.
A2. An electric sequencing type draft control shall be provided by boiler manufacturer, complete with damper motor and linkage, draft gauge to measure over-fire draft, low draft switch, diaphragm type draft sensing element and sequence relay. Sequencing shall provide for full open boiler damper purge prior to main burner ignition, damper modulation during burner operation to maintain constant over-fire draft, open damper during post-purge and close damper during burner of period. In addition to the above sequencing requirements, the boiler damper shall return to an adjustable starting position for burner ignition before switching to damper modulation during burner operation.
Section Instructions: A - Use A1 for a barometric draft control, A2 for an electric sequencing draft control. The above section should be omitted in its entirety where boiler(s) will be supplied with stacks of 15 ft. high or under. On stacks of 30 ft. or higher, a draft control will definitely be required.
http://www.easco.com/pdf/easco-suggested-spec.doc
Cleaver Brooks
Cleaver Brooks calls for neutral pressure/draft at the smokebox.
Stack/Breeching Size Criteria
The design of the stack and breeching must provide the required draft at each boiler flue gas outlet. Proper draft is critical to burner performance.
Although constant pressure at the flue gas outlet of the Model 4WG is not required, it is necessary to size the stack/ breeching to limit flue gas pressure variation. The allowable pressure range is -0.25" W.C. to +0.25" W.C.
http://www.cleaverbrooks.com/products-and-solutions/boilers/firetube/4wg/4wg-boiler-book.aspx