Coil Tube Boiler Operator's Manual: Models CB-200-CTB, CB-350-CTB, and CB-500-CTB
8-2
Checking for Correct Draft Over Fire (continued)
Backdraft is caused by the following conditions:
•
Poor draft caused by improper stack design. (See Section 4.)
•
Poor draft caused by improper adjustment of the barometric damper.
•
Incorrect combustion air band setting on the burner. (See Section 6.)
•
Boiler flues are plugged with ash. (See Section 9.)
•
Improper seal on clean-out door on back panel. (See Section 9.)
•
Exhaust fans in your building are sucking gases down the stack.
Understanding the Effect of Exhaust Fans on Draft
Any type of exhaust fan, paint booth, or exhaust system in a building will create negative pressure in the
building unless there is a source of make-up air (i.e. fresh air which enters the building and replaces the
air removed by the exhaust fans.)
If there is insufficient make-up air, the exhaust fan will suck air and combustion gases down the coil
tube boiler stack and create backdraft in the boiler. After the burner is shut down, the backdraft may
suck the residual heat out of the combustion chamber and into the burner causing damage to the burner.
Even if the exhaust fan is on another level of the building or in another room away from the boiler, the
exhaust fan will still create backdraft at the boiler.
Checking Draft Overfire to Determine Severity of Backdraft
The following procedure is an accurate method of determining how much backdraft is created by the
exhaust fans. Once this is determined, you can select the correct method for resolving the backdraft.
1.
Turn off ALL exhaust fans and close ALL doors and windows in the building (any open door or
window will allow make-up air to enter the building and will negate the test).
2.
Start the coil tube boiler and adjust the barometric damper so that the draft overfire is -.02 w.c.
3.
Check the draft overfire again. Now have someone start the exhaust fans.
4.
Note how much the draft overfire has changed.
ATTENTION:
If the draft overfire changed towards positive, it is mandatory that make-up air
is provided to the building or severe damage to the boiler and burner will occur (voiding the
warranty). If the draft overfire remained constant at -.02 w.c., there is sufficient make-up air
entering the building, and the exhaust fan is not adversely affecting the draft.
CAUTION:
Under no circumstances should the equipment room ever be under a negative
pressure. Look for exhaust fans, compressors, air handling units, or anything else that may take air
away from the boiler. Also look for chemical fumes, solvents, refrigerants, etc. that may be in use around the
boiler and cause damage to it.
Summary of Contents for CB-200-CTB
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