CLEAN BURN CB-3500 series, Operator'S Manual

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Operator's Manual: Models CB-3500 & CB-5000
8-3
Solving Draft Overfire Problems
WARNING! If you cannot achieve proper draft overfire, do not operate your furnace!
Contact your Clean Burn dealer for assistance.
ATTENTION: Backdraft must be resolved or your furnace will not operate correctly!
Under backdraft conditions, draft overfire readings will show positive pressure in the combustion
chamber. Backdraft means that oil spray, combustion products, and heat are blown back against the burner.
Backdraft results in oil-fouled retention heads and electrodes. Severe backdraft will force heat back against the
burner and result in heat damage to the cad cell and transformer.
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.)
• Furnace flues are plugged with ash. (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.) Refer to Figure 8C on the following page.
If there is insufficient make-up air, the exhaust fan will suck air and combustion gases down the furnace stack
and create backdraft in the furnace. Even if the exhaust fan is on another level of the building or in another room
away from the furnace, the exhaust fan will still create backdraft at the furnace.
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. Refer to Figure 8C on the
following page as needed.
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 furnace 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 furnace 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.