Understanding The Effect Of Exhaust Fans On Draft; Checking Draft Over Fire To Determine Severity Of Backdraft - Saturn 125 Operator's Manual

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 Over Fire 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 over fire is -.02 w.c.
3. Check the draft over fire again. Now have someone start the exhaust fans.
4. Note how much the draft over fire has changed.
ATTENTION: If the draft over fire 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 over fire 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.
Operator's Manual: Models Saturn™ 140 & Saturn™ 230
8-3