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Symptoms Possible Cause
Check/Remedies
7. Thermostat cuts furnace off
Thermostat heat anticipator
set too low
Thermostat heat anticipator
setting should be total control
ampere load or may require
addition of 100 ohm, 1 O watt
resistor across terminals "W"
and "C" on furnace control board
Thermostat needs calibrating
Calibrate per instruction or
level subbase
8.
Excessive blower cycling
Improper setting of fan control
Check setting of IFC switches
or screws
Blower motor overload trips
Check current draw and compare
with nameplate amperage; replace
motor if faulty
Low voltage
Check voltage
Loose electrical connection Check and tighten connection
Failed integrated furnace control
Replace
Limit control cycling
Check for control fault
9.
Excessive blower noise Blower wheel loose on motor shaft
Tighten set screws
Blower running too fast Check CFM, temperature rise,
and adjust speed
Vibration of ducts or undersized ducts
Check duct work and correct
Damaged blower or unbalanced wheel
Replace
Blower wheel excessively dirty
Remove and clean blower wheel
Motor bearing worn Replace motor
Noisy blower motor Replace if necessary
Blower compartment door not secured Secure door
10. Air noise from register Register too small or closed Determine CFM requirement
and size register accordingly
or adjust vanes
Wrong type of register Check for vibrating vanes;
replace if necessary
Air velocity too great Reduce fan speed and/or
increase duct
size
11. Insufficient heat Unit undersized
Calculate heat loss
Blower running backwards Check rotation
Limit control cycling
Check for control fault
Unit underfired Check and adjust input
Dirty filters
Clean or replace
Air volume low
Check filters; check fan speed;
and check static pressure
Poor combustion
Check for sufficient combustion
air supply or blockage in
combustion air supply
Wrong main burner orifices Check orifices; refer to orifice table
Thermostat failed
Test and replace if required
12. Delayed (noisy) ignition
Igniter out of position
Check and correctly position
Pilot hood out of position
Check and correctly position
Gas pressure too high or low
Check and adjust manifold pressure
Incorrect orifice
Check orifice table
119
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