035-17473-000-B-1101
Unitary Products Group
25
Table 12: System & General Troubleshooting
Problem
Possible Cause
Remedy
Furnace will not start
Thermostat not calling for heat.
Check thermostat and adjust. Also, check thermostat
for accuracy; if it is a mercury switch type, it might be
off level.
No power to furnace.
Check furnace switch, main electrical panel furnace
fuse or circuit breaker. also look for any other hand
operated switch, such an old, poorly located furnace
switch which was not removed during furnace replace-
ment.
Thermostat faulty.
Remove thermostat wires from R7184 control termi-
nals T1-T2. Place a jumper across T1-T2. If furnace
starts, replace thermostat, thermostat sub-base (if
equipped), or both.
Oil primary control faulty.
Check reset button on oil primary control. Remove
thermostat wires from oil primary control terminals T1-
T2. Check for 24v across T-T. If no voltage is present,
check for 115v to oil primary control. If 115v is present,
go to Table 10.
Photo Cell wiring shorted or
room light leaking into photo
cell compartment.
Check photo cell (cad cell) wiring for short circuits.
Also, check for room light leaking into cad cell com-
partment. Repair light leak if necessary. See Table 10.
Open safety switch.
Check for open limit or auxiliary limit. also, check inter-
nal wiring connections; loose connectors, etc.
Furnace will not start without first pushing oil primary control reset
button.
(Happens on frequent basis)
No fuel oil.
Check fuel oil supply. Check that all hand operated
fuel oil valves are in the open position. Fill oil storage
tank if necessary.
Clogged nozzle.
Replace nozzle with high quality replacement. Use
rating plate or Table 2 as a guide.
Clogged oil filter.
Replace oil tank filter or in-line filter if used.
Low oil pump pressure.
Connect pressure gauge to oil pump. Adjust pump
pressure, or replace oil pump if necessary. Ensure that
erratic pressure readings are not caused by defective
fuel oil line.
Air getting into fuel oil lines, or
fuel oil line dirty, clogged, or in
some manner defective.
Check fuel oil lines. Replace any compression fittings
found with high quality flared fittings. Check for any
signs of oil leaks. Any oil leak is a potential source of
air or contaminants.
Defective burner motor.
Check burner motor. If burner motor is cutting out on
over-load, determine why. Replace if necessary.
Furnace starts, but cuts out requiring manually resetting the oil
protector reset button.
Photo Cell (Cad Cell) defective.
If cad cell is dirty, clean it. (Determine why cad cell is
getting dirty). If cad cell is poorly aimed, realign it.
NOTE: the photo cell shuld have a resistance of
100K
Ω
in absence of light; a maximum of 1500
Ω
in
the presence of light. Ensure that room light is not
leaking the cad cell compartment. (See diagnostic light
section).
