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Furnace Starts Then Stops - Troubleshooting _ Repair and Replace

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Language: en

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Hi, I'm Vance and welcome to Repair and Replace.
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If your furnace starts, then stops, and does
this several times,
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then it's likely a safety switch
has tripped or has failed.
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In this episode, we'll look at the causes
of short cycling.
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We'll briefly cover each repair, but if you
need more information
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then watch the videos linked in the description.
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To troubleshoot short cycling, we're going
to look at a few specific scenarios,
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1: the furnace shuts off before ignition,
2: the furnace ignites but the flames won't stay lit,
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and 3: the furnace ignites, blows warm air
but shuts off after a minute.
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Lets begin.
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Modern furnaces have a set of safety switches
that shut down the furnace
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when it detects an unsafe condition.
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This includes the pressure switch, flame sensor,
flame roll out switch, and limit switch.
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If a switch is tripped, it will cause the
furnace to lockout.
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It will cycle on and attempt ignition several times
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before going into a hard lockout
for up to an hour.
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Every time a lockout happens, the furnace
will use flashing lights as codes
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to identify the source of the shutdown.
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These codes are unique to each model, so check
your furnace manual to see what each code means.
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Even without any furnace codes, you can still
troubleshoot the problem by observing the furnace.
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Keep in mind, if you are going to troubleshoot
your furnace, always be safe and turn off
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the power and gas to your furnace before doing
any repairs.
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If the draft inducer fan runs, but the furnace
shuts off before igniting,
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it could be a problem with the pressure switch.
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When the thermostat calls for heat, the draft
inducer pulls fresh air into the burner.
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This clears the burner of exhaust gases and
allows for safe combustion
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When the pressure is low,
the switch will stay open
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and will prevent the furnace from igniting.
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Try to identify anything that might cause
a change of pressure inside the heat exchanger.
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Check to see if there's any obstructions in
the flue vent or in the inducer motor port.
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Next check to see if there are any blockages
or water in the pressure switch tubing.
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If the airflow is good, then the pressure
switch might be faulty.
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Alternatively, if the burners still don't ignite,
then the furnace ignitor might of failed.
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The ignitor can be tested for continuity
with a multimeter
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to see if a continuous
electrical path is present.
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If there is no continuity,
then replace the furnace ignitor.
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If the burners shut off shortly after igniting,
then the flame sensor could be dirty or defective.
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When the airflow is good, the hot surface
ignitor will heat up,
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the gas valve will open
and the burners will ignite.
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The flame sensor monitors the burners,
and will shut the furnace off
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if there's no flame present after 7 seconds.
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If the flame sensor is dirty, the furnace
might shut off even when there's a flame.
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Remove the sensor and clean it with a soft
abrasive pad.
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Now if the burner still doesn't stay on, then
replace the flame sensor.
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After ignition, the blower fan will run and
circulate the warm air around your home.
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If the furnace ignites, the blower fan runs,
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but the furnace shuts off
before the house warms up,
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then the filter could be dirty,
or the limit switch may be faulty.
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The limit switch monitors the temperature
of the air passing over the heat exchanger
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and will shut off the furnace if the system
overheats.
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Any restriction of airflow might cause the
heat exchanger to overheat.
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This is why it's essential to replace the
furnace filter every 3 - 6 months.
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If you've replaced the filter, and the furnace still continues to shut off,
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then check the limit switch.
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The limit switch can be tested for continuity.
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If the switch has continuity, the multimeter
will show zero ohms.
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Even if the limit switch has continuity, it
might still be faulty.
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As a limit switch fails, it can start shutting
the furnace off at lower and lower temperatures.
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So if the limit switch has failed
or is starting to fail,
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then it's best to replace it.
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If you liked this and want to see more tutorials
and informational videos,
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then subscribe to our channel.
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And if you need help, you can call or visit
an AMRE location
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to talk with our
knowledgeable staff.
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Thanks for watching.

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