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