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High-efficiency (92 percent and
higher) condensing gas furnaces
are complex. They have multiple
sensors and safety systems, all run
by a computer. Some of the components
are complex in themselves
and require special
equipment to diagnose and repair.
But other parts are pretty straightforward
and easy to replace.
We talked to the experts at
Superior Heating, Air Conditioning
& Electric in Anoka, MN, to get the
lowdown on which furnace problems
generate the most service
calls, which of those repairs can be
performed by a DIYer and which
ones are “pro only.” It turns out
that most of the problems are
caused by a clogged furnace filter
or any of three easy-to-fix parts—the igniter, the flame sensor and
the high-limit switch.
If you’re willing to risk $120 and
buy all three parts ahead of time,
you may be able to fix your own
furnace in less than an hour and
save a $125 service call. If the parts
don’t fix the problem, at least
you’ve replaced the most likely
suspects before calling the pros.
We’ll show you what the parts
look like and where they’re
installed on a typical furnace. But
every furnace brand is different. So
your best bet is to find a service
manual for your particular furnace
online. We think these repairs are
relatively easy for a DIYer, but if
you feel uncomfortable with any of
the steps we show here, call a pro.
DIY-friendly furnace repairs.
Figure A: Where The Parts Are
High-efficiency furnaces are jam-packed with valves, sensors, motors and
flue pipes. We’ve removed many of those larger components from this furnace
so you can see what the parts look like and where they’re located. They
won’t be as easy to see in your furnace. Here are some tips to help you find
the igniter, flame sensor, high-limit switch and pressure switch tubing.
The igniter and flame sensor are located inside a sealed combustion
chamber and aren’t usually visible. But they can be replaced without removing
the combustion chamber cover. If you don’t have a service manual showing
their location, consult the furnace-wiring diagram to identify the wire
colors for the igniter and flame sensor. To locate the sealed combustion
chamber, follow the gas piping from the gas valve to a box with a cover and
an inspection window. Then look for the wires going to the igniter and flame
sensor. The high-limit switch is usually located below the sealed combustion
chamber. The pressure switch will always have flexible tubing running to it.
Before you start
The igniter and the flame sensor are like tires on
your car—they wear out every four to five years. So
it makes sense to keep spares on hand. Igniters and
flame sensors cost about $50 each at appliance
parts stores and furnace dealers. Or you can find
them for less online. But first you’ll need the make,
model number and serial number of your furnace.
Find all that information on the manufacturer’s
label (Photo 1).
Then contact an authorized furnace dealer or
visit a local appliance parts store and buy the parts.
Some furnace dealers won’t give out parts numbers
or sell parts to DIYers. And appliance parts stores
don’t always stock parts for every brand. In that
case, search the Internet for “furnace parts” and
enter your furnace brand and model number to find
the right parts (theignitorstore.com is one source).
Shut Off The Power!
All furnaces have a power cutoff switch inside the blower compartment
that disconnects power to the furnace controls when the front panel is
removed. But there’s still power coming into the furnace, so always turn
off the furnace power at the switch on the side of the furnace and at the
circuit breaker panel before disconnecting any wires, or testing/replacing
any parts. And never run the furnace with the front panel off.
Check the power, then the filter
Repair experts tell us that they often show up at a residence
only to find that the unit isn’t getting power because of a
tripped breaker or a flipped switch. So check that first. Flip the
switch on the side of your furnace, and flip the breaker off and
on again before you even think about replacing any parts.
Next, check the filter. A clogged air filter restricts airflow
through the heat exchanger, causing the furnace to overheat.
The high-limit switch detects the dangerous overheating and
signals the computer to shut off the burners and run the
blower fan to cool off the heat exchanger. Once the furnace
cools, the computer tries to fire up again. But if the filter
remains clogged and the furnace overheats four or more times
(the actual number varies by manufacturer), the computer will
shut down the furnace until it’s repaired.
If the filter is filthy, you’ve most likely discovered the cause
of the shutdown. Replace it with a new one. Then reset the furnace
(Photo 2). If the furnace won’t restart, the repeated overheating
may have damaged the high-limit switch. Consult the
wiring diagram on the furnace door or your service manual to
find its location. Then test it (Photo 3). If the switch is bad,
remove the retaining screws and pull it out of the heat
exchanger, noting the position of the sensor. Install the new
switch with the sensor facing as it was.
Leave These Repairs to the Pros
Replacing an inducer fan motor looks like a simple job. But the fan requires special sealants and procedures to ensure it doesn’t leak
any carbon monoxide into your home. Screw this up and you could kill somebody. Leave this one to the pros.
Replacing a gas valve isn’t difficult—it just screws into place. But you can’t install a new one without calibrating it to your home’s
gas pressure and your furnace specifications. And that’s going to require a service call from a trained expert with the proper equipment.
Don’t think you’ll save money by slapping one in yourself and skipping the calibration, because improper calibration can burn
up your heat exchanger, fill it with soot, run up your gas bill or all of the above.
Replacing the igniter and flame sensor
Most high-efficiency furnaces use a “hot
surface” igniter that heats up to 1,800
degrees F to light the burners. Once lit,
the burners then heat a flame sensor.
The furnace’s computer uses the signal
from the flame sensor to confirm a successful
ignition and turn off the igniter.
However, over time, the constant
heat/cool cycles cause the igniter to crack and
fail. And the flame sensor can develop
surface corrosion, causing it to send an
incorrect signal to the computer. Or it
can simply wear out.
The igniter can be held in place either
by screws or by a snap-clip arrangement.
Use a lighted flexible mirror to discover
the method used on your furnace. Then
remove the screws or unsnap the
retainer and remove the old igniter. Use
care when you install the new igniter—it’s brittle and can crack or shatter easily
Next, remove the flame sensor. If the
sensor element is covered with corrosion
and you don’t mind replacing the sensor
later, you can try cleaning it (Photo 5).
Otherwise, just replace it.
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One last preventive measure
Condensing gas furnaces attain their
efficiency by extracting water from the
exhaust gases. Sometimes condensation
from that exhaust can form in the pressure
switch tubing. This silicone tubing
runs between the flue and the heat
exchanger and the safety pressure
switch. Experts tell us they usually
remove those tubes and blow them out
with compressed air as a preventive
measure (Photo 6).