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Basic Troubleshooting

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Here is another from my archives. :D

Blower turns on but there is no heat.

Thermostats only control a fan relay, fan center or circuit board to turn on the blower motor.

Although these relay's are necessary for cooling to work, they have nothing to do with the heating operation or outdoor cooling section of the equipment.

What is a flame sensor?

The flame sensor,  is typically located in front of the pilot or one of the burner flames.

It is used as a safety device that insures a flame is present.

When a flame is present the sensor passes a small electrical current through the flame and tells the circuitry what to do.

If the sensor gets dirty the current passing through the flame is to weak for the circuitry to work properly.

To clean the flame sensor, disconnect power to the furnace, remove the flame sensor, clean it with some crumpled up aluminum foil or some very fine sandpaper or steel wool.

Reinstall it and reconnect the power to the furnace.

What is a pressure switch?

  Pressure switches are a safety device.

They insure the flue pipe and inducer motor are functioning correctly.

If the flue pipe becomes obstructed, or the inducer fails to start, the furnace will not ignite.

Also be aware, if the vacuum hose or vacuum ports become obstructed, or fill with water, the furnace will not start.

Often furnaces are installed with improper flue pipe sizes and can take a year or two for the problem to surface.

Sometimes the pressure switch can simply go bad and need replaced.

Where is the diagnostic light?

Diagnostic lights are typically found on the printed circuit board

If the circuit board detects there is a problem the lights will begin to blink a code. ex... 3 blinks and a pause, then 3 blinks-pause again, and so on.

The codes are then located on one of the doors, the circuit board or in the blower compartment, describing the problem.

Note... on most furnaces when the power is turned off, these codes are lost, you will have to wait until the furnace malfunctions again to retrieve the diagnostic code.

[align=center][/align] Where is the fuse?

There are several fuses used in most furnaces.

Fuses protect wiring and components from receiving to many amps or power.

High voltage fuses can be in a breaker box or fuse panel.

They can also be located on the side of the furnace by the switch that looks like a light switch, typically called an SSU.

Low voltage fuses can be on the printed circuit board  or in what is a fuse holder.

Fuse holders are typically found in the 24 volt wire leading from the transformer.

Some transformers are equipped with a mini circuit breaker

Note... Many digital thermostats also have a fuse in them.

When a fuse blows, quite often, there is a reason. Either a wire has short circuited to ground or a component has failed.


What is a transformer?

Transformers are devices that typically step down voltage in an appliance.

In most furnaces they step 120 volts down to 24 volts.

The 24 volts then is used as control voltage to the thermostat, other safety devices and relays in the furnace or air conditioner.

What is a main limit?

Limit are safety devices. When they get to hot, they open and turn off the gas.


Is my furnace overheating?

Furnaces contain overheating protection by means of a limit.

Typical symptoms are...

  • Burner turns off before thermostat temperature is reached.Burners ignite again in only a few minutes after they went out.Diagnostic light says main limit open.Furnace keeps turning off and on when thermostat still should be calling for heat.

To correct this problem be sure...
  • Filters are clean, Blower motor is running at correct speed, Secondary heat exchanger is clean.Cooling evaporator coil is clean.Cold air returns and registers are open.

Roll out limit.

This is just one example of many. However, all roll out limits must be manually reset.

Purpose: Safety device.

These limits trip if flames ever roll out of the heat exchanger, or flames burn where they are not supposed to be.

If a roll out trips the potential of having a major problem with the burners are high and a service tech should probably be called to diagnose the problem.


[align=center] Should I put the fan switch on "ON" or "AUTO"

There are several benefits to continuously running the blower. By continuously running the fan, you can even out the temperature in all the rooms of your home.

Then the negative side of the issues are. Blower motor can consume 1 dollar a day or more to operate. Flexible ducts, in attics are poorly insulated, moving air continuously is inefficient and has been known it to cause ice damming in some newly built homes. Filtration becomes more critical. Poor filtration, grit and dust will build up faster in the furnace, filter, blower motor, blower wheel, and evaporator coil. Dust in motor bearings cause most premature motor failures.[/align][align=center]


[/align][align=center] [/align]

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Bobby what yas should do is post a picture with each subject so the common folk can know what you is talking about. .



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Good idea to post these tips. Keep it up.

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The files I have all have pictures but somehow are left out when I copy and paste.

Not too computer smart. :P

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[user=43030]Bobice[/user] wrote:

The files I have all have pictures but somehow are left out when I copy and paste.

Not too computer smart. :P

The picture size in the file may be too large for this forum...

If you could email the files (picture & text) to me, I should be able to re-size them ...



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Samurai Appliance Repair Man

An even better way to do pictures is to use one of free online photo hosting services and make an album with comments, if you like, for each photo. Then you would simply post the link to the album on your post.

Some online photo services:





Flickr will even let you make annotations directly on the photos, like this one (mouse over the photo to see the annotation boxes).

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I've replaced my Goodman ignitor with a universal Honeywell model from Home Depot. It installs and operates just fine, but this design doesn't completely cover the hole into the firebox (if it's called that).

Does this need to be completely enclosed?


I could use aluminum tape, a piece of sheet metal & screws (or aluminum tape). Or, maybe nothing?

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Yes,it should be sealed.

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