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90+% furnace no heat troubleshoot and diagnosis.
These furnaces are similar to the 80% efficient furnaces for troubleshooting, however, there are additional components and complexities.
When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas. If you do not have the ability to do these operations safely, do not attempt them.
This page covers problems with electrical supply voltage. If you know voltage is available, click here
Check for power
Most of these furnaces have some type of error code light or lights. If it is off, there is no low voltage power to the unit. If the furnace was built within the last 25 years, it will probably be controlled by an IFC. Most IFCs have an automotive type fuse mounted on the IFC. If the fuse is blown, there has been a short in the low voltage. You can replace the fuse, however it may blow again. The pics below show the types of fuses found on these furnaces.
inline fuse
The fuse installed should be 3 or 5 amp to match the original fuse.
If the fuse continues to blow, there is a short. The procedure here is first check all wiring. Many low voltage transformers have one side of the circuit hooked to chassis ground. If any wires are in contact with the chassis of the equipment, the fuse will blow. If you are proficient with a volt-ohm-ammeter, continue. (If you have a clamp on ammeter, you can bypass the fuse with a jumper wire. Place the clamp around the jumper wire. If, when the power is applied to the circuit, the amp draw exceeds about 1.6 amps, there is a short). Do not leave this circuit on with excessive amp draw. This will save on fuses.
If the wires are ok, follow the wires to the various components. Remove the wire from each component in turn with the power on and the thermostat calling for heat. If it blows with the wire removed from the component, the part is ok. Continue removing wires until the fuse does not blow. Then you have found the problem. In these furnaces, there are many components and you must do a thorough job. The key here is double check everything and be sure you are checking what you think you are.
If the wires are ok, follow the wires to the various components. Remove the wire from each component in turn with the power on and the thermostat calling for heat. If it blows with the wire removed from the component, the part is ok. Continue removing wires until the fuse does not blow. Then you have found the problem. In these furnaces, there are many components and you must do a thorough job. The key here is double check everything and be sure you are checking what you think you are.
If upon replacing the fuse, it does not blow and there still is no light, (remember to tape the door switch closed), the transformer may be failed. You must then find the transformer.
On the left, are a number of transformers. It is usually near the IFC. |
The wires from the transformer should be checked for voltage. There will be at least 2 wires on the high voltage side. There should a black and a white. Check voltage between them with a voltmeter. It should be 120v. If the voltmeter reads 0 the problem is not in the furnace. You must recheck breakers, fuses and power switches. Next, the output voltage should be checked. These wires could be any color. The wires should enter the IFC. It should be 24v. If the reading is zero, the transformer has failed. You can double check the transformer by isolating it electrically, and using an ohmmeter to check continuity of the windings. Be sure you are checking the black to the white on the primary. Then check the secondary. Both windings should show a resistance. If either winding shows an open, the transformer has failed. Some transformers have a fuse installed in the secondary winding, so if you have a short in the system, it will blow the internal fuse again. I recommend using a transformer without the fuse, then installing an inline fuse with the new one. Do not install a replacement transformer without a fuse. The transformer will smoke. (it sounds like bacon frying).
Once any transformer problems are repaired the IFC light should come on.
If the light does not come on, check all wires from the transformer to the control board. If all is ok, the board has probably failed.
Once any transformer problems are repaired the IFC light should come on.
If the light does not come on, check all wires from the transformer to the control board. If all is ok, the board has probably failed.