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If still no joy, then you will need to dig a little deeper. Electric furnaces typically have two key types of safety components to prevent a fire. One is a high limit switch that disengages the coils any time the safe high temperature limit is reached and the other is an airflow switch that requires air be flowing for the coils to be energized. Some units have two sets of high limits, one that is a self-resetting bimetal device and the other that is a one-shot thermal fuse at higher temperature, possibly wired in series with heating elements. Although not labeled in the schematic, the safety switches included in the control circuit are indicated in your photo of diagram #1, connected in series to the gray common wire leaving the W2 contactor. If all contactors cycle together despite adequate airflow, it is a likely there is an issue involving high limit or airflow switch. Some older furnaces do not have an airflow switch and instead rely on an interconnect between control power to the fan and heat contactors, but since yours does not appear to have this interconnect, I would expect your furnace does have an airflow switch. In my experience high limit switches are the most likely to become intermittently bad because they suffer abuse when the filters are not changed regularly. To clarify, if your unit is equipped with thermal fuses then they would not be the cause of intermittent behavior, but if present for each heating coil they should be checked to confirm that a portion of them are not blown due to chronic overheating.

If still no joy, then you will need to dig a little deeper. Electric furnaces typically have two key types of safety components to prevent a fire. One is a high limit switch that disengages the coils any time the safe high temperature limit is reached and the other is an airflow switch that requires air be flowing for the coils to be energized. Some units have two sets of high limits, one that is a self-resetting bimetal device and the other that is a one-shot thermal fuse at higher temperature, possibly wired in series with heating elements. Although not labeled in the schematic, the safety switches included in the control circuit are indicated in your photo of diagram #1, connected in series to the gray common wire leaving the W2 contactor. If all contactors cycle together despite adequate airflow, it is a likely there is an issue involving high limit or airflow switch. Some older furnaces do not have an airflow switch and instead rely on an interconnect between control power to the fan and heat contactors, but since yours does not appear to have this interconnect, I would expect your furnace does have an airflow switch. In my experience high limit switches are the most likely to become intermittently bad because they suffer abuse when the filters are not changed regularly. To clarify, if your unit is equipped with thermal fuses on each heating element in addition to a central high limit then the thermal fuses would not be the cause of the intermittent behavior, but if present they should be checked for each element to confirm that a portion of them are not blown due to chronic overheating.

If still no joy, then you will need to dig a little deeper. Electric furnaces typically have two key types of safety components to prevent a fire. One is a high limit switch that disengages the coils any time the safe high temperature limit is reached and the other is an airflow switch that requires air be flowing for the coils to be energized. Some units have two sets of high limits, one that is a self-resetting bimetal device and the other that is a one-shot thermal fuse at higher temperature, wired in series. Although not labeled in the schematic, these safety switches are indicated in your photo of diagram #1, connected in series to the gray common wire leaving the W2 contactor. If all contactors cycle together despite adequate airflow, it is a likely there is an issue involving high limit or airflow switch. Some older furnaces do not have an airflow switch and instead rely on an interconnect between control power to the fan and heat contactors, but since yours does not appear to have this interconnect, I would expect your furnace does have an airflow switch. In my experience high limit switches are the most likely to become intermittently bad because they suffer abuse when the filters are not changed regularly. To clarify, if your unit is equipped with thermal fuses then they would not be the cause of intermittent behavior, but if present for each heating coil they should be checked to confirm that a portion of them are not blown due to chronic overheating.

If still no joy, then you will need to dig a little deeper. Electric furnaces typically have two key types of safety components to prevent a fire. One is a high limit switch that disengages the coils any time the safe high temperature limit is reached and the other is an airflow switch that requires air be flowing for the coils to be energized. Some units have two sets of high limits, one that is a self-resetting bimetal device and the other that is a one-shot thermal fuse at higher temperature, possibly wired in series with heating elements. Although not labeled in the schematic, the safety switches included in the control circuit are indicated in your photo of diagram #1, connected in series to the gray common wire leaving the W2 contactor. If all contactors cycle together despite adequate airflow, it is a likely there is an issue involving high limit or airflow switch. Some older furnaces do not have an airflow switch and instead rely on an interconnect between control power to the fan and heat contactors, but since yours does not appear to have this interconnect, I would expect your furnace does have an airflow switch. In my experience high limit switches are the most likely to become intermittently bad because they suffer abuse when the filters are not changed regularly. To clarify, if your unit is equipped with thermal fuses then they would not be the cause of intermittent behavior, but if present for each heating coil they should be checked to confirm that a portion of them are not blown due to chronic overheating.

If the thermostat is in “fossil” mode (the usual default) then it will not activate the electric furnace blower in "auto" when heat is called for. Reconfigure the settings on the thermostat to “electric” mode.

If still no joy, then you will need to dig a little deeper. Electric furnaces typically have two key types of safety components to prevent a fire. One is a high limit switch that disengages the coils any time the safe high temperature limit is reached and the other is an airflow switch that requires air be flowing for the coils to be energized. Some units have two sets of high limits, one that is a self-resetting bimetal device and the other that is a one-shot thermal fuse at higher temperature, wired in series. Although not labeled in the schematic, these safety switches are indicated in your photo of diagram #1, connected in series to the gray common wire leaving the W2 contactor. If all contactors cycle together when they should not despite adequate airflow, it is a likely there is an issue involving one of these safety components. Some older furnaces do not have an airflow switch and instead rely on an interconnect between control power to the fan and heat contactors, but since yours does not appear to have this interconnect, I would expect your furnace does have an airflow switch. In my experience high limit switches are the most likely to become intermittently bad because they suffer abuse when the filters are not changed regularly. You would need to find the bad component by troubleshooting and replacing as required, which I will not go into in more detail here. DO NOT…REPEAT..DO NOT leave the unit running unattended with any of the safety devices bypassed.

If the thermostat is in “fossil” mode (the usual default) then it will not activate the electric furnace blower in "auto" when heat is called for. Reconfigure the settings on the thermostat to “electric” mode. In an acknowledgement of ThreePhaseEel, the schematic for your unit does indicate that this thermostat change should not be needed, but changing the mode on the thermostat from "fossil" to "electric" is usually trivial and making that change will overcome a possible failure of the NC contacts on the blower relay.

If still no joy, then you will need to dig a little deeper. Electric furnaces typically have two key types of safety components to prevent a fire. One is a high limit switch that disengages the coils any time the safe high temperature limit is reached and the other is an airflow switch that requires air be flowing for the coils to be energized. Some units have two sets of high limits, one that is a self-resetting bimetal device and the other that is a one-shot thermal fuse at higher temperature, wired in series. Although not labeled in the schematic, these safety switches are indicated in your photo of diagram #1, connected in series to the gray common wire leaving the W2 contactor. If all contactors cycle together despite adequate airflow, it is a likely there is an issue involving high limit or airflow switch. Some older furnaces do not have an airflow switch and instead rely on an interconnect between control power to the fan and heat contactors, but since yours does not appear to have this interconnect, I would expect your furnace does have an airflow switch. In my experience high limit switches are the most likely to become intermittently bad because they suffer abuse when the filters are not changed regularly. To clarify, if your unit is equipped with thermal fuses then they would not be the cause of intermittent behavior, but if present for each heating coil they should be checked to confirm that a portion of them are not blown due to chronic overheating.

You would need to find the bad component(s) by troubleshooting and replacing as required, which I will not cover in more detail here. DO NOT…REPEAT..DO NOT leave the unit running unattended with any of the safety devices bypassed.