I have an electric heater central air/heat system with a standard thermostat. I have replaced the thermostat with no change (that was some time ago), so I believe it's not the thermostat causing the problem.

I put the left dial obviously on "Heat" and the right dial on either Auto or On. For Auto it doesn't come on (fan or heat). For On, the fan runs, but the heating element clicks on and off about every 3 seconds.

Make: Frigidaire Model: B3BM-042K-B

Does anyone know what this problem is? Also, is there some way I can override for now and just make the heating element turn on to warm up the house? Thank you

enter image description here

-- EDIT/UPDATE #1 --

I have removed the access plate and there are 6 basic components that I can see: 1. Two-element slide-in panel - top back. Two large black and two large red wires coming out 2. Single-element slide-in panel - bottom back. One large black and red each. 3. Contactor (?) Left front - where the thermostat wires lead into 4. Transformer (?) Left back 5. Circuit breakers - front center 6. Blower motor (below everything) - 3 wires lead into it.

picture of components #1 - elements in back, breakers front, trans. and relay left

picture of components #2 - elements in back, breakers front, trans. and relay left

--- EDIT/UPDATE #2 --

OK, so on the blower motor is the diagram for everything that is NOT on the front panel, including the blower relay and the 24VAC transformer. The diagram and the blower relay part number are shown in these photographs:

blower motor and relay/transformer and, the relay.. enter image description here

  • What make and model is your furnace? Dec 2, 2016 at 5:08
  • Frigidaire B3BM-042K-B - added to post Dec 2, 2016 at 14:04
  • Could be a bad contactor, bad safety switch (limit), or maybe a bad control circuit. Can you hear the contactor opening and closing when the elements turn on/off? Are any of the breakers tripped? Do you own a multimeter? Are you comfortable poking around in the system and taking measurements? If you don't know much about electrical work, stop now and call a professional.
    – Tester101
    Dec 2, 2016 at 15:59
  • I can hear a click when the heating elements engage and also when they disengage. No breakers are tripped. I own a multimeter and have installed electrical myself years ago, so I'm comfortable, just don't know where to start testing. Dec 2, 2016 at 16:55
  • What connects to that "heater kit plug" in the posted wiring diagram? Dec 13, 2016 at 5:06

3 Answers 3


Before making any adjustments to the unit internally, please confirm that all main disconnects and/or breakers feeding the unit are switched OFF. Do not attempt this work unless you are sufficiently skilled to do it safely.

I expect you may have two separate issues:

  1. The thermostat must be configured to control an “electric” heater and not a “fossil” heater.

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.

  1. There is an issue with the performance of the electric furnace itself.

First check the airflow when the blower is switched “ON”. If the airflow out of the registers feels weak, look for an obstruction such as a clogged air filter or closed seasonal damper. Once you have confirmed there are no obstructions, change the blower speed setting to increase the airflow temporarily based on the wiring diagram under Update #2 (you can move the connector back to its original position later). First move the blower connector from pin #6 to pin #5 to increase the blower from low to medium and then eventually to pin #4 to increase from medium to high. Each time the airflow is increased, check if the furnace will engage without the coils abnormally cycling off. For normal operation, the discharge air should be about 20F-30F warmer than the room air.

If that does not work then next temporarily disconnect each of the three heat coils one at a time. DISCONNECT ALL POWER BEFORE PROCEEDING. Since you have a two stage heater, begin with the lower contactor. Disconnect W2 and fire the heater. If you do not have W2 connected, then proceed to the next step by installing a jumper between W1 and W2 and then remove one red wire from the top contactor. Safely insulate the disconnected wire, re-energize the power and fire the heater. If the unit functions with one of the heat coils disconnected, there may be an issue with that coil, but you can continue to run at reduced capacity with the problem coil disconnected. . If either coil connected to the W1 contactor is found to be bad, then leave W1 and W2 connected together and leave W2 from the thermostat disconnected so the all remaining coils will fire with stage 1 heat.

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.

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.

Photos of a typical high limit switch and thermal fuses: enter image description here enter image description here enter image description here

  • His unit is wired so that it will work with a thermostat set for either "electric" or "gas/oil" mode (note the fact it feeds power from the switched side of the contactor to the NC terminal on the blower relay). Dec 18, 2016 at 22:39
  • Also, it's clearly not a one-time thermal fuse because than the unit would be deader than a doornail, and note in the wiring diagram that there are multiple high-limits as well. Dec 18, 2016 at 22:40
  • TPE - regarding power through NC blower relay from W1; I see that now that you mention it, but it clearly is not working as intended if the blower only runs with the fan switch in "ON." One easy solution might be change the thermostat fan mode another might be fix the fan relay. Regarding thermal fuse; I do realize that is not causing intermittent operation, but if the unit is damaged due to past overheating then a portion of the elements may also not function due to a blown thermal fuse. I am identifying the part for reference.
    – user39367
    Dec 19, 2016 at 2:00
  • TPE - I edited my answer to clarify the points you made.
    – user39367
    Dec 19, 2016 at 2:59
  • what thermostat did you install?
    – d.george
    Dec 21, 2016 at 11:28

I initially misread the symptoms -- if the fan is indeed running properly with the system ON, then your system is cycling on its high limits. I'd check for air obstructions (such as bad ducts or a dirty/too-tight filter) or a sluggish fan motor. You can also temporarily jumper the high limits for testing purposes, DO NOT LEAVE THE SYSTEM UNATTENDED when you do this, and keep an ABC or BC dry chemical extinguisher handy in case it does start to smolder.

As to the fan issue, that's an issue with the NC side of the fan relay -- it can be bypassed by setting your thermostat from "gas" or "fossil" mode to "electric" mode, where the thermostat is responsible for calling for both fan and heat instead of letting the furnace call for the fan.

If the symptoms were what I thought they were, with the fan and the heat cycling together, than they'd sound like a faulty connection at the C (0V/return) end of the air handler's blower relay -- the return from both heater contactor coils goes to the blower relay return end instead of returning directly to the C (0V) end of the transformer. When the G (fan) line is off, the connection between the grey C wire from the heat kit connector and the rest of the grey wires is not made, causing the heat to not turn on at all as the contactor does not energize. When the G (fan) line is on, the operation of the blower relay causes the dodgy connection to make for a short period of time before it comes undone again, and this cycle repeats itself as long as the blower relay is on.

The "click" you are hearing is most likely the contactor itself, and that's normal -- the short cycling is the problem. And yes, you can safely jump R to W and/or G on your unit -- that won't hurt it, as that's what the thermostat's contacts do anyway. As to troubleshooting the problem, I'd try measuring the voltage between the grey wire coming from the heat kit connector on your unit and the grey wire at the 24VAC transformer. It should be very close to 0V at all times; if it reads near 24VAC, then the connection at the blower relay isn't working.

P.S. your air handler is really a Nordyne -- a wiring diagram for a similar unit can be found here.

  • I believe that is correct. Still trying to get head around it. I removed the thermostat and contacted red (power) to white (heater) - contactor sounded and element heated [and perhaps the fan ran at lower power]. Lasted about 20-30 seconds, then went out. Next I contacted red to green (blower). Blower operated normally. AFTER that I contacted red to white again, and the contactor/element engaged again, this time for a shorter period. I'm leery about doing the element too long without the fan blowing. I'm now opening the system up to look for the contactor. Dec 18, 2016 at 17:47
  • @OliverWilliams -- find the blower relay first, it's the center of this problem. Also, jumping R to W will cause the fan to engage anyway whenver the contactor's on through the blower relay NC terminal and the orange wire from the heat kit plug. Dec 18, 2016 at 17:50
  • Can you suggest a "jump" to bypass the problem/verify it is in fact the problem? I'm actually in (pictures loaded in my post) Dec 18, 2016 at 18:24
  • The cable with the thin Yellow, Green, White, Red, and Blue wires is the thermostat cable -- jumper the Red to the Green for an R to G (fan on) jumper, and the Red to the White for an R to W (heat on) jumper. Also, I'm having trouble reading the wire colors in the photos -- you may want to tidy up/pull out the wire bundles inside with the breakers off and take a couple more shots. Dec 18, 2016 at 19:36
  • TPE, as it turned out the diagram for everything else was on the blower motor in the lower compartment. I added this to the OP. This clarifies what the G and C are (at least I think). I still could use advice on which to jump for a test though I believe that it should be some combination of the vertical paths shown? All connections to the relay appear sound. Dec 18, 2016 at 20:18

You have gotten a lot of help with this problem, so my question would be What is the brand and model of the new thermostat? And If you put a jumper on the wires from the thermostat R and W1 during a normal call for heat does the clicking stop? If it does the problem is in the thermostat. If the thermostat is the problem Knowing what thermostat you have will help. If the thermostat is not the problem I would look for a bad wire or a bad contacter or relay. If you have or can find the instructions for the thermostat look for an adjustable slide wire, (may be marked L for longer or S for shorter or maybe a low to high number) this would be what is called a heat anticipater. It has to be set to the amperage draw of the item it is controlling. If you can measure the amperage between R and W1 and set the heat anticipater to that amperage that may correct the problem. If you do not have the means to measure the amperage try to adjust to a higher setting. If it set to low it will cause the problem you have.

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