Jumper on Fan/Limit Switch

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Old 12-19-07, 12:01 PM
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Jumper on Fan/Limit Switch

I have a multifuel furnace and would like to install a battery backup (inverter and transfer switch) for the blower in my furnace. I am looking to only power the blower since I will use wood or coal (as I normally do) if the power goes out (along with the blower) to keep the heat up longer than if I were to burn oil which requires twice the energy draw. I believe I have the wiring solution figured out, and I have just one question.

The furnace has a Honeywell L4064 fan/limit control. The hot lead is jumpered between the fan and limit sides of the control (two connections on the bottom of the switch). Is there any problem with removing the jumper and running separate 110V leads for each of the fan and limit sides? This will allow me to isolate the battery backup to just the fan/blower side.

I ask since all documentation and references I have seen show that the jumper is to be removed if differing voltages are used for each side, but make no mention in the case of wanting to isolate each side of the switch while maintaining the same voltage circuit on each side. I assume it will work, but want to get another opinion before I do so, as it seems there are others on these forums who are familiar with this fan/limit control.

Thanks in advance for the help!

Chris
 
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Old 12-19-07, 05:24 PM
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Do not remove the jumper. The blower and the limit are on the 120 volt side. If you remove jumper you will have to rewire so the limit is on the 24 volt side. Does your L4064 have a push/pull switch to run the blower continuosly? If no you only need to install a bypass switch across the fan side leads on the switch. Of course the blower will continue to run until switch is opened.
 
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Old 12-20-07, 06:04 AM
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Thanks for the quick reply! From your answer, I take it the only time I want to remove the jumper is if I am to power one side with 24V and one side with 110V? Not if I'm using two 110V circuits? I could rewire with 24V on the limit side. However, you're right that is a lot more work than it's probably worth.

An alternate solution may be to leave the furnce and fan/limit switch wired as they are and put the battery backup/inverter in-line between the furnace and circuit breaker. When the main power goes out, to prevent the oil burner from firing and draining the batteries too quickly (or just tripping my inverter), I could install a relay in-line with the burner that is normally open, but closes when there is power in the system. This relay would need to receive power separate from the main power feed to the furnace so that if power goes out, the inverter doesn't keep powering the relay. This may actually be a simpler solution than what I had previously envisioned.

Does anyone see a problem with this setup, or have other suggestions? Again, my goal is to power the furnace blower motor, but not the oil burner, in the event of a power outage.

Thanks again!

Chris
 
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