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Electric Heat Relay Wiring Diagram
What is an isolation relay or isolation switch used in heating or cooling systems and how is the relay connected? An isolation switch allows a low voltage (eg 24VAC) circuit such as a thermostat to control high voltage equipment such as electric heaters and a fan in an electric oven.
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This series of articles details the wiring and adjustment of HVACR controls and answers most central heating system troubleshooting, inspection, diagnosis and repair questions. We describe how to inspect, troubleshoot, and repair heating and air conditioning systems to inform homeowners, buyers, and home inspectors about common heating system malfunctions.
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Connection of an isolation relay on an electric pipe Question: connection of an isolation relay on my electric pipe heating system
I have an older electric oven with heat and AC. I replaced the sequencers last year and the fan isolation relay last week. the wiring was suspect before the repair, I had to rewire the 24 volt wires.
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The only wire I’m interested in now is the 240v wire from the thermal sequencer to the isolation relay. The only problem I have is: only the high speed fan runs. the low speed fan does not run when the heating is on, the high speed fan continues to run. this is automatic fan and fan on. What is the correct wiring between the sequencer and the fan isolation relay to have a low speed fan for heating? I have the high speed AC and the fan on like a cable.
Bob, sorry to be less than helpful, but I think the risks involved is to identify the make and model of the furnace and then deal with the wiring diagram. If you have one, we need to identify the terminals on the isolation relay to specify the wiring.
Above and at the top of the page I show two White Rodgers isolation switch control relays; the other pretends to be the isolation switch you have.
Thanks …. I have the wiring diagram for the original furnace (Wesco 1980 era). The unit was retrofitted with crane AC in 1998, so the wiring had to be modified at that time. the original schematic only had a high speed wire to the switch, no isolation relay, and only had the red and white thermostat wires. the isolation relay is a 91-132006-13302 with a 24v pair of connections and two 240v switch connections, the 240 is connected to the bottom of the first switch with a high speed cable connected to the top.
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The low speed cable is connected to the second switch by cable 240 from the top of the sequencer bag to the center pole of the second switch. That’s as close as I can describe it since the unit was retrofitted.
For other readers: an isolation relay is simply an electrical control that isolates different voltages in an HVACR application, so for example a 24VAC thermostat can control a 120VAC or 240VAC powered cooling or heating device. Some isolation relays also support multiple switching, such as multi-speed fans.
Do you think that the fan is a model that can work with multiple speeds, but the controller is designed for only one speed? If so, by adding a switch you will be able to change the existing single fan cable and manually switch between the two fan speeds.
I found these White Rodgers 1F90 / 1F97 isolation relay wiring diagrams that might help. Click to enlarge any image:
Emerson White Rodgers 24a01g 3 Relay Electric Heat New Open Box
I don’t know much about the White Rodger Isolation Relay, but if it helps with the fan speed issue, Honeywell’s R8285 is sold in various models that only support fan speeds of 1 vs. These instructions also show the location and wiring for the isolation relay. In 2-speed fan operation with this control, the fan runs at low speed until a demand for heat occurs, then the fan increases.
It shows the circuits on the relay I have. My fan has a multi-speed motor with 3 speeds, of which I use two. I’ll study the link above and see if I can figure out how to enable low speed for heat. I am currently connected as stated in the previous message. This link should allow me to determine if I can change it or not. I’ll let you know when I study it.
Mod Answer: The article shows a 12 volt relay (or in some systems another voltage eg 24VAC) used to switch higher voltage circuits. The user must identify each terminal for proper wiring; they are usually stamped or marked on the relay housing:
Reader follow up: white and green thermostat wires not working as expected: why heat won’t turn on low speed fan but will run high speed fan.
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The ultimate solution to the Fan Isolation Relay/Fan Speed problem. After reviewing the isolation relay links above, I can confirm that my heating and cooling are connected correctly to the relay terminals.
After confirming, I retested the thermostat wires to see why the heat wouldn’t turn on the low speed fan, but kept the high speed fan running.
To my surprise, I found that the thermostat was heating the green wire of the fan while also heating the white wire of the heater. (I thought the white wire being hot would leave the green cold and the sequencer would turn on the low speed fan.)
I also thought the green would only be warm if I used a fan on the thermostat). When the green wire is hot during the thermal cycle, the relay prevents the hot wire from the sequencer from being connected to the low speed terminal. So problem solved, I know why the low temperature won’t activate.
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My solution will be to add another isolation relay between the heat and the fan only to achieve a low fan speed when heating, a high speed fan when fan only and a high speed fan when cooling. Thanks for all your help. I added this conclusion to share how these isolation switches work and can help with troubleshooting.
All right, Bob. This diagnostic is sure to help other readers dealing with thermostat or isolation relay wiring issues. Thanks for taking the time to watch.
PS: before adding another isolation relay I would double check that the white and green wires aren’t shorted somewhere along the way. Disconnect the wires at both ends of their circuit and then check for continuity. – Ed.
My fan limit says push man and arrow, no white button, just an empty hole. What should I do? I set the numbers to the correct position, but the fan is still on for a while and then turns on again after turning it off. I was lucky enough to replace the fan motor myself when I ordered it online. NO HELP FROM ANY MAN! The fan limiter bothered me the least. Now, after a month of heat, I need help! Thanks
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If your limiter does not have an actual controllable button or switch, then it is a model that has omitted this feature. The switch still works automatically. Manual switch is optional, not mandatory.
Be careful not to tangle, squeeze or bend the switch parts – this can be dangerous.
Have an olsen ultramax NG oven that turns on for 3 seconds and turns off for 4 seconds. It starts for the 3rd time. going to change the pilot mounting kit, hope that’s it. he runs a lot. Put the new thermostat in now. I hope you will hear from me. standard
I don’t blame you for thinking of the pilot unit because it sounds like the unit is having trouble starting. Keep in mind that the problem can be dirt or debris on the igniter or pilot, as well as on the sensor or the thermocouple itself. We can often tell what is going on by looking at the flame at startup. For example, if the pilot lights up but then goes out, I suspect the thermocouple is bad.
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Q: Does the Honeywell fan and limiter have any role in controlling the fan in A/C mode?
Does the Honeywell fan control and limiter play a role in controlling the fan in A/C mode or is it just the fan relay and thermostat? I just installed a new fan and limited my furnace and the fan is still running for 10 minutes after the a/c condenser shuts off?
If someone switches the main switch to the manual FAN ON position, the fan simply continues to run.
I need a new fan limit switch L4064w1296 19538c. It is no longer listed. I use a dual energy system (oil/electricity) of my choice. Where I can
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