White Rodgers relay switch 90-340 at Isolation Relay Control Switch Wiring
Furnace / Control Relay Isolation Switches, eg. White Rodgers 90-340

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Isolation switch purpose, wiring, use:

What is an isolation relay or isolation switch used in heating or cooling systems & how is the relay wired? An isolation switch allows a low voltage (e.g. 24VAC) circuit such as a thermostat to control higher-voltage equipment such as electric heaters and the blower in an electric furnace.

Shown above: White Rodgers 90-340 24VAC Relay switch, model 90-340. This article series describes in detail the wiring & setting of HVACR controls and answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

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Wiring the Isolation Relay on an Electric Furnace

Question: wiring the Isolation Relay on my electric furnace heating system

White_Rodgers_91-132006 Isolation Relay 90-340 control switch at

[Click to enlarge any image]

2016/09/16 Bob said:

I have older electric furnace with heat and ac. I replaced the sequencers last year and the fan isolation relay last week. the wiring was suspect before the repairs, I had to rewire the 24 volt wiring.

The only wire now I am wondering about is the 240v wire from the heat 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 heat is on, it continues to run the high speed fan. this is is fan auto and fan on. What is the proper wiring from sequencer to fan isolation relay to have low speed fan for heat? I have high speed for ac and fan on as wired.

This question was originally posted at FAN LIMIT CONTROL SETTINGS


Bob, sorry to have to be less than helpful but I think that considering the risks involved, what's needed is to identify the furnace brand and model, then pick up the wiring diagram. If you have that we need to identify the terminals on the isolation relay to be able to specify the wiring.

Above and at page top I'm showing two White Rodgers isolation switch control relays; the second one purports to be the isolation switch you have.

Reader follow-up:

Bob said:

Thanks .... I do have the wiring diagram of the original furnace (wesco 1980 era). The unit was retrofitted with trane AC in 1998 so the wiring must have been modified at that time. the original diagram had only high speed wired up to a switch, did not have the isolation relay and only had red and white thermostat wires. the isolation relay is a 91-132006-13302 with 24v pair of connections and two 240v switch connections, 240 is connected to the bottom of the first switch with the high speed wire connected to the top.

The low speed wire is connected to the second switch with a 240 wire from the top of the sequencer sack to the center pole on the second switch. This is as close as I can describe since the unit was retrofitted.

Moderator reply:

For other readers: an Isolation Relay is simply an electrical control that isolates different voltages in an HVACR application, so that, for example, a 24VAC thermostat can control 120VAC or 240VAC powered cooling or heating equipment. Some isolation relays also support multiple switching such as multiple speed fans.


Do you think the blower fan is a model capable of multiple speeds but the controller is designed for only one speed? If that's the case, by adding a toggle switch you might be able to switch the existing single fan-on wire to swap manually between the two fan speeds.

I think the relay you cite is a 12 V White-Rodgers-rbm-91132009113200690340 isolation relay.

I found these White Rodgers 1F90 / 1F97 isolation relay wiring diagrams that might help. Click to enlarge any image:

White Rodgers 1F90 1F97 isolation relay wiring at  White Rodgers 1F90 1F97 isolation relay wiring at

I don't know much about the White Rodgers isolation relay, but if it helps sort out the fan speed question, Honeywell's R8285 is sold in different models that support just 1 vs multi fan speeds. Those instructions show the position and wiring for the isolation relay too. In a 2-speed fan operation with this control the fan runs in low speed until there is a call for heat, then the fan speeds up.

Also see these instructions illustrated below:


Bob said: how to wire double-pole double throw relay switch in a circuit

..., your comments helped me with a search string that found this link:

It shows the circuits on the relay I have. My blower does have a multi speed motor with 3 speeds of which I am using two. I will study the link above and see if I can figure out how to activate low speed for heat. I am currently wired up as specified in my previous message, this link should allow me to determine if I can change it or not. I'll let you know after I study it.

Mod reply: The article shows a 12-volt relay used to switch higher voltage circuits. The user needs to identify each terminal for proper wiring; usually they're stamped or marked on the relay case:

  1. Coil terminal - a 12V circuit that closes a relay that closes (or opens) the switch
  2. NO - a switch that is Normally Open (or "off") - this switches a 120V (or higher voltage depending on the switch and application) device "ON" when the relay is powered and closes
  3. NC - this is a Normally Closed (or "on") switch. This switch in your application would run a low speed fan, if used. The switch will open (turn "OFF") when the relay is powered and closes
  4. COM - identified as the common wire for the relay; the relay may have different voltage outputs on different COM wires or terminals. Some relays such as the one in this example have eight terminals, sporting two of each of the types listed here.

Reader follow-up: thermostat white & green wires not acting as expected: why heat would not turn on low speed blower but left high speed blower running.

2016/09/18 Bob said:

Final resolution of my fan isolation relay / fan speed issue. Having reviewed the above links on the isolation relay, I can confirm my heating and cooling are properly connected to the relay's terminals.

Having confirmed that, I tested the thermostat wires again to see why heat would not turn on low speed blower but left high speed blower running.

I found, to my surprise, that the thermostat heats up the green fan wire at the same time it heats up the white heat wire. (I thought the white wire hot would leave green cold and the sequencer would turn on the low speed blower.)

Also, I thought green would only be hot if I used fan on the thermostat). With green wire hot during heat cycle, the relay keeps the hot wire from the sequencer from connecting to the low speed terminal. So problem, solved, I know why low heat doesn't activate.

My solution to this will be to add an additional isolation relay between the heat and fan only to achieve low speed blower on heat, high speed blower on fan only and high speed blower on cool. Thanks for all your help. I added this conclusion to share how these isolation switches work and can help solve problems.

Moderator reply:

Good going, Bob. This diagnosis will surely help other readers working on thermostat wiring or isolation relay wiring problems. Thanks so much for taking the time to follow-up.

PS: before adding another isolation relay, I'd double check that the white and green wires are not shorted together anywhere enroute. Disconnect the wires at both ends of their circuit and then check them for continuity. - Ed.


Continue reading at FAN LIMIT SWITCH - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see BLOWER FAN OPERATION & TESTING if your heating or cooling system blower fan itself appears not to be working,



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