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Ralph III 03-29-2012 01:56 PM

Fan Relay Wiring, Purpose of #5 terminal???
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Hello All,

I had to troubleshoot some issues with my air handler (defective sequencer) but have a general question in regards to the Relay. This is just FYI in trying to undertand what purpose the #5 terminal on a spdt fan relay serves, as I cannot figure it?

#1 (green) goes to the tstat and is used to energize the relay.
#3 (brown) goes to the Sequencer 24v terminal and supposed to serve as the common for the 24volt, as I understand.

#2 (black) is 120v direct from the breaker.
#4 (brown) is 120v and comes from the fan. It gets it's power from the other leg of the breaker, via capacitor. These two terminals are normally "open" to each other (no continuity). However, when the relay is energized they "close" (continuity) and thus supply 240v to the fan which starts it.

#5 (black) is designated as low speed fan heat. A wire runs from it to the top of the Sequencer on left side.

* When the system is idle it is normally closed (continuity) with the #4 terminal. You thus get a useless flow, nothing is being asked to occur, of 120volts from the fan up-to the left side of the Sequencer.

* When the system is energized (heat or cold) the #4 and #5 are then open (no continuity) between each other, because the #4 is then switched to the #2(which starts fan, as noted). The #5 then becomes a dead-end and again apparent useless terminal? If you turn on the heat, the sequencer will close, and you will get a reverse flow of 120v down-to the #5 terminal but again useless, as it is a dead end when the relay is energized?

Am I missing something or does the #5 terminal indeed serve a purpose?


hvactech126 03-29-2012 05:26 PM

it changes the blower speed.

Houston204 03-29-2012 08:13 PM

It insures that the fan will become energized with a heating demand, even if you replace the stat and forget to configure the new stat to energize G (fan) on demand for W (heat).

Ralph III 03-29-2012 09:34 PM


Originally Posted by Houston204 (Post 888147)
It insures that the fan will become energized with a heating demand, even if you replace the stat and forget to configure the new stat to energize G (fan) on demand for W (heat).

Ok, that makes sense. So as long as everything is hooked up or working properly with the thermostat (g wire energizes fan) then it really only serves as a redundant feature.

That would be useful if the Relay ever froze in it's normally idle/closed position (#5, #4). With that type failure, the #2 and #4 couldn't close together in order to engage the fan as normal. Therefore, you could have an air handler heating up with no moving air. With the redundant feature it will be reversed energized from the Sequencer, through the #5, with a call for Heat. This helps insure you always have air flow with a call for Heat, which could avoid a possible fire hazard.

People think of everything....

God Bless

Houston204 03-30-2012 10:53 PM

Your G (fan) signal will open as soon as the call for heat ends but the heating elements will have a delay on break because of the sequencer. Wiring the fan to the sequencer will add this delay on break for the fan in heating mode, increasing efficiency and possibly extending the life of your heating elements.

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