Hooking up Nest to a Lennox G8Q3-90 Fan/Blower


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Old 08-08-16, 10:15 AM
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Hooking up Nest to a Lennox G8Q3-90 Fan/Blower

Hi all, I have replaced my old 2 wire mercury thermostat that was in my house and hooked up a nest with a common wire to the furnace, but am having trouble figuring out how to hook it up the control the fan on/off only without the heat. I have included the wiring diagram, the fan is one of those silver honeywell boxes inside the furnace with one button auto/manual push/pull. On the diagram I believe my fan unit is the one in the diagram, not the 'alternate'. Any thoughts on how I might make this happen?, my nest is a gen3 if that matters

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Old 08-08-16, 10:56 AM
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Welcome to the forums.

Assuming that you have an additional wire inside the thermostat cable you can add a relay to do what you want. The following modification to your diagram illustrates it.

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You could also buy a fan center which is a bit more than you need here.
 

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Old 01-03-17, 07:03 PM
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Hi,

I have the same setup. Would this relay work?
https://www.grainger.com/product/WHI...RS-Relay-4E659

Thanks
 
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Old 01-03-17, 09:08 PM
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Welcome to the forums.

The relay you linked to says it's a 90-293Q which is a five pin relay.
What they have in the picture is a 90-290Q.

Either will work.
 
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Old 01-04-17, 09:44 AM
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Thanks,

I'm planning to run the fan only during the summer to recirculate the air from the cool basement to the warm upper floors.

I don't want the fan to run at full speed. It is possible to put this variable fan speed controller (from a roof mounted attic fan) on the yellow or the black wire coming from the new fan relay so that I can control the fan speed?Name:  IMG_20170104_093638.jpg
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Old 01-04-17, 10:42 AM
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You could certainly try it without even connecting the relay.

However, you should have a multi tap - multi speed blower motor and you should be able to use a different (slower) speed tap.

Check out the motor and let me know what colors are in use.

In order to use a different speed tap you would need the 90-293Q with the five terminals.
 
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Old 06-29-17, 02:40 PM
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Hi Pjmax,

Could you please reupload that picture. It's summer now, and I'm planning on doing this modification
 
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Old 07-02-17, 11:55 PM
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You could modify that furnace to add fan control.

The internal transformer isn't sufficient for anything but the gas valve so you can't just add a relay without putting a transformer with a much higher volt-amp rating.

But then you might as well get a fan center with it's own relay and transformer.

You'll need two R wires to the t-stat.

There are different ways to wire it, depending on which speed. and if you go for the lowest speed for continuous fan, some creative wiring/a low voltage relay will be needed to remove the call for fan during a call for heat.

Otherwise it will stay on low speed with a call for heat in continuous fan mode and you risk overheating the furnace.

The fan center is made for adding a/c to an old furnace, which normally needs a higher speed than heating - the t-stat brings the fan on and the fan center is wired so a call for fan energizes high speed. So in continuous fan mode there's no risk of the furnace overheating in a/c applications or if u want it running on high in continuous fan mode.

Also, wiring the fan to run at a lower speed would involve re-wiring the furnace's limit to keep the heating transformer live when there's a call for low speed continuous fan. (the fan center cuts power to the heating fan control and when continuous fan is energized since u can't energize multiple taps at once - in your case power for the heating transformer is tapped off the same thing)

The wiring would look something like this, take a look at the right side of the pic below. except yellow would go to the fan control rather than red and and red would be wired to the normally open side of the fan center's relay. There would have to be a way to cut off G with a call for heat - low voltage relay can do that:




However, it would be a total waste of time and money to add a fan center.

The G8 hasn't been made in the early 1990s in canada and probably was discontinued for longer than that in the us.

You can check the manufacturing year - it's the third and fourth digit of the serial number.

It's in the 60 to 70% efficiency range and the heat exchanger likely cracked.

You would see significant savings by putting a condensingm 95% furnace in unless you're in a climate with mild winters.

it would have the controls for continuous fan built in and be much safer than any old standing pilot/natural draft furnace. They don't have rollout switches, just a high limit and backdraft safety switch on the draft hood.

If you choose to keep your G8, the heat exchanger needs to be inspected carefully.

They crack at the back where the cell starts to curve.
 

Last edited by user 10; 07-03-17 at 12:22 AM.
 

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