Add Continuous Fan Setting to an Old Furnace

ac_aidsc · January 3

I have this old furnace, A Lennox Merit Series G40UH-36C that I have been trying to turn the Continuous Fan mode on from the wifi thermostat but will cannot do so. I checked the board and on the terminals where it says Cool, Heat, and Fan. On the Fan, there is no wire plugged in. Beside it on Park is a red wire. I am no HVAC guy so I am not sure about this stuff. I saw on another post with a slightly similar model that moving the wire to another terminal can change the fan speed of the fan. I would really appreciate anyone's help on this. I feel like when I changed my thermostat to the new smart wifi one, the furnace was not running as much which left a lot of stale air in the house. Thank you so much!

HVACNUT · January 4

Normally I would say no problem. But I'd like to know where the wire on the heat terminal is going. It's looks like the factory wiring might have been altered. If it's a 3 speed PSC motor, then Black on Cool is High speed. Red on Park is Low speed. And it's usually a Blue that would be Medium speed. It also doesn't look like a factory spade connector on the Heat terminal.

Big Ed_4 · January 4

All furnaces have manual fan . On the board the thermostat terminals , R is the power , power to G turns the fan on..

EBEBRATT-Ed · January 4

The fan terminal on the furnace board mut have a wire on it to run the fan continuous. The fan terminal on the furnace must be wired to the "G" terminal on your thermostat. Check and see how many wires you have from the furnace to the thermostat.

You need min 2 wires for heating only
you need 3 wires to run heating and to run continuous fan
you need 4 wires minimum to heating cooling and fan (if you have ac)

Big Ed_4 · January 4

They used the fan wire for the common , I got it ...

The Nest Mess

HVACNUT · January 4

Big Ed_4 said:

They used the fan wire for the common , I got it ... The Nest Mess

That's probably exactly what happened.

ac_aidsc · January 4

HVACNUT said:
Normally I would say no problem. But I'd like to know where the wire on the heat terminal is going. It's looks like the factory wiring might have been altered. If it's a 3 speed PSC motor, then Black on Cool is High speed. Red on Park is Low speed. And it's usually a Blue that would be Medium speed. It also doesn't look like a factory spade connector on the Heat terminal.

It goes to this black box.

ac_aidsc · January 4

EBEBRATT-Ed said:
The fan terminal on the furnace board mut have a wire on it to run the fan continuous. The fan terminal on the furnace must be wired to the "G" terminal on your thermostat. Check and see how many wires you have from the furnace to the thermostat.

You need min 2 wires for heating only
you need 3 wires to run heating and to run continuous fan
you need 4 wires minimum to heating cooling and fan (if you have ac)

I checked and there are 5 wires from the thermostat to the furnace.

HVACNUT · January 5

Someone added a relay. Why?
On the bottom of the relay, the thin low volt wires, the Brown wire looks like it's going to C on the board. Where is the Blue wire going?
Humidifier?

Electricity is colorblind but on the board, Y has a Green wire connected. Is that the way it is at the thermostat? Typically Y is Yellow for cooling. G is Green for fan. W is White for heat, where yours is black. Make sure all wires are were they're supposed to be.

109A_5 · January 5

Hello @ac_aidsc,
Looks like it should have been fairly easy just the Green wire between the Thermostat and the Furnace (24VAC thermostat control wires, near the bottom of the control board). Not the 120 VAC stuff first pictured.
The relay may just change fan speed between Heat and Cool.

This looks like your unit by the numbers on the document but the board layout looks different. Still the G terminals furnace to thermostat.

mattmia2 · January 5

HVACNUT said:
Someone added a relay. Why?
On the bottom of the relay, the thin low volt wires, the Brown wire looks like it's going to C on the board. Where is the Blue wire going?
Humidifier?

Electricity is colorblind but on the board, Y has a Green wire connected. Is that the way it is at the thermostat? Typically Y is Yellow for cooling. G is Green for fan. W is White for heat, where yours is black. Make sure all wires are were they're supposed to be.

The relay is factory, it selects high and low heat, my 2 stage Lennox from about 2000 is wired the same way, the 2 stage control is a separate board.

They need to connect something to the fan relay terminal on the main board to get a g only call to run the blower.

ac_aidsc · January 5

Here is a photo of the whole board and the back the thermostat. I hope this helps.

109A_5 · January 5

Hello @ac_aidsc,
OK , Yes, it looks like the Blue wire would handle the functionality you want.
With the thermostat disconnected or otherwise off and you jump R to G at the furnace does the blower come on ?

If not, I wonder what the small black wire that runs off to the right is for (not on wiring diagram), maybe it disables the function you want ? Does it go to another relay that disables the blower ?

mattmia2 · January 5

It looks like someone removed the 2 stage board and bypassed the circuit breaker for the low energy control transformer. They also set it to always run high heat.

Fan should work if you connect one of the motor taps to the fan terminal on the control board.

HVACNUT · January 6

mattmia2 said:

Someone added a relay. Why?
On the bottom of the relay, the thin low volt wires, the Brown wire looks like it's going to C on the board. Where is the Blue wire going?
Humidifier?

Electricity is colorblind but on the board, Y has a Green wire connected. Is that the way it is at the thermostat? Typically Y is Yellow for cooling. G is Green for fan. W is White for heat, where yours is black. Make sure all wires are were they're supposed to be.
The relay is factory, it selects high and low heat, my 2 stage Lennox from about 2000 is wired the same way, the 2 stage control is a separate board. They need to connect something to the fan relay terminal on the main board to get a g only call to run the blower.

IDK, the diagram the OP put up shows only W1 and a single stage gas valve.

mattmia2 · January 6

@HVACNUT the op never posted a schematic, someone else did. Their control board looks like mine.

Here is the board(26 year old me wasn't as neat about wiring as i would be now). The 2 stage board is at the bottom, the 2 stage blower relay is sort of off to the left:

Here is the schematic:

109A_5 · January 6

Hello @ac_aidsc,
I wonder if the G wire only energizes one of the relays at the top of the control board, do you here it click ? Since the G wire is connected through to the thermostat and none of the documentation matches up exactly (even the circuit breaker is not connected and I don't see a fuse on the control board and the motor wire colors don't seem to match the connections and the Blower wiring chart).

Maybe if you move the Red wire from Park to Fan would get you what you want. Which is kind of what you wanted to do in the first place. Did you try it ?

ac_aidsc · January 7

It works! Hahaha! Thanks so much everyone for your expertise.

Add Continuous Fan Setting to an Old Furnace

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