Lennox fan turning off while burners are on.

[Highfly]

First some history..... Lennox Natural gas furnace Model G8Q3-120-1

Filter "basket type" has been changed. The squirrel fan motor has also been replaced. Woke up one morning to a stinky cold house and found the motor seized sitting there humming and not spinning, hot as hell.
I replaced the motor with a direct replacement with same RPM. Three wiring speeds, currently wired to high speed as old one was. I also replaced the motor starter capacitor while I was at it. The fan spins noticeably faster, we can hear a lot of wind blowing with the new motor. Seems like the old motor was struggling for some time.

After installing the new fan motor I turned the heat up and checked everything out. The furnace burners came on and shortly after the fan kicked in. To my surprise after a very short amount of time the fan stopped but the burners continued to stay on. Just as quickly as the fan turned off it came back on again. I thought to myself furnaces are not designed to run with the covers off so I dismissed what I just witnessed.

Now usually notice this at night, it seems to be doing the same thing. Furnace comes on (Fan, can't hear the burners from up stairs) and the fan turns off only to come back on again and then usually it will stay on for the remainder of the cycle or how ever long it takes to bring the house up to temp.

I can not confirm if the burners are staying lite the entire time as this all seems to be intermittent, and even if I was to try and run down stairs when I hear the fan quite buy the time I got down there it would be running again anyway. My gut tells me the burners are staying on or the fan would not kick back in.

Fan is a direct drive , no blets.

Any ideas?

[paul52446m]

First some history..... Lennox Natural gas furnace Model G8Q3-120-1

Filter "basket type" has been changed. The squirrel fan motor has also been replaced. Woke up one morning to a stinky cold house and found the motor seized sitting there humming and not spinning, hot as hell.
I replaced the motor with a direct replacement with same RPM. Three wiring speeds, currently wired to high speed as old one was. I also replaced the motor starter capacitor while I was at it. The fan spins noticeably faster, we can hear a lot of wind blowing with the new motor. Seems like the old motor was struggling for some time.

After installing the new fan motor I turned the heat up and checked everything out. The furnace burners came on and shortly after the fan kicked in. To my surprise after a very short amount of time the fan stopped but the burners continued to stay on. Just as quickly as the fan turned off it came back on again. I thought to myself furnaces are not designed to run with the covers off so I dismissed what I just witnessed.

Now usually notice this at night, it seems to be doing the same thing. Furnace comes on (Fan, can't hear the burners from up stairs) and the fan turns off only to come back on again and then usually it will stay on for the remainder of the cycle or how ever long it takes to bring the house up to temp.

I can not confirm if the burners are staying lite the entire time as this all seems to be intermittent, and even if I was to try and run down stairs when I hear the fan quite buy the time I got down there it would be running again anyway. My gut tells me the burners are staying on or the fan would not kick back in.

Fan is a direct drive , no blets.

Any ideas?

You are moving a little more air than you was before. So when your fan switch makes, it cools the fan switch down fast and it turns off.
Fan switches get weak over the years so they need to be adjusted.
I don't know which fan switch you have. When putting in a new fan switch,
i set the fan on temp. at 120 degrees and the fan off at 90 degrees.
What fan switch do you have? Cam Stat or Honeywell?
How many wires going to the fan switch, and are they all line volt or is there a couple of small wires? Is your furnace a up flow or down flow?
If your fan switch has a adjustment on it, try turning the on and off temp up
5 degrees. Later Paul

[Highfly]

Thank you for your quick response.

Fan switch is a Honeywell. I see three large wires coming out from it.

It is an up flow furnace. I'll look for some adjustments on the Honeywell box. I know it has a push pull button to manually turn the fan on and off but didn't notice any adjustment, I'll look closer.

[Highfly]

The Honeywell box had a cover on it and was removable. Low was set at 85, high was set to 110.

I upped the low from 85 to 90 so we will see what happens.

Thanks again for your support!!!

[paul52446m]

The Honeywell box had a cover on it and was removable. Low was set at 85, high was set to 110.

I upped the low from 85 to 90 so we will see what happens.

Thanks again for your support!!!

the flat side of the pointer is the setting It is coming on to soon. you might have to set the on to 120 and off at 95. later Paul

[Highfly]

You are right on the money 90 didn't work, cut out again so I bumped it up to 95 and upper to 115.
Will see what happens now.
Will go to 120 if I have to......................
Thanks again!!!!!!

[HayZee518]

there is a helix element inside your plenum that operates like those cheap thermometer gizmos. essentially a bimetal that expands at a different rate and imparts a rotation to the moveable contact inside the control. the proportional band is what you are adjusting. good luck with it.

[Highfly]

3:00 AM this morning ON, OFF, ON, OFF, ON, OFF. So I went down and pulled the Honeywell cover off and watched. I could totally see the little element in action.

The fan was coming on on the high switch and I could see as soon as the fan started the coil switch direction and head for the low switch and trip it. It made sense to me to drop the low back to 80 so it would not get tripped.
So my settings now are 125 on the high and 80 on the low keeping the switches nice and far apart.
The furnace has been cycling properly this morning so I think I got it.


What is the High temp normally set at? My setting is pegged at 240 (highest the setting will go).

[HayZee518]

it could be set anywhere but I wouldn't let it go above 180-200. that's HOT for a plenum! if its any help, wood starts to smolder at 512 degrees.

[Highfly]

Thanks. I was a bit uncomfortable seeing it so high. I'll back it off a bit.

Edit

Set it to 190, thanks again......

[paul52446m]

3:00 AM this morning ON, OFF, ON, OFF, ON, OFF. So I went down and pulled the Honeywell cover off and watched. I could totally see the little element in action.

The fan was coming on on the high switch and I could see as soon as the fan started the coil switch direction and head for the low switch and trip it. It made sense to me to drop the low back to 80 so it would not get tripped.
So my settings now are 125 on the high and 80 on the low keeping the switches nice and far apart.
The furnace has been cycling properly this morning so I think I got it.


What is the High temp normally set at? My setting is pegged at 240 (highest the setting will go).

a furnace is made to have about a 70 degree temp. rise across the heat exchanger, so if return air was 70 the out going would be about 140. I would
not set the limit more than 180 degrees. Later Paul

[Highfly]

Thanks Paul, readjusted to 180.