[Spydey]

I just replaced the blower motor on an old Lennox forced air furnace and now the blower runs continuously while the burners cycles on and off. When the thermostat is satisfied, I can hear weird clicking noise, then I can hear burners shut off for several seconds. The clicking noise starts again and then the burners fire up. After a min or so the cycle starts again. The clicking sounds like an electrical shorting type noise. If I turn the thermostat down the blower will shut off but will not fire up again till I turn the thermostat up. There's 2 separate limit switches. One is a fixed limit switch and the other is a fan control. Would both of these switches be faulty? Any help would be appreciated. Attached are some pictures of the furnace, which is a Lennox.

The clicking seems to be coming from the gas valve looking component where the thin copper tubing is connected. The tubings are part of the flame sensors.

 

[scottmcd9999]

Can you show a pic of the blower motor and wiring?

Is there air conditioning on that beast?

 

[Spydey]

Furnace Blower

The furnace seems to stop cycling right now and I don't hear the clicking. The burner just came on again and I heard a slight click but the blower hasn't stopped. Below are the pics of the blower motor and the wiring leading to the fan limit switch. There's no AC on the furnace.

 

[beenthere]

The copper line goes to a reg, doubt its making the clicking noise. probably either the gas valve or the limit.

Are you sure the blower motor is spinning the right way.

 

[Spydey]

The copper line goes to a reg, doubt its making the clicking noise. probably either the gas valve or the limit.

Are you sure the blower motor is spinning the right way.

I'm leaning towards the limit. Yes, the motor is spinning the correct way. I know because there's heat blowing out of the vents. I cleaned the pilot and flame sensors with a scotch pad, wonder if that had anything to do with it.

You think its the fixed limit or the fan limit? If either one is faulty, would it cause the symptoms the furnace is having?

 

[beenthere]

Probably the limit.

Is it blowing out as much air. more air, or less air then the old blower motor.

A motor spinning backwards will still blow out air.

 

[Spydey]

Probably the limit.

Is it blowing out as much air. more air, or less air then the old blower motor.

A motor spinning backwards will still blow out air.

There's only 2 wires on the motor to change the rotation and the diagram says to interchange the wires to reverse rotation. I left the wires in its factory state when I first installed the motor and it was spinning the wrong way because very little air was coming out of the vents. So I interchanged them and now its spinning the right way.

The circled prongs in the photo is where the 2 rotation wires go. The other 2 wires in the photo attached by the nuts, I guess, are the power wires. I interchanged them once to see if that changed anything but it did not so I connected them the same way as the old motor.

Which limit? The fixed or the fan?

 

[beenthere]

Fixed. The fan switch/relay, is not a limit per say.

 

[Spydey]

Fixed. The fan switch/relay, is not a limit per say.

Ok, thanks a lot for the replies and advice. I'm gonna take the limit switch out and check for continuity.

 

[beenthere]

Probably won't tell you much. Unless the contacts are pitted or carboned up.

Limits may be weak and opening early. Is the air coming out of the registers hotter then it use to be, or about the same or cooler. the limit may have become weak from when the old motor stopped working.

 

[Spydey]

Probably won't tell you much. Unless the contacts are pitted or carboned up.

Limits may be weak and opening early. Is the air coming out of the registers hotter then it use to be, or about the same or cooler. the limit may have become weak from when the old motor stopped working.

Air seems to be about the same as before. You might be right about the limit becoming weak because I noticed yesterday that it ran like it was suppose to for a short time. Seems the limit hasn't completely failed just yet. I should just change it as it's not a very expensive part and see if that fixes it.

 

[beenthere]

Yep, they're cheap parts.

 

[Spydey]

Yep, they're cheap parts.

I'm still wondering about the blower running continuously. Would a faulty limit cause that also?

 

[beenthere]

No, that would be the fan switch on that furnace.

 

[Spydey]

No, that would be the fan switch on that furnace.

Ok, so you think both my switches are faulty? The fan stays on continuously while its doing the whole clicking and cycling thing. The fan will only turn off when I lower the temp at the thermostat. The furnace doesn't seem to turn back on until I raise the temp at the t-stat. Basically I turn up the t-stat to just under 70 and I can hear the burners light, then the blower comes on and it will stay on constantly. When the t-stat is satisfied, the rapid clicking starts and the burners shut off for a few seconds and then I hear the rapid clicking again and then the burners fire up again. Burners stay on for about a min or less and then the hole cycle repeats itself with the clicking each time before the burners light and just before they shut off. During this cycling the blower is on constantly until I turn the t-stat down to about 60 and then the blower will shut off after about a min or two. At 60 or slightly above the furnace doesn't start again on it's own it seems. But if I turn the t-stat up a little it will fire up again.

 

[kirwinjd]

Wow. Looks like the original gas valve and milivolt pilot burner. Based on the age i would definitely replace the fan and limit controls. If the wiring is cracked or brittle, which is common on older furnaces, replace all the 110 volt wiring. If it was my furnace, I would be tempted to also change the milivolt valve and pilot to a 24 vac valve which also means installing a transformer, thermocouple and new thermostat. Id even consider going with spark ignition. However, if your paying retail for all your parts and not comfortable doing the work yourself then it would probably more expensive than its worth. Plus with the age of the furnace, the chances of the heat exchanger cracking in the near future is also a possibility.

 

[beenthere]

Can't say for sure. it may be working fine. Replace the limit and see how the blower works after doing that.

 

[Spydey]

Thanks for the replies. Good points, the burner fires up fine and the pilot is good still so I don't think those are the issues. It seems like a switch issue in my case here. The furnace is, I'm sure an original from when the house was built, if not, its pretty old but in good working condition. Probably better to just replace the whole unit in the near future.

 

[how]

OK first things first.
Attach a ground wire on the green screw of that new motor and attach the other end to the frame of the furnace. People can die from the lack of that ground.
That furnace is 45yrs + in age so I am suprised everyone here isn't saying..change it.
Your furnace fan is controled by an adjustable fan control. If the fan is coming on (within 5 minutes of the gas coming on) and is continuing to blow after the thermostat is satisfyed until the air coming out the register is starting to cool, this is the way it was designed to work.
The gas should be coming on whenever the thermostat is calling for heat.

If you wish to identify where the clicking sound is coming from..Take your longest screwdriver and put the handle against your ear and the metal end against the part you want to test. (stethescope). Obviously don't touch anything that could shock you.
I'd start with the gas valve!

 

[Spydey]

OK first things first.
Attach a ground wire on the green screw of that new motor and attach the other end to the frame of the furnace. People can die from the lack of that ground.
That furnace is 45yrs + in age so I am suprised everyone here isn't saying..change it.
Your furnace fan is controled by an adjustable fan control. If the fan is coming on (within 5 minutes of the gas coming on) and is continuing to blow after the thermostat is satisfyed until the air coming out the register is starting to cool, this is the way it was designed to work.
The gas should be coming on whenever the thermostat is calling for heat.

If you wish to identify where the clicking sound is coming from..Take your longest screwdriver and put the handle against your ear and the metal end against the part you want to test. (stethescope).
I'd start with the gas valve!

The ground is attached to the new motor already. You can see from the picture. Its the green/yellow wire attached to green screw on the outside of the motor and I attached the other end to the black cover plate. Initially the other end of the ground was attached to the motor bracket that came with the motor but the bracket doesn't fit the original set up on the furnace so I took off the bracket and attached the ground to the cover plate. It's still touching metal to metal and the motor is attached to the bracket on the furnace which is all metal. Is that ok? Or should I drill a small hole to attach the ground to the bracket on the furnace?

I understand what you're saying about the way the furnace is suppose to operate but the the fan continues to blow continuous. It should shut off after the furnace cools which it doesn't. All the while i'll hear clicking then the burners light while the blower is still blowing and then the clicking again as the burners shut off. Today, however, its been running normal so far after 2 cycles. I suspect an intermittent problem which leaves me to believe that the limit and or fan control is starting to go.