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Snailspace 01-31-2008 09:05 PM

furnace cycling / shut down
HI. My Lennox is doing a few weird things and I' just wondering if anyone can diagnose!: I have a Lennox Elite, about 10 years old, with a programmable thermostat
Weird #1: Late at night the blower cycles on for 2 seconds, off again, back on 4 seconds later. I counted 46 times once before it stayed on and the heat came on.
Weird # 2: Several times, the above happens when the thermostat is not displaying a call for heat. (Thermostat set for 17 C, temp showing 19 C for example).
Weird # 3: It shuts down for lack of ignition once every couple of days - and three times just today. Turning off at the breaker for a few minutes sometimes solves this one, but not always.
I had the flame detector replaced last year...

thanks for any ideas!

bigMikeB 01-31-2008 09:17 PM

Clean the flame rod with steel wool and it should end the nuisance lockouts. As for the blower, I would look to see if the motor is over heating and cutting off on the internal overload or if the fan control is broken. How cold is the house when you hear it cycling on and off?

Snailspace 01-31-2008 09:23 PM

temperature varies - that long 46 cycles it was a cold night - maybe 16C / 60F. How can I check if the blower is overheating / cutting off?
Thanks bigMikeB

bigMikeB 01-31-2008 09:27 PM

It may not be necessary to check the motor. The blower comes on when the heat exchanger is hot. If the air being pulled in the return is low @60F, it will make the blower go on and off because it is dropping the heat exchanger temp rapidly, so with a cold return air that is normal behavior.

Snailspace 01-31-2008 09:37 PM

I hope I am talking about the right thing - the 'blower' I am talking about isn't the fan (that blows air round the house) the blower I hear cycling is, I guess, the exhaust fan, blowing the exhaust up the flue. Does that still make sense?

bigMikeB 01-31-2008 09:39 PM

Oh, sorry about that. I thought you meant the blower not the inducer fan.

bigMikeB 01-31-2008 09:41 PM

How many pressure switches are in that unit?

Snailspace 01-31-2008 09:53 PM

Hi bigMikeB
my fault on the blower! Inducer fan - that sounds better. You're losing me on the pressure switches - any clues what to look for? What does a pressure switch look like!?

bigMikeB 01-31-2008 09:55 PM

look at the pics in this post and you'll see a pressure switch. Item "A" in the first pic.

Snailspace 01-31-2008 09:59 PM

got it - there's just one on mine

bigMikeB 01-31-2008 10:02 PM

Then I would see if the motor feels hot to the touch when it's going on and off like that. And see of you can check it with a volt meter and see if it is getting power but not running.

TobyU 02-01-2008 02:55 AM

As soon as the thermostat calls for heat, the inducer fan is supposed to come on and stay. So you have a flashing light on one of the control boards? Many furnaces will flash the trouble code.
Inducer blower is a common one.
The motor has to turn and the flue has to be open enough to give the switch its signal from the inducer blower wheel turning.
I have pulled many birds out of the inducer blower wheel.
You can check with a volt meter at the connector going to the inducer motor to see if it has power when the cycle starts and the poser stays on.
This will tell you if the motor is bad, or if the system is telling the draft inducer motor to stop.
The motors usually don't spin then stop.
If they are spinning freely, they are usually good and the problem is in the sensor switch or a blockage.

Snailspace 02-04-2008 09:08 PM

Thanks for these leads. I've been watching it closely these last couple of days. The repeated 2 or 3 second cycling (up to 40 odd times) of the inducer fan is followed by a longer period of inducer fan during which the spark fires, but doesn't ignite a flame. This then happens twice more, and the furnace locks out (Flashing light matches lockout code that flame wasn't detected 3 times). All this is happening when the thermostat shows the room temperature is higher than the set temperature. So it's usually at night, when we have the thermostat set at 15 C and the room temperature is around 17 or 18 C. (I assume the thermostat isn't calling for heat in that case). Should some of the furnace still cycle without a call for heat from the thermostat?

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