Weird heat/blower cycle on furnace...

[seamlyne]

We've got an ancient Lennox furnace, more than twenty years old though I'm not sure as to the exact age. A couple of years ago, we had a new thermostat installed hoping to better regulate the temperature in the house. Last winter I replaced that with a digital thermostat, and now when we set the thermostat at 70 degrees, we get 70 degrees.

But that's not the issue. After the first new thermostat the furnace/blower cycle changed, and this has continued with the digital thermostat. The furnace/blower comes on, delivers good heat for a "normal" cycle and shuts off. After three minutes or so (and this is the substance of my question), the blower comes back on and runs for an additional minute without heat before shutting off until the next cycle. In all other ways the furnace is running normally, we're warm and toasty. Well, warm anyway.

So, what's with this cycle? Is it a function of the thermostat?

Curious, but not terribly concerned,

- Wm

Measure with a micrometer
Mark with a crayon
Cut with an axe.

[LazyPup]

It may not be a function of your thermostat.

Some furnaces have a thermotat that senses the air plenum temp to control the fan. That way it can insure the air is hot before the fan starts. Sometimes at the end of a cycle the temp in the plenum cools enough to turn the fan off, but once its off the residual heat in the firebox will heat the air enough to restart the fan for a few moments until the warm air is discharged.

[Mike3]

It sounds like your fan control may be set a little high especially if this is a counterflo furnace. At the end of the blower cycle if there is hot air left in the supply plenum it will rise & if it is warmer than the off temp setting it may start blower & push out that air. What is the current setting of your fan control? Are filters clean-

[Hube]

In some cases,depending on the type of fan control, this is a normal cycle, because of a highly oversensitive fan control.(really no problem,as the unit is getting "all" the worthwhile heat blown out to you.)
Note; The T stat has no bearing on this fan re-start.
Initially, the fan will run until the fan control setting in the plenum area has been reached. If your unit has an adjustable fan on/off setting, try lowering the "off" setting down to approx 90 or slightly lower.
The fan "on" setting should be approx 130-135.
Normally a differential of 30-35 degrees between on and off should be sufficient.

[seamlyne]

quote:Originally posted by Hube

In some cases,depending on the type of fan control, this is a normal cycle, because of a highly oversensitive fan control.(really no problem,as the unit is getting "all" the worthwhile heat blown out to you.)
Note; The T stat has no bearing on this fan re-start.
Initially, the fan will run until the fan control setting in the plenum area has been reached. If your unit has an adjustable fan on/off setting, try lowering the "off" setting down to approx 90 or slightly lower.
The fan "on" setting should be approx 130-135.
Normally a differential of 30-35 degrees between on and off should be sufficient.

Alright-y then. Plenum? I've been in the guts of my furnace to tighten the grease nuts on the blower, change the filters (of course), even once replaced the thermocouple and re-lit the pilot light, so I'm not afraid to get my hands dirty. Is there a diagram of a prototypical furnace somewhere I could look at?

Measure with a micrometer
Mark with a crayon
Cut with an axe.

[Hube]

what parts do you want to know about or whereto look.?
Did you locate the fan on/off control? If so, what is the setting at?

[LazyPup]

I couldn't find a cross section of a gas furnace so i drew up the attached illustration,,,,i hope it will help you...

Normally the control section is in the blower compartment.

Easiest way to find the control section is locate the thermostat wire and see where it enters the furnace.

[imeduc]

If the blower control is set right for the furnace and home just about all of them will do this on and off thing at the start of the winter. Just the standing pilot light and mild weather will let the blower come on and off like that.

ED

My mistakes dont define me they inform me.

[seamlyne]

quote:Originally posted by LazyPup

I couldn't find a cross section of a gas furnace so i drew up the attached illustration,i hope it will help you.

Normally the control section is in the blower compartment.

Easiest way to find the control section is locate the thermostat wire and see where it enters the furnace.

Wow. Thank you, LazyPup...thanks for the answer, and for taking the time to make up the drawing.

And while we're at it, thanks to everyone who posted. It's been a good learning experience.

Measure with a micrometer
Mark with a crayon
Cut with an axe.