Gas Fired Heater - Fan kicks back on briefly after normal cycle completes

Greetings. New here so I will probably step in it a few times as far as usual and customary behavior goes. Case in point; my long title to this thread. Good, bad, don't care? By the way, I am a home owner, reasonably handy but, no where near familiar with HVAC ;-) On to my issue:

I live in a desert basin area of southern California. We don't often require heaters here so they get short seasonal use. The mounting floor of the indoor closet where the unit is installed has been modified so the unit is probably not original. The house is 25+ years old. It is a Payne with a long model number but, why not be complete; 394JAN036045. On top of the Payne is a Carrier unit that then ducts into the ceiling. They match in dimensions and color and I assume they were marketed as one of a selection of matched a units. The Carrier has an even longer model number; 518CXX030000MAAA. Thermostat is a Honeywell of the same era. Trouble shooting with Honeywell's online presence (pretty decent actually) I was able to 99% eliminate the controller as the cause.

Behavior is that the heater will perform a regular cycle. At the end of the cycle the fan (only) will kick on almost long enough to get up to speed and then shut off. It then immediately kicks on again almost long enough to get up to speed and shuts off. All is well until the next normal cycle where the routine is repeated. Sometimes the fan will only re-kick (is that a word?) on once but, mostly twice. Behavior began just after the first of the year. This is my 8th winter in the home.

I have all the registers open and the filter is clean so I do not believe I am causing the unit to run too hard or hot. The side of the blower was quite dirty which surprised me as I am pretty diligent about keeping the filter clean. I believe I found the cause when I noticed an 8" stub from the ceiling of the closet into my unfinished attic area. What's up with that?

There is also an opening dressed into the lower area of the rear wall about 5" high and a foot or so wide. This is just a cutout in the drywall with flashing around the edges allowing entry to the area between the inside and outside walls. Again I am clueless as to the function of this opening.

Glad to have found this forum and hope my problem will add some excitement to an otherwise boring Saturday

I know little to nothing about HVAC but, have researched enough to start thinking the following (uh-oh, now I'm thinking again):

There is a sensor function that tells the fan that the air is warmed and it should turn on to blow that warm (and continuing to be warmed) air through the system. When a cycle first starts I can hear the burner come on and shortly thereafter the fan kicks on. There may or may not be a sensor function to tell the fan to continue running for a bit after the burner shuts down (I have not tested this to a conclusive result yet) to flush the ducts of warm air(?).

I am guessing (still really, really guessing) that this sensor may be shutting off the fan as per normal at the end of a cycle. Then something (fan limit control?), correctly or incorrectly, decides that the air in the exchanger (new term for me) area is still too warm and kicks it back on, passes the decision to the part of the circuitry that decided the fan should be off and so turns it gets turned back off and so on, for a couple of times.

You are on the right track. There is a limit swicth in the upper plenum area, which keeps the fan running as long as there is warm air, and depending on the design,may actually turn ON the fan when you don't want or need it. Sometimes, folks will see their fan turn on in the summer time, just because it is hot in the attic!

This is the control board: http://www.air1supply.com/ICM271-5595-Fan-Control-Center-REPLACES-CARRIER_p_117.html
( your original is on the supercede list....HH84AA0020)

I could not find a wiring diagram, but you may have that on the inside of one of the cover panels.

Thanks jimbo, you are right on target. There was a diagram inside the lower panel. With your link providing a visual on the control board and the panel diagram it is quite clear where the sensors are. Like many things, now that I have found them, they were right in front of me . I am not sure if I should just order a control board or if there is a chance that the sensor is bad instead. I have a meter and am comfortable with it but, I assume this is a normally open or normally closed situation that just toggles under the right (or in this case not so right) conditions.

The heater is doing it's job. My concern here is unnecessary motor start-ups shortening the unit's life (and burning electricity). I figure it is better to be proactive on something like this than wait for a failure. I'm still curious about the stub between the heater's "closet" and the attic and the additional opening on the back wall. I'm sure they make sense in regard to something that I am ignorant of. Thanks again.

My quest on the additional openings into the closet continues. I have gotten some info from a semi-knowledgeable source that the air return (through the filter) should be the only return air into the heater's closet space. This would mean I should plug the 8" stub into the attic (just a foot long piece of 8" duct open into the attic and into the closet) and the opening at the rear of the closet with insulation or some such. Any opinions on this?

Now you are getting into trouble. THAT tube is probably combustion air, and if you mess with combustion air it can be dangerous if not malfuncioning!

Ah, thanks Jimbo. Serves me right for thinking too much about something I know so little about.

Conditioned air should be self-contained...all the heated air from the furnace outlet should be directed back to the furnace via the return. You don't want to be pushing that conditioned air out or pulling in unconditioned air. But, as was mentioned, when a combustion device is in an enclosed place, it MUST have enough air to burn the fuel AND that path must be free enough so that it doesn't try to suck conditioned air into the burner as well. Sizing both the heating ductworks and the air makeup for the burner need to be properly balanced to have a properly working system. Many of the newer, high-efficiency burners are what is called closed combustion...they pull their own combustion air from outside on a dedicated path directly into the burn chamber, and then exhaust the fumes on a second, separate path. In this manner, it should never suck in outside air through cracks in the walls or try to use conditioned air. An open duct into the attic may be a means to get combustion air into the closet, but if the closet isn't well insulated and sealed, you will be dumping heated air out there as well, and the cold air will leak into the house, potentially, significantly decreasing efficiency of the whole thing. It might be good if that opening had an automatic damper in it to limit air intrusion when not running.

Jimbo, you are the man. I just installed the new control board and the heater is acting normal again. Many thanks to all.