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Old 02-06-2012, 01:58 PM   #1
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Gas Fired Heater - Fan short-cycles after normal cycle


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.

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Old 02-06-2012, 03:31 PM   #2
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Gas Fired Heater - Fan short-cycles after normal cycle


Are you saying that after the burners have been on for a while, that the main blower comes on but but never gets up to full speed before shutting off again and then continues that same cycle a numer of times
Or
Are you saying that after the main burners are off and the heating cycle is finished that the main blower comes on & off a few more times?

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Old 02-06-2012, 04:22 PM   #3
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Gas Fired Heater - Fan short-cycles after normal cycle


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Are you saying that after the main burners are off and the heating cycle is finished that the main blower comes on & off a few more times?
Exactly.

The heater runs a normal cycle and ends; heat off, fan off. Heat remains off (no call for heat from thermostat and no sound of burner heard) but the fan kicks on and almost reaches full speed before shutting down again, then the fan kicks on again (still no call for heat on the thermostat display and still no sound of the burner firing), almost reaches full speed and shuts of. Once the house cools off enough to trigger the thermostat to ask for heat again, the cycles runs as expected but, does the double fan kick again after the cycle ends.

I know little to nothing about HVAC but, have researched enough to start thinking the following: 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. Way outside my area of expertise and thank you all in advance for any assistance.

Last edited by glenn bradley; 02-06-2012 at 04:25 PM.
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Old 02-06-2012, 05:08 PM   #4
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Gas Fired Heater - Fan short-cycles after normal cycle


??????
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Old 02-06-2012, 05:27 PM   #5
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Gas Fired Heater - Fan short-cycles after normal cycle


I actually had a similar issue on my Trane furnace in my last house. The house/furnace was about 4-5 years old when it happened. Very similar to what you are describing, after a cycle there would be repeated, short-run, on-off from the blower before everything would turn off.

I had the repair guy out and he pulled out a circuit board with a hole burnt right through it. Part was under warranty, but I had to pay the labor. I guess it's such a common problem in that model that he actually had spare boards on his truck. He plugged in the new circuit board and I didn't have another problem while I lived there, another year or two.
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Old 02-06-2012, 05:46 PM   #6
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Gas Fired Heater - Fan short-cycles after normal cycle


If your furnace is 25 years old it may well be a fan control adjustment that is needed. There is sometimes enough latent heat in the exchanger to trip the fan control into turning on the blower again but ususally it's just the controls that need to re adjusted again. Look for a box attached to the wall of the furnace that may also have a summer fan contol switch on it . It will probably have 3 wires attached to it. It is usually positioned above the burners and has an internal probe that measures the temp of the exchanger. It's cover should just pull off. Inside you may see a round dial with temp markings on it and 3 nibs representing the temp when the fan turns off, on and a safety limit temp. Holding the dial in place, move the low temp nib a bit lower. You want the temp coming out the vents to be barely warm to the hand when the blower shuts off. You can also stop the short cycling by turning the middle nib a little bit higher but that might cost you a bit in fuel efficiency. A typical nib setting would be 90F off/ 125F on & 200F for the limit.

With the power off, also clean off that dirt you mentioned on your blower fins.
Let us know what you find?

Last edited by how; 02-06-2012 at 06:05 PM.
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Old 02-06-2012, 06:08 PM   #7
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Gas Fired Heater - Fan short-cycles after normal cycle


if you don't have a control board within the furnace it is just left over heat on the heat exchanger then it stays off...silver covered HONEYWELL controller with a dial within with 3 pin settings,might have a thermal disc mounted just above the burners...over the years the heating/and coolng of it takes its toll...making it short cycle afer a heating cycle..see the thermal fan disc or fan limit might be one you have http://www.bestbuyheatingandaircondi.../fan&limit.htm

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Old 02-06-2012, 06:18 PM   #8
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Gas Fired Heater - Fan short-cycles after normal cycle


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Originally Posted by how View Post
Let us know what you find?
Thanks guys. I already cleaned out the blower fins and chamber. I believe I have seen the box above the burner area you mention and will give it a look after work and report back .

I am still curious about the stub going from the heater "closet" into the attic area and the opening on the back wallboard allowing air from between the studs(?). Any thoughts on these?

I am reasonably sure the unit is newer than the house as the floor area was re-structured to accommodate the different footprint of the current unit. I think I realize now that the Carrier unit on top of the heater is part of the A/C unit. I am learning more about this than I ever wanted to know

Last edited by glenn bradley; 02-06-2012 at 06:18 PM. Reason: sp?
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Old 02-06-2012, 08:09 PM   #9
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Gas Fired Heater - Fan short-cycles after normal cycle


the return should only be returning conditioned air that was discharged at one time.ny other lower temp air from a crawl space or wall cavities will cause the unit to run longer.WHY...the furnace is only designed to raise the return air so high it's easier to reheat 70F return then 40F return.
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Old 02-06-2012, 08:23 PM   #10
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Gas Fired Heater - Fan short-cycles after normal cycle


I have a control board like this: http://www.air1supply.com/ICM271-559...IER_p_117.html

There is a wiring diagram on the inside of the lower cover that made it easy to find the sensors. The one in the exchanger is screwed into a rectangular hole with a gasket of some felt-like material. No adjustments apparent on it, just two wiring terminals that are in use. There is another sensor just below the vent which is like a metal button that pokes through into the discharged air path (not the house air path).

It sounds like I am hearing what I suspected. The additional 8" stub into the attic and the opening at the rear are probably left over from the previous unit. The previous owner may have installed this thing himself. There were/are many signs that he was a bit of a handyman with varying levels of success depending what he was doing. He may have not known or not cared about sealing them off. I can just pack the openings with fiberglass insulation if you think that is OK(?).

Last edited by glenn bradley; 02-07-2012 at 11:02 AM. Reason: sp
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Old 02-06-2012, 08:54 PM   #11
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Gas Fired Heater - Fan short-cycles after normal cycle


If you have a board, you can ignore my post about the nibs and fan control. The devise you see that you think may be the fan control is actually a limit control that will not help you.
Your fan will be activated by timers on the board, not temps.

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