Sears Direct Vent Wall Furnace fans cycling?

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Old 12-29-07, 03:04 PM
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Sears Direct Vent Wall Furnace fans cycling?

We have a Sears Model 867.778030 Direct Vent Wall Furnace, and the fans have started cycling after the burner shuts down.

Recently, the pilot refused to light, so I lightly cleaned the tip of the tubing and the thermocouple. I also noticed that many of the burner jets were not working, and found that one of the needles my wife uses to hold turkeys together, just fits in the holes that were burning, so I used it to clean out those holes I could get my hand to. Everything was working until yesterday, when the fans started cycling (it starts at about 15 second intervals, and gets down to about 2-3 seconds, but it never stops).

A couple of other items. First, we replaced the mechanical thermometer some years ago, with a digital programmable thermometer. Second, there is an adjustment screw on the front of the unit, which seems to have no effect on the operation of the unit.The tag attached to the screw says, "Use this screw to adjust main burner flame".

Any help and guidance will be greatly appreciated.
 
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Old 12-29-07, 07:26 PM
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Originally Posted by froglips View Post
We have a Sears Model 867.778030 Direct Vent Wall Furnace, and the fans have started cycling after the burner shuts down.

Recently, the pilot refused to light, so I lightly cleaned the tip of the tubing and the thermocouple. I also noticed that many of the burner jets were not working, and found that one of the needles my wife uses to hold turkeys together, just fits in the holes that were burning, so I used it to clean out those holes I could get my hand to. Everything was working until yesterday, when the fans started cycling (it starts at about 15 second intervals, and gets down to about 2-3 seconds, but it never stops).

A couple of other items. First, we replaced the mechanical thermometer some years ago, with a digital programmable thermometer. Second, there is an adjustment screw on the front of the unit, which seems to have no effect on the operation of the unit.The tag attached to the screw says, "Use this screw to adjust main burner flame".

Any help and guidance will be greatly appreciated.


Unfortunately, your post illustrates the hazards of having unqualified people doing repairs on heating equipment.


If you think you have a dirty pilot burner, it should be disassembles and cleaned completely, and the orifice replaced if the result is doubtful.

If the burner is dirty, it needs to be removed and throughly cleaned, inside and out, then reinstalled and adjusted properly.

The adjustment screw is there to control the amount of primary air is supplied to the gas burners. It's similar in function to the adjustable air shutters founf on many bunson type burners.

At least the fan problem is a nuisance at worst, rather than being a safety hazard at worst. It may be an adjustable fan switch with the temperature set too low, allowing the fan to cycle on and off. Or the fan switch may be defective and need replacement.


Worse than not doing these repair jobs properly is the significant risk that you will fail to recognize hazards that exist but aren't causing symptoms you notice, or that you will overlook maintenance work that needs to be done which you don't know about.
 
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Old 12-29-07, 08:06 PM
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First, thank you for your response.

Second, the past two time we have needed to call on "professional, certified, and qualified" HVAC mechanics to work on this heater (both from local Heating/Fuel companies) it has resulted in both poor ineffective quality, yet VERY EXPENSIVE repair work! Yes I was aware of the need to completely disassemble and clean the entire unit, but at 13 degrees outside, I was trying to ease out another 2-3 months of heat from it. I want to replace the unit with a high efficiency unit in the spring, and did not want to pay for a complete rebuild of this unit first.

Again, thank you for your time and knowledge.
 
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Old 12-29-07, 08:26 PM
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The 13 degree weather may account for the fan shortcyling. It thinks it is satisfied and opens, then the burner(s) continue to heat and the fan switch closes. The cool air the blower is pulling across the fan switch once again fools the switch and it opens. Should stop doing that after the room warms up a bit.

IF that is all it is and not a mechanical problem
 
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Old 12-30-07, 07:34 AM
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Per mbk - yes.

And also if standing pilot, the heat generation from this, coupled with a weak or otherwise out of adjustment fan switch could cause this:

Heater flame and fan runs, flame shuts off when set temp met, then fan shuts off. Internal heat gets hotter in heater after shut down of flame(they all do that), fan comes back on and cools heater down again, fan shuts off for a bit, then pilot heat makes fan come back on again due to low "fan-on" setting or weak fan switch, depending on kind you have. Then there is finally a re-call for heat, and fan winds up coming on as it should, but now you can't tell the difference if it is from short cycling or normal operation due to all the overlaps.

You may just have thermodisc fan switch and limit system. Those go bad. Old mobile home furnaces were set up like this and gave similar symptoms as yours. New fan thermodiscs or thermodisc limit switches always cured my short cycling or limiting conditions.
 
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Old 12-30-07, 08:59 AM
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ecman51 & mbk,

Thanks, and yes all there is in the circuit is a Therm-o-disk (although it states in the parts manual that you can use a TI Limit Switch). This would make the most sense to me ... 30+ years as an Electronics Technician, so I guess that I will try and find one locally (highly doubtful in our area).

Again, thanks for your time and help.
 
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Old 12-30-07, 09:44 AM
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An appliance parts store that sells parts for washers, dryers, refrigerators and such may have such a part, as well as furnace repair companies and distributors of furnaces and such.
 
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