I am 99% sure it is the fan limit switch, causing frequent furnace blower cycling, (even when the burner is off).
It was replaced 20 years ago for the same issue.




fan limit switch
Lennox# 15P38
or
Honeywell# L4064W1098

As a momentary fix, changing the fan limit switch setting 5° F seems to have reduced the frequent blower cycling issue.


.

Assuming it is the common bi metallic version. They do fatigue and tire with time. Usually in my limited experience fan controls slowly lose their range of movement. Or lack of actual response in degrees moved for thermal changes if you wish. They have just lost their rotational motion torque to a large degree so their range of movement becomes very weak and impaired. It takes work to close contacts and they cannot do it reliably or accurately when tired.

Twenty years may even be a little longer than usual for an average lifespan of a fan control these days. You can lose some efficiency of heat delivery with a shakey old control. Once you get down to doing minor adjustments to get by with it. It is time.

Now the fan control on a really high delivery modern gas furnace may not rely on a bimetallic control. I have no familiarity with more modern gas furnaces.

That unit of yours looks like a simple bi metallic fan switch with the high limit incorporated. Judging by the wire count. I did find the appearance of it somewhat different so looked up the part numbers. Pictorially they did not show the same control as you have. It is almost like someone may have installed a different unit than was original. Can you have a look at the fan switch itself for a part number? The lennox control has been discontinued but suspect it was probably a Honeywell relabel version anyways. Also the Honeywell part number has been replaced as well.

I usually buy heating system parts as needed at local trade suppliers. Some plumbing and electrical wholesalers suppliers have a furnace controls section.

Before spending money on a new fan/limit control I would check the condition of the heat exchanger. After 20 years those old Lennox models are riddled with rust.
Just get a flashlight and mirror and check the top of the heat exchanger through the burner ports in front.

Back when I was working as a sub-contractor for the gas utility in Toronto I would have to disconnect and red tag at least 5-6 every month.

__________________

OLD & RETIRED
_.... Don't mess with old farts - - age and treachery will always overcome youth and skill! Bulls**t and brilliance only come with age and experience!____________
'96 E320
'79 450 SEL
'78 300 CD

I always thought it prudent to have at least a carbon monoxide detector in any dwelling where any form of fossil fuels where burnt. Early detection of inner body or flue issues are caught. Probably code in a lot of juristictions on new construction where any form of fossil fuel is used. Hope you do not find your older heat exchanger broached yet.

Call it paranoia.
https://www.google.com/search?q=defective+carbon+monoxide+detector&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-USfficial&client=firefox-a

Having lost several relations to carbon monoxide, I feel this is rational.

I have two carbon monoxide detectors, purchased five years apart.

They are from different manufacturers.

They are tested twice per year.

I replace one unit every ten years.

http://www.trulia.com/blog/KathleenTurner/2010/12/help_it_s_1am_and_my_carbon_monoxide_detector_is_going_off

defective carbon monoxide detector
https://www.google.com/search?q=defective+carbon+monoxide+detector&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-USfficial&client=firefox-a

.

I will inspect it again asap, four years since last checked.

I am looking at replacement this year, increasing heating capacity to 125,000 btu.
The old Lennox unit never has been adequate under -10° F.

Note:
Please don't suggests sealing / insulating the house better, the walls are R-30, attic is R-60, floor is R-19, foundation crawl space walls R-13, windows are twenty year old thermal double pane.

.

How many -10F days do you see? May not be worth upgrading just to solve that problem. If it has trouble at that temperature, a couple of $30 electric oil radiators could supplement it for the few days of the year it's working hard.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

House exposed to a lot of winds? Possible distribution issues rather than heat generation capacity? Efficiency of the old furnace would be less than a new one today anyways. Perhaps in a very big way.

Insulation does not do much good if substantial penetration by wind is occurring. The gentleman previous posting may know something about those early furnace efficiencies in comparison to todays.

Back approximately 40 years ago when your furnace was new. If it was that long ago. Natural gas was generally cheap. No reason for a manufacturer like lennox to give great efficiency then.

The last new gas furnace our son in law installed looked loaded with computer type driven functions. It actually surprised me how complex it seemed. When they get down to only needing a plastic pipe through the wall outlet they have to be efficient.

Well if the detectors are not firing off it sounds like no breeches exist yet and that is a good thing. A shakey fan switch or one not set right will seriously waste heat. Plus 125,000 btu is a a lot of capacity up to about 2500 square feet floor area in a house as well insulated as yours usually.

That's is if the ductwork is not choking it off . There are simple ductwork guides on the internet that do not require a lot of math calculation. A quick separation test is to read the plenum temperature with a spot heat gun. It should dip and stay lower after the fan starts. If it rises further I would assume the ductwork is inadequate for the furnace output. Or the fan is too weak.Then too much heat is going up the flue.

Old, low-efficiency heating systems:

Natural draft that creates a flow of combustion gases
Continuous pilot light
Heavy heat exchanger
56% to 70% AFUE.

Mid-efficiency heating systems:


Exhaust fan controls the flow of combustion air and combustion gases more precisely
Electronic ignition (no pilot light)
Compact size and lighter weight to reduce cycling losses
Small-diameter flue pipe
80% to 83% AFUE.

High-efficiency heating systems:

Condensing flue gases in a second heat exchanger for extra efficiency
Sealed combustion
90% to 98.5% AFUE.
Furnaces and Boilers | Department of Energy

__________________

OLD & RETIRED
_.... Don't mess with old farts - - age and treachery will always overcome youth and skill! Bulls**t and brilliance only come with age and experience!____________
'96 E320
'79 450 SEL
'78 300 CD

The blower died, pulling it now for teardown, inspection, repair or junk.
Will buy a replacement unit tomorrow Lennox# 89C37

Electric space heaters for tonight.

.

I've replaced a number of blower motors. Not a difficult job and worth doing if the heat exchanger is intact and the furnace otherwise is functional. In a pinch you could put a box fan in there and wire it to the blower wires.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I will try this, there are several box fans on hand.

.

I don't think a box fan will move enough air and the over temp sensor will trip over and over because the heat exchanger will over heat.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

.

ugh... looks like two or more fields are burned out. does the dataplate have an RPM or HP rating?

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!