high limit switch on clayton 7.1

i'm new to the forum/wood furnace stuff (though we had one in my childhoold home). my GF has an older clayton 7.1 wood furnace; the backplate says "july 1983". the stove has the problem of the fan running for around 30s then shutting off for quite some time (could be minutes or what seems like hours) even though the stove is fired and quite hot. (aside from not heating the house efficiently, i worry about the stove overheating. a legitimate concern??)

it has what looks to be a honeywell L4064 high limit switch; see pics below. (the clayton manual doesn't give part numbers; google helped me figure out it was a honeywell L4064.) i took the cover off and observed that the scaleplate sticks in places as the temp moves up or down (e.g. when i put the blower on manual, for example) then it'll suddenly jump, giving off broken/stuck spring sounds. this switch on this stove uses a 5" temp probe. i'm planning to replace it with the same model but the L4064 looks like it comes in many specifications. any advise on which one to get? or maybe there's a different/equivalent part to consider; i.e. a drop-in replacement that fits in the same size hole used by the current switch? many thanks.

thanks for the speedy response. i'm out of town and don't have the clayton manual with me but i'll ask her to scan the schematic. my recollection is the high limit is 290*F (per the manual). the only other thermo on the system (the standard round household thermo) is upstairs and it is wired down to the motorized draft on the front of the stove. when the house temp is below the requested temp, a motor opens a draft vent in the front of the stove.

concerning the wiring in the picture, there are actually three wires to the switch. there is a black wire (unseen) behind my index finger in the left picture. so the white and black are both on what i believe is the fan side of the switch. you may be questioning that green wire on the limit side? from my reading of the stove wiring and schematic (as well as the honeywell wiring document for this switch (link below), the limit side of the switch (that green wire) supplies line voltage to a 120v-to24v transformer, which controls the draft motor. i'm guessing that when the stove temp reaches the limit, it shuts off the 120v supply to the transformer, which shuts down the draft motor, thereby closing the spring-loaded draft. (but again, i'm new to how this stuff works so i may be way off.)

here's honeywell's diagram. i think i'm using configuration C in figure 9, page 4.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-0117.pdf

i'll double-check for restrictions next time i'm in town.

BeGreen said:

What turns the fan on/off? There needs to be some sort of thermostatic control for this or the blower would run continuously.

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this high limit switch, which has the thermostatic element (that 5" probe on the back), controls the fan. the on/off point for the fan is around 90-100*F. at the limit, it shuts the draft damper. (if the switch was used on a gas furnace, it would shut off the gas to prevent overheating.)

BeGreen said:

PS: 290F is quite high for the limit. It would be good to see this whole setup to be sure there are not other problems causing the high-limit to trip. Hopefully it is just a failing fan thermodisk (snap switch).

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hope so too. i'll post the schematic; hopefully this this evening.

laynes69 said:

I had a limit/control go bad after 20 years on our old furnace. The limit controls both the main blower and draft damper. Where are the temperatures set at, off on? They will click when turning on or off. If the blower is short cycling it could be a few things. If the limit control was manipulated manually, the spring could be off or it's just weak from age. For all they cost, I would just replace it.

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yeah, i'm thinking it's just old age (almost 30yrs) and just replace it. the on/off set is 90 and 100. even though it sticks and jumps when the temp does up/down, the fan will eventually shut off when the wheel jumps below 90/100. i'm guessing the sticking must be worse when the temp rises, which explains why it can take a long time for the fan to kick on.