Hope someone can help with advice. I have an old (not sure of the year) Sears Homart model 867 with LAU blower (80K or 90K BTU, I think). Within the past year or so I've replaced the main gas control and the blower motor and thermocouple. Now, sometimes the blower comes on several minutes before the furnace ignites and blows cold air until the furnace ignites. Other times, they both come on as they should, but the blower continues to run for several minutes after the furnace shuts off. Then, from time to time, the entire unit works as it should. The auto/man.(?) switch has been replaced at some point,too, but that was done before I moved in approx. 2 yrs. ago. Any help would be appreciated.

 

You mention an "auto/man. switch". Does this switch have a dial inside the cover with numbers on it which range from about 70 to 200 & have pointers set at various numbers? On this furnace is the blower on the top or bottom?

 

The switch for the blower does have numbers and pointers like you described, but they actually go from 50 to 300. I don't understand what the numbers mean or their purpose, so maybe you can help. The blower is on the bottom of the furnace.

 

On the dial, there should be either two pointers & a stop or three pointers. At what numbers are the pointers set?

 

The stop is set at 200. There is three pointers, and appear to be set at 100,150,and 200 where the stop is. They kind of appear to be just lined up behind the stop,but it's a little hard to tell exactly where they're at because the pointers are kind of thick.

 

The device to which you are refering is called a fan & limit (or fan/limit) switch. When the furnace fires & heats to the center pointer it turns the fan on. The pointer at the highest setting is the limit. If the furnace gets this this hot, the limit will shut the burners off as a safety device. The pointer at the lowest setting is the "fan off" setting. Once the furnace cools to this point, the control shuts off the fan. If you want to adjust this setting, hold the dial so it does not turn (Very Important) & push the pointer up to about 110-115.
I presume your control has four wires connected to it (2 at the bottom corners & 2 at the top corners). Are there by any chance two other wires connected just below the dial?

 

I think the switch is a generic replacement unit. All the wires are on the bottom below the dial. There's one wire on each bottom corner, and a single wire is attached to both top poles from one to the other(a ground,I assume). The wiring has never been changed or adjusted since I've been here, so I guess it's wired correctly. The dial is turned clockwise quite a bit, though. I wonder if perhaps the furnace did limit and the switch needs to be reset? I looked closer at the pointers, and they appear to be set at 110, 150 and limited at 250. The furnace did come on a few times before I replaced the blower moter(the blower wouldn't come on),so it could have built up some serious heat in the furnace that could've caused it to limit. I would think the guy that serviced the blower would have checked that but who knows. When the blower went bad, it made several loud clunks as the moter attempted to start but then shorted out or burned out from heat or whatever caused it to fail. Eventually,after 12-15 times of trying to start, the motor quit altogether. Could the switch have been thrown or even damaged by the motor shorting out like that?

 

It is possible the motor overload damaged the fan/limit. If you elect to replace it, get one with the same length probe. Standard lengths are 5,8,&11 inches.