I realize you don't do DIY advice here; I'm just looking for some insight.
My house has an an older high-efficiency furnace, "first generation 90%" according to an HVAC pro I know who dropped by casually but did not diagnose. It's a Kenmore model 867.769010, sold by Sears in (I think) the early '80s. We do not have central air. The furnace operates the way it always has over the 5 years we've owned the house, but a couple of friends recently pointed out that it's not acting as it should.
First, the blower kicks on instantly when the thermostat turns on the furnace. I've been told that the blower should not come on until after the burners have started to warm the heat exchanger, but this is not what happens; blower start-up is instantaneous.
While the furnace is operating, it always seems as if the airspeed in the ducts is faster/louder than it should be. The wiring is set up so that the blower should be running on low speed, but it sure is moving a lot of air. It's a 50k btu furnace in a small house (600 square foot first floor, no ducts to finished attic) and has only four 6" ducts coming off of it, so perhaps it simply moves more air than it should for so few ducts, even on low speed.
Lastly, when the thermostat shuts the furnace off, the blower shuts off instantaneously too, rather than continuing to operate until the exchanger cools off as I understand it should. There is always a several-second delay of silence, then the blower (but not the burners) kicks back on briefly -- I assume this is the limit switch doing its job.
So there you go. Yesterday I made a copy of the wiring diagram and traced every wire on it, crossing off each as I went (this was a suggestion of the hvac pro I chatted with). All of the connections inside the furnace are as they should be. The only discrepancy between the diagram and what I actually have is the way the thermostat is hooked up. The diagram shows a 3-wire connection to the thermostat; red, white and green. but it appears that the installer used the old 2-lead wire that was already threaded through the walls up to the thermostat; the red and white are correct, the green was omitted.
Having acquainted myself with the various parts I intuitively wonder about the fan relay and limit switch, but what I really need is a sense of how complex/expensive this will be to fix. It's already an older unit, so I'm disinclined to pour a lot of money into service calls if it will be such a tricky diagnosis that a new unit would be smarter. Any thoughts or advice?