A friend of mine just spent $100 having her Armstrong Ultra SX-80 furnace fixed. According to the repairman, this is a common problem with this model and is easily fixed by the homeowner -- so I thought I'd post the fix here and, hopefully, save some future reader a few bucks.
Her problem was that the furnace would not light. This unit has an electronic ignition that will not function unless conditions are OK in the combustion area. When the thermostat calls for heat, the combustion fan motor turns on, drawing in fresh outside air. The pressure caused by the fan is detected by a separately mounted pressure switch. If the switch is not activated, no ignition.
The pressure switch is connected to the combustion area by an 18 inch length of 1/4 inch rubber hose. It turns out that there is a lot of condensation in the combustion chamber in the area of the hose fitting and particles of rust can clog that fitting. Once the serviceman realized the pressure switch wasn't activating, the first thing he did was pull the hose off the fitting near the fan and ran a wire down the fitting bore to clean it. He reattached the hose, hit the power switch and the furnace fired right up. Total time: 10 minutes, tops.
I now know what to do next time -- and I expect there will be a next time.
Hopefully, I've saved you a service call.