Heating, Air Conditioning, Fridge, HVAC/gas heater blower & burner control
Residential Trane gas furnace >15 yrs old with White-Rodgers Mod 50A50-404 controller, 1200 sq ft 2nd floor of 1-1/2 story house. After turning unit off and on at the thermostat switch (reset the problem), unit would work okay for 1 or more cycles, then blower would come on w/o burner, so it was blowing recycled air cooled by attic temp to about 10 deg below room ambient. Upstairs gets colder and colder. Turn unit off at thermostat and on again (which is essentially a reset) but same thing happens - repeated this maneuver many times. When malfunctioning, the LED code on controller blinked message 2 (system lockout-no flame). The inside thermostat was electronic w/batteries; changed to Honeywell round Hg switch - still the same problem. Person who installed the unit 18 years ago has been out four times (1st) couldn't see anything wrong (unit has just been turned on; (2nd) cleaned the flame sensor (unit worked for about an hour); (3rd) replaced controller with White-Rodgers Mod 50A55-486 (called "3797" on invoice)--Worked for an hour and then same problem; (4th) he jiggled wires, tested repeatedly, said it was bad connection but fixed now (unit worked worked for several cycles then same old cold air thing). The LED error code on the new controller is also 2 blinks. It may be that every time the unit appears to be fixed, it is because it has just been turned back on, and it will work for some cycles (including testing) no matter what fix has been done. When I interrupt power to the controller, something resets and the LED blinks slow steady (=normal no call for heat). But if the heat is turned on, the problem will happen again the next cycle or soon after. So I cut power to the blower and left it off. The real problem hasn't yet been addressed. It is something other than controller and thermostat that resets with power turned off and on. Repairman is now out of town until next year. Any suggestions what I could do next to diagnose the problem? Thanks.
Did he check the gas pressure at the valve? Did he clean the burners and crossover? If he didn't he should have. I would call someone else to look at it. When he jiggled the wires and it worked for a while I would have looked for a bad ground.
About all you can do now is check the wires to make sure they are connected tightly.
Let me know if you have any other questions.