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Ducane Furnace Question

try2hard
try2hard Member Posts: 26
Hi everyone. I'm currently suffering this winter with an existing Ducane forced air furnace. In the Spring, I'll be tearing it out and preparing for my radiant installation.

The Ducane has been working well until today. We're in a colder-than-usual pattern right now, single digit highs and 0 or below lows. But, my heat loss calcs are about 70,000 BTUH at -10/+72 design temp and the 75,000 BTU input HE furnace has been keeping up with the demand, without a problem.

Tonight the burner started short-cycling, but not rapidly enough to make me suspect the flame sensor. The thermostat calls for heat, the combustion fan kicks on, burners ignite quickly with good flame color and pattern. The burners continue to run about 10 minutes, meanwhile all operation is normal. Next, the burner kicks off, despite the continued call for heat.

The gas valve light reads normal operation per the manual. Solid green while in standby, slowly flashing constant during operation. The furnace control board lights also indicate no errors.

I tried turning off the power, waiting a few minutes, then turning it back on to no avail. About 10 minutes after the burner has shut down, it restarts the process. However, if I manually shut off the thermostat and turn it back on right away after the shut down, it relights right away (no lockout).

If it were a flame sensor issue, wouldn't the burner stop within seconds after ignition? Any other ideas?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,065
    What safety sensors are on that unit? I'm not familiar with it. It almost sounds as though something is tripping off, and then resetting after a bit.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • try2hard
    try2hard Member Posts: 26
    I'm not sure about the safety sensors. It's very much a "low-end" brand but my background is hydronics so the forced air is a little different. It's just strange there are no faults being reported.

    I think I'll pull the flame sensor out tomorrow after work and check it out and clean it anyway. What could it hurt?

    I'm just trying to make it through the winter without putting much money into it (more for my new boiler, controls, tubing, etc.)
  • Ironman
    Ironman Member Posts: 6,562
    Check your condensate drain and vent system for partial blockage.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • unclejohn
    unclejohn Member Posts: 1,749
    If no errors are coming up check the tstat batteries or just crank up the stat as high as it will go and see if it stays on.
  • Alan Welch
    Alan Welch Member Posts: 255
    Possibly shutting off on high limit, dirty air filters?
    try2hard
  • try2hard
    try2hard Member Posts: 26
    I did change the thermostat batteries and even tried swapping it out for another stat, same result. I can hear that the thermostat contacts don't open when the burner shuts down.

    @Alan Welch -- good idea, I didn't think of that, all though I just changed the filter about a month ago.

    Like many forced-air systems, it's very poorly designed, has undersized supply and (especially return) ducts. I can't wait to get back to having a hydronic system again. I know I'll be comfortable next winter, lol. And of course, it's oversized much of the year since it's not multi-stage or modulating.

    I'll check out the air filter and check the flame sensor when I get home from work today.

    Thanks for everyone's comments -- I'll reply back later.
  • Mike
    Mike Member Posts: 94
    Check the pressure switch. I've had a bunch that would act as you're description. Check the switches pressures, vent motor could going.
  • John Mills_5
    John Mills_5 Member Posts: 935
    Nothing wrong with Ducane furnaces. New ones are just a Lennox with a different board. Could be cycling on limit. We find that people don't know it is happening til it's so cold they miss the heat while it is off.
  • try2hard
    try2hard Member Posts: 26
    Well, it appears to have been a thermostat issue. Although I didn't hear the thermostat contacts opening, they must have been. I switched it out for a new Honeywell and the furnace ran until the call for heat was satisfied.

    Thanks for everyone's suggestions.
    DZoro
  • ratio
    ratio Member Posts: 2,925
    This is the third got-it-fixed followup I've read today—thanks!
    DZoro