Hello,

My natural gas furnace has an intermittent problem. Occasionally it won't turn on when there is a demand from the thermostat.

Furnace: Tempstar Model NUG5125BHB1, About 18 years old.
Ignition MOdule: Honeywell S8600M
Fan Timer/Furnace Control: Honeywell ST9120G4004
Fuel: Natural Gas
Thermostat: Hunter 44155C (electronic programmable)

Batteries were replaced on thermostat about one month ago. Problem started 4 days ago.

Area: Denver, Colorado. Outside Temp: 30-40 day, 10-20 night.

Normal Operating Sequence: (manually raising temp on thermostat)
Thermostat makes "clicking" noise, the call for heat.
Exhaust blower comes on within few seconds. It runs for a few seconds then, the igniter activates (making a very rapid clicking noise), gas comes on and the burners are lit. About 30 secs later the fan comes on. Furnace works until it reaches 1 deg above set temp, then gas shuts off, burners shut down, and about 60 seconds later the fan stops.

Intermittent Problem Sequence:
Thermostat makes "clicking" noise, call for heat.
Nothing at the furnace. Exhaust blower doesn't come on. Firing sequence never initiates. Door switch appears to be working. There doesn't appear to be any indicator lights on the control modules, so no error code to check.

If I turn off the main power switch (located on the outside of the furnace), wait a few minutes, then turn it back on, the furnace will go through it's normal firing sequence. This problem has occurred three time now, at night or early morning, when its colder.

Let me know if you need anymore information. Any help is greatly appreciated. Any ideas on what could be wrong? Thanks.

 

Is the main burner ignited directly from the ignitor ?

OR is there a pilot light that gets lit by spark module ?

 

It appears the gas control valve has a small tube leading from it, to behind the burners and is next to the spark module igniter. It occurs so fast that I don't see a separate pilot light ignite, then the main burners. It appears to happen simultaneously (at least from my perspective)

 

This is the service info on your control board.

https://www.google.com/url?sa=t&rct=...42965579,d.dmQ

 

Thanks for the info and link. I looked at the start-up sequence and troubleshooting for the S8600M. It doesn't show the exhaust motor coming on just prior to the ignition sequence. This appears to be controlled from the Honeywell ST9120G4004 fan timer/furnace control module. I assume that this module gets energized before the ignition module. There appears to be a pressure switch connected to the exhaust motor. Could the problem be either a bad fan timer/furnace control module, or a bad pressure switch? Unfortunately, since the problem is intermittent I haven't had the opportunity to check voltage from the pressure switch or the furnace control module. If the problem occurs again, where would I check voltage or resistance and what values should I expect. Thanks

 

I have a tappan 90% furnace that is around 12 years old. Its runs with propane and is forced air and vented out side of house. I can get model # if I need to but its hard to get to with the snow.my problem is that when I turn the thermostat on for heat my furnace will go thru all the steps until after the igniter glows and as soon as I hear gas valve open it shuts off and blinks twice for stuck pressure switch.i replaced the pressure switch and still get same problem. I noticed that there was a lot of water inside the drain hole on furnace so I hooked up a wet dry vac and sucked out all the water and when I started it up again it ran for few minutes then blinked twice again. I then decided to leave the vacuum cleaner hooked to furnace drain running and when I started the furnace again it turned on and would run like normal and turn on when it called for heat.so now after burning out two vacuum cleaners cause of having to leave them on so I have heat. Could someone please tell me why my furnace will only run with a wet vac connected to furnace drain.all parts are working.only thing I could think of would be control board thank you