We have a natural gas-fired Tempstar furnace that sometimes will not start-up. It is controlled by a setback thermostat. In the morning it normally fires up fine for the first heating cycle, but usually gets "stuck" on the second cycle. By stuck, I mean we get a yellow flame (coil heating up?) for a few seconds, then a blue flame (burner jets?) for a few seconds, then a click and the blue flames go out. The cycle is repeated a few times. Sometimes the burners successfully light and other times not. If the burners do not light within about five tries, a red light on a module inside the furnace comes on - which I'm guessing is an error light of some kind.

If I turn the system off (toggle switch on side of the furnace) and back on, it sometimes will get the system running again. Sometimes not...

We've had service technicians out twice. The first said everything looked fine, did a cleaning and things seemed to work okay until the end of the season when the same issue started occurring. This year the system ran for a few weeks without issues, but now it seems to be occurring again. A technician was out to investigate last week and said he found a loose wire, which he reinstalled. However, the next morning we had the same issue... it fired up for the first cycle and failed to fire up on the second. I turned the system off and it ran.

Any chance this is related to the thermostat itself? I think not, but I'm not a technician.

Any good advice on where to go with this would be appreciated. I hate to keep paying $100 per trip for technicians who don't seem to be able to find and resolve the core problem.

Kindly post the model and serial number off the ID tag of the furnace for me... THANKS

TSA: The model number is NULK100DH06 / 867.779435
The seriel number is l902368 511. Thanks in advance for your interest.

FYI, I do not intend to try to repair this unit myself. I'm simply trying to understand the problem and to direct a qualified technician to do the repairs.

Yoppdk

I wasn't sure it it was clear from my previous answser, the seriel number is L902368 511.

I'll post my help answer Tues. AM, my manual for that unit is at work...

The most likely cause is the flame proving, the original manual shows the ignition system as a HSI ( hot surface ignitor) with a flame sense rod proving flame back to the control module. And after a number of tries to prove flame as you stated, it will lock out, and you will have to remove the power to clear the lockout. You may be able to remove the flame rod ( with power off ) check that it does not have a crack in the ceramic, and clean it with sand paper or rough steel wool, to get the oxidation off it... which usually happens due to damp basements, and as well with burning LP gas, and be sure that the wire going to the flame rod and control is tight and not corroded, and in good condition.. . To find the flame rod, follow from the control wire marked "sense" to the flame rod which is located at the burners. If that does not solve your problem... it is a bad control..

tsa7man:

Thanks, I really appreciate your time and advice. The service tech who did not successfully get this resolved last week is coming back tomorrow. I will ask him to check the item(s) you mentioned. I'll let you know how it turns out.

yoppdk

TSA7man:

The service tech cleaned the flame rod and also found a poor connection on a grounding wire, which he corrected. This has taken care of the problem. Thanks for your help.

yoppdk

The most likely cause is the flame proving, the original manual shows the ignition system as a HSI ( hot surface ignitor) with a flame sense rod proving flame back to the control module. And after a number of trys to prove flame as you stated, it will lock out, and you will have to remove the power to clear the lockout. You may be able to remove the flame rod ( with power off ) check that it does not have a crack in the ceramic, and clean it with sand paper or rough steel wool, to get the oxidation off it....which usually happens due to damp basements, and as well with burning LP gas, and be sure that the wire going to the flame rod and control is tight and not corroded, and in good condition... . To find the flame rod, follow from the control wire marked "sense" to the flame rod which is located at the burners. If that does not solve your problem...it is a bad control..


I am having the same issue with my furnace. AirPro model# BGU07512A Ser.# 99167789. I too have a hot surface ignitor. Is the flame rod the one to the left of the burner that looks like about 2 inches long? How do I remove it to clean it? Do I have to turn off the gas? Not sure I know how to do that.

Thanks,
Cory

Cory, turn off the power to the furnace... take out the flame sensor ( your descriptions seem correct... that should be it.) Clean it with real rough steel wool, and or sand paper to remove the corrosion. While you have it out, check that the white ceramic insulator is not cracked, and re-install it back in the correct position.

Cory, turn off the power to the furnace...take out the flame sensor ( your descriptions seem correct...that should be it.) Clean it with real rough steel wool, and or sand paper to remove the corrosion. While you have it out, check that the white ceramic insulator is not cracked, and re-install it back in the correct position.


I took out the flame sensor and there was some corrosion on it. I cleaned it all off and reinstalled it. The furnace as worked like brand new since.

Thanks everyone for your insight.

Cory

Since the other two problems were solved so well, I thought I might as well try my problem out.
I have a Tempstar model #NUGM050EFB1 high eff. Furnace. First I noticed that the fan was running with no flame. The igniter (heat style) wasn't glowing so I figured that may be the problem. Pulled the igniter and the heating element looked crusty so I lightly touched it with an exacto knife to scrape it clean and it crumbled apart. Replaced the igniter, and now the pilot lights, then the burners do ignite, but they immediately turn back off. It will do that three times, then the burners won't fire up again until I kill power to the furnace and switch it on again. Had a tech out and he said now it's most likely the gas valve. Replaced the valve part #SV9501M2528, but it still does the same thing. Did some research online, and found that the circuit board that's mounted to the fan housing also controls some of the sensors. Replaced the board, part #ST9120U1003, same problem. There are two sensors that are in series, one in the firebox and one in the pilot box. I bypassed both of those sensors and connected the red wire that comes up from the circuit board to the one that goes back down so that I have a direct connection without any sensors. No difference either. I also pulled out the primary blower fan and took apart the housing to see if there was any obstruction in the drain area. That was fine, and so is my drain line going to the floor drain. I also pulled the drip line pipe in my gas line, bone dry. The vacuum hoses are in good shape. The vacuum modulator clicks on shortly after the blower fan turns on and ignites the pilot, so that is working. There is another hose coming from that modulator and goes to a "t" fitting on the side of the pilot box, then another hose goes down to the gas valve. Inside the box, that "t" fitting is open which appears to be right, but what does that part of the vacuum line do? Or is that pressure? I tried disconnecting that hose, holding my finger over the open "t" and blowing and sucking on the hose while it was still connected to the gas valve, but it either did the same thing or the burners wouldn't fire at all. I've replaced practically all the suspect components, and bypassed others. I've run out of things to look for, there's not much left! Any help beyond what I've already done would be greatly appreciated.

Try cleaning the flame sensor. Some look different than others but here is a reference pic.

It's brand spankin' new, I just put it in. But for kicks I'll give it a look.

If the burner lights and then turns off righ away it is usually a flame sensor problem or the wire on the flame sensor is not sending the signal back to the board or valve. This is a safety device and the only item that I am aware of that would kill the flame immediately unless the valve/board is defective.

OK, I'll give it a good once over again. I hope it does the trick, I'll let you know.

One more thing, is it possible that my burners are bad. If they are rusty, would that prevent them from sending heat back to the sensor, or is the sensor registering overall air temp?

The sensor senses the flame and creates a signal that all is OK so the burner will continue running.