Kenmore furnace flame sensor question

______________________________________________________________________ Yesterday, I cam home to find that my furnace would not bring the temperature in the house up from 61 degrees to 67 degrees. After reading a lot of posts on here, my problem seemed to be of the dirty flame sensor variety. The furnace would fire up, light the main burner, and then approximately 10-15 seconds later, the flame would go out. The cycle kept repeating. This morning I went down and removed the ignitor/sensor assembly (Sears p/n Q345A1313 (612921)). The first thing that I noticed when I removed the large HV wire with the spark plug type boot on it was that the actual metal tip that screws onto the threaded end of the metal rod running through the center of the ceramic tube of the ignitor assembly was loose. I was certain that this was my problem since the sensor works in the microamp range. A loose connection could certainly cause this problem. I used steel wool and some 220 grit sandpaper to clean up the ignitor tip and the metal tab that curves over top of it. I also made sure that the tip was on tight when I reinstalled the assembly. The furnace fired up and ran for approximately 5 minutes. When I came upstairs, I noticed that I could hear the gas valve cycling. I went back down to the furnace and watched the next cycle. Approximately every 20 seconds, the gas valve closes and the main burner goes out. A small flame stays burning near the ignitor assembly. Approximately 10 seconds later, the burners re-ignite. The main blower fan never stops running until the thermostat stops calling for heat. I set the thermostat to 74, and the furnace has been running for 14 minutes straight. The temperature in the house is now up to 70 degrees. My question is: Is it normal for the burners to cycle off/on 2-4 times per minute during a long heating cycle? I do not recall hearing the valve opening and closing like that before, but I wasn’t really paying attention. You never do until there’s a problem. The fact that I now have heat indicates that I (at least partially) fixed the problem. Do you think I should replace the ignitor / sensor with a new one, or does it sound like the valve or control unit has a problem? Thanks, Michael Replies: Freon “Re(1):Kenmore furnace flame sensor question” , posted Tue 1 Mar 10:27 ______________________________________________________________________ If it’s igniting and staying on for as long as you indicate, I would suspect the high limit temperature sensor is shutting off the gas. To test if it is, in fact, the high limit sensor, locate the sensor and with the furnace doing its 20 second on, 20 second off routine, use an insulated wire with alegator clips on each end (Radio Shack) and jumper the sensor about 5 seconda after the flame goes off. If the valve opens immediately and the burner lights, it would appear the high limit sensor is the issue. DON’T leave it jumpered for more than enough time to verify the flame reignited due to your jumpering the sensor. If it is the high limit sensor, either the heat exchanger is getting too hot due to inadequate air flow (dirty filters, closed supply or return registers or a dirty AC coil) or the sensor itself is out of calibration. I suggest you check the temperature of the house air just as it leaves the heat exchanger and compare that to the recommended temperature rise shown on the furnace’s data plate. The difference between entering air (room temperature) and exiting air is usually between 35 and 55 degrees. skibum “Re(2):Kenmore furnace flame sensor question” , posted Tue 1 Mar 11:24 ______________________________________________________________________ Thank you for your reply. Is the “high limit” sensor the same as the “vent limit” switch. I’m looking at a parts diagram, and I do not see anything labelled as a “high limit” sensor. An exploded view of my furnace can be found here: =0582&modelDesc=GAS-FIRED%20FURNACE&modelNumber=867762181&documentID=0 0043771&documentTypeID=PLDM&documentClassCode=PM&titleType=FUNCTIONAL% 20REPLACEMENT%20PARTS&titleID=00001&” (include the last ” when you paste the link into your browser) Can you see a “high limit” switch on there? Thanks again, Michael Freon “Re(3):Kenmore furnace flame sensor question” , posted Tue 1 Mar 12:40: ______________________________________________________________________ Look at part # 19. The cover of the 3X5 box should come off by pulling it toward carefully. Hopefully you’ll see a dial with 3 sliders and numbers stamped in the dial. If this is the case, there’s your fan and possibly one high limit switch. Watch the dial rotate as the main burner burns. The last slider (set at the highest temperature on the dial) is the high limit setting. See if the dial rotates far enough, while the furnace is heating up, so this slider turns off the gas. You can hear the sliders “click” as the make/break a circuit. I suspect if your filters etc are clean, this high limit will not be exceeded but check it anyway. Part #20 is another candidate. I suspect that sensor measures the actual flue gas temperature. That’s the sensor you’ll want to bypass if the high limit on the slider (measuring air temp flowing across the heat exchanger) is not responsible for shutting down the furnace. [this message was edited by Freon on Tue 1 Mar 12:46] skibum “Re(4):Kenmore furnace flame sensor question” , posted Tue 1 Mar 14:32 ______________________________________________________________________ Thanks Freon, I really appreciate the help. OK… There’s no cover on the dial with the sliders. I can see that it does rotate while the burner is burning, but it never makes it to the 200 degree mark, where the cutoff slider appears to be. It must be #19 in the diagram, but it certainly isn’t located where the diagram reports it to be. It’s to the right, below the metal “ramp”, and located 3-4″ above where the rightmost 2 burners go into the heat exchanger. I just ran the furnace for 20 minutes and got the house up to 78 degrees. Observations. With the upper front cover off, the burners stay lit longer. Put the cover back on, and within 1-3 minutes, the burner starts cycling again. I think fixing the ignitor / sensor just uncovered another problem. I think your first reply has the answer – clogged or dirty AC coils. I measured the AC voltage across part #20. Little to no change in resistance or ac voltage drop until after the burner goes out, then 0 vac but the resistance was unchanged at about .6 ohms or so. Jumpering it after the burner went out did not restart the burners. The dial stays pretty much parked on 160 degrees or so (if the tick mark at the bottom is right). It gradually creeps up close to, but not quite to the 200 mark before the burner shuts down. (It actually looks like 170-180 to me, but maybe the switch is not as linear in operation as the tick marks.) After the burner goes out, the dial moves back down to the “on” slider (near 150), and the burner reignites. It looks to me like the temp is building up and the high limit switch is doing it’s job. Come to think about it, my cooling wasn’t so great towards the end of last summer, either. I can’t find a way to disassemble the sheet metal around the cooling coils. Guess the installer ddn’t think we needed access. I have a flight to catch tonight, so I’m throwing in the towel. My Dad lives across the street, and knows a good HVAC guy. He’s going to come over and let him in after I’m gone today. I replaced the filter, so I think he’ll clean the coils and I’ll be good to go. I let you know how it turns out. Thank you, Michael Freon “Re(5):Kenmore furnace flame sensor question” , posted Tue 1 Mar 15:18: ______________________________________________________________________ If the dial is stable around 160 degrees then the other switch, #20, is more likely your problem. For the dial to affect the burner, it has to get to 200, where the high limit slider is now set. Since the dial temp stabilizes below 200, as you said, and the furnace still goes off makes me suspicious about the flue temp sensor. You can test the dial by gently twisting it, after it’s stabilized around 160, up to the 200 mark and see if the furnace burner shuts down. Be gentle and don’t force it but you should be able to get the 200 degree slider to “click” and shut the burner off. That’s one way to test the dial switch. The flue sensor is usually a simple SPST switch that is closed unless the flue temperature reaches a danger point. Last test. Turn off the furnace. Use the jumper wire with ‘gator clips and jump the switch or remove the leads to the switch and tape them together–FOR TEST PURPOSES ONLY. Now turn on the furnace and call for heat at the thermostat. If the furnace runs longer than the 20 minutes it did before or reaches the thermostat set temperature, turn it off and disconnect the jumper or untape and reconnect the switch wires and get a new sensor ASAP. You can try removing and cleaning the switch but I doubt it will help… but who knows… Nothing ventured, nothing gained. If this switch isn’t the problem, we’ll keep digging. [this message was edited by Freon on Tue 1 Mar 15:28] skibum “Re(6):Kenmore furnace flame sensor question” , posted Mon 7 Mar 10:18 ______________________________________________________________________ Freon, thank you for your help. It is fixed now. The HVAC guy (Nick) showed up and took a look at it before I left for the airport. He found 3 things that needed adjustment, not including the first one that I fixed. Problem #1 – Dirty flame sensor causing furnace not to stay lit and causing no heat condition. This is the one I fixed. Problem #2 – There is a little curved metal “hood” over the pilot tube outlet that was cocked to the left. Apparently, this part has 2 functions, it directs the pilot gas towards the ignitor, and it also serves to direct some of the flame towards the flame sensor / ignitor. When I removed it for cleaning, I though that it looked strange as it was not completely covering the pilot gas orifice. The closest main burner to the ignitor / sensor is to the left of the ignitor / sensor, so I assumed that this was normal and served to direct the pilot flame towards the main burner. Nick showed me (by looking through the vents in the front cover) that when the cover was put in place, the flame near the sensor moved about 1/8″ to the left of where it is with the cover off. The airflow changes slightly with the cover in place. He removed the ignitor /sensor, and used a pair of pliers to rotate the little metal hood to line up over the pilot gas orifice. He had to clamp the assembly in my bench vise and use some real pressure to get it to turn, so I am sure that I didn’t move it while cleaning it. When he reinstalled it and fired up the furnace, we could see that the flame stayed much closer to the sensor / ignitor with the cover on. The furnace stopped cutting off the gas every few minutes after he did this. The flame moves very little, if at all, when the cover is put back on now. After fixing this, the system would run for at least 5 to 7 minutes before shutting off with the cover on. The thermostat was indicating that it was still calling for heat after the system shut down, though. Nick looked at the dial, and said that it did not shut off the system, so we still had an issue. Problem #3 – There are 7 large wing nuts at various points in the supply ducts going to each room in the house. I always thought that they were on the end of some allthread or something attached to the joists, and were used to hold the ducts in place. Wrong. They are actually holding dampers in place. Every last one of them was at least 40% closed. This was apparently was causing the hot air that was supposed to blow into the house to “back up” inside of the area with the heat exchanger. The dial started climbing towards the 200 mark, but the system was cutting off before it got there. THe heat was apparently backing up, and overflowing into the flue area. The flue sensor was doing it’s job – cutting off the furnace when it got too hot. He did what you said – jumpered the switch. The system would now stay running until the dial hit the 200 slider. The switch tested fine, so he checked the dampers. After opening up them up, this stopped happening as well. Now the system would run until the thermostat told it to stop running. My thermostat (Honeywell CT8602 Chronotherm IV programmable) is set from the factory for 6 cycles per hour, so the tstat calls for heat for about 10 minutes at a time. He said that there is a way to change it to 3 cycles per hour, but Honeywell has nothing in the manual or on their web site about how to do this. Issue #4 – The dial that we were discussing had the sliders set as follows: the low temp fan cutoff slider was set to about 120, the “on” slider was set to 150, and the high cutoff was set to 200. Since I moved in, I noticed that my furnace always ran for only 4-5 minutes whenever it came on. I never thought that this was a problem. It also kept turning the fan on 30-40 seconds after the heating cycle stopped. The fan would run (no flame) for about 30-40 seconds before shutting off the second time. I though that this was normal for this system. Nick didn’t think so. He adjusted the bottom slider down to about 75 degrees. Now, the system runs until the tstat stops calling for heat, and the fan stays running until the dial hits the 75 degree slider. The fan no longer stops and then comes back on after the burner goes out. Everything seems fine now. Good heat, better air flow to all rooms, longer cycles. I really appreciate your help, and all of the great info on this this site. Michael