AdrianeS
Member
I'm troubleshooting my Heil/ICP 80% single stage gas furnace (Model N8MSN0902120120A) and feeling a bit stumped at this point. Over the last couple weeks, my furnace has been intermittently exhibiting a limit circuit fault which leads to a limit circuit lockout and cold air blowing through my house. It will generally run fine for several cycles and then I'll notice the house getting colder and that's when I find the control board flashing the limit circuit lockout code. Reading the furnace manual, it has this to say:
During the lockout, I checked for continuity on all of the switches on the furnace including the flue pressure switch, main limit switch, two flame rollout switches and the blocked vent switch. The pressure switch was not listed as a cause, but I checked it anyways just to be thorough. All switches had continuity. The rollout switches and the blocked vent switch both need to be manually reset but neither had tripped so I did not need to reset them. If it were the flue pressure switch or the main limit switch, they are supposed to automatically reset after the unit cools down and the furnace should re-ignite at that point, but this doesn't happen. I checked the temperature at all of the switches, and they were all at room temperature.
I decided to sit and watch the furnace through a few cycles to test the switches while it was operating normally and found they all had continuity then as well. I also checked the voltage across all of the switches, and then tested for resistance with the furnace powered off. Everything tested normal. I took the temperature at each of the switches and at the vent throughout several cycles and nothing was out of the ordinary. Of course, I've been unable to actually see the furnace at the exact time of the limit fault which has been super frustrating. It seems as soon as I walk away and forget about it for a moment, the next thing I know the house is getting cold again.
This has me somewhat stumped because I assumed one of the switches tripped, causing the lockout. If a switch didn't trip, where do I go from here? I can't get onto the roof to visually check the outlet (it's way to high and I'm too nervous to go up there), but I did order a manometer and will check the pressure when it gets here. However, wouldn't the vent switch trip if there were a blockage? The same goes for the duct work. I feel a normal amount of air coming out of each duct but will need to confirm this when the manometer arrives. As far as I can tell, that leaves me with one last possibility, a defective blower motor. Is it possible that the motor does not shut down when it's supposed to, and just continues to blow? This doesn't seem likely to me because how would that cause a limit circuit fault?
I'm not sure what, if anything, I can do while I wait for the manometer to come. When it does, I plan to check the vent and the ductwork for proper airflow, along with the gas pressure. I still feel like a switch is somehow at fault, though all the tests I did with the multimeter say otherwise. I should also mention I have a CO meter that displays any level of gas near the furnace and it reads 0 every time I've looked at it.
If anyone has any thoughts about what might be causing this, I would really appreciate any help!
The first thing I checked was the air filter which looked clean but I replaced it anyways just in case. I looked over the furnace and nothing seemed out of place. The heater has been running just fine for the last 7 years with no issues until now. The blower wheel is not loose, and the vent is properly sized.
During the lockout, I checked for continuity on all of the switches on the furnace including the flue pressure switch, main limit switch, two flame rollout switches and the blocked vent switch. The pressure switch was not listed as a cause, but I checked it anyways just to be thorough. All switches had continuity. The rollout switches and the blocked vent switch both need to be manually reset but neither had tripped so I did not need to reset them. If it were the flue pressure switch or the main limit switch, they are supposed to automatically reset after the unit cools down and the furnace should re-ignite at that point, but this doesn't happen. I checked the temperature at all of the switches, and they were all at room temperature.
I decided to sit and watch the furnace through a few cycles to test the switches while it was operating normally and found they all had continuity then as well. I also checked the voltage across all of the switches, and then tested for resistance with the furnace powered off. Everything tested normal. I took the temperature at each of the switches and at the vent throughout several cycles and nothing was out of the ordinary. Of course, I've been unable to actually see the furnace at the exact time of the limit fault which has been super frustrating. It seems as soon as I walk away and forget about it for a moment, the next thing I know the house is getting cold again.
This has me somewhat stumped because I assumed one of the switches tripped, causing the lockout. If a switch didn't trip, where do I go from here? I can't get onto the roof to visually check the outlet (it's way to high and I'm too nervous to go up there), but I did order a manometer and will check the pressure when it gets here. However, wouldn't the vent switch trip if there were a blockage? The same goes for the duct work. I feel a normal amount of air coming out of each duct but will need to confirm this when the manometer arrives. As far as I can tell, that leaves me with one last possibility, a defective blower motor. Is it possible that the motor does not shut down when it's supposed to, and just continues to blow? This doesn't seem likely to me because how would that cause a limit circuit fault?
I'm not sure what, if anything, I can do while I wait for the manometer to come. When it does, I plan to check the vent and the ductwork for proper airflow, along with the gas pressure. I still feel like a switch is somehow at fault, though all the tests I did with the multimeter say otherwise. I should also mention I have a CO meter that displays any level of gas near the furnace and it reads 0 every time I've looked at it.
If anyone has any thoughts about what might be causing this, I would really appreciate any help!
