Furnace flame limit switch tripped

wdn

Messages
1,639
Location
NH
Got an HVAC issue with the furnace I figure at least one of you guys works in the trade or knows more about it than I do as the homeowner and could help out with some knowledge or advice. It might be a bit long so please bear with me. On my York 95.5 furnace the furnace fan was on but was getting no heat. Flame was off and the furnace impeller fan was also off. The magic eye was blinking 5 red LED. Looking it up that read "flame limit switch open circuit". Cycling the power, the impeller fan came one, then quickly turned off, igniter never glowed so (obviously) gas did not come on, and no flame. Main furnace fan stays on so unheated air coming out of the furnace outlets upstairs. I did some reading and saw a pretty good explanation on youtube. On mine one of the two causes listed was the evaporation tank pressure hose was clogged or bad switch. I cleaned out the hose it was filthy but that not it. Applying a bit of pressure hear the switch trip. The other and main reason for 5 red LED blinks was flame limit switch open. One of the furnace limit switches above the burner box was indeed tripped. It is a push to reset switch. I reset it. The furnace then cycled on, igniter lit, gas opened and I have heat. I looked at the flame and it seemed like nice cones not visibly backing up into the furnace which is the safety feature this thermal switch is supposed to catch. Yet sometime, today, the switch indeed tripped. The videos showed very wavy flames when there's a problem but mine are not like that. To be honest I would not know a good from bad flame but saw no flames backing up into the furnace. It also said a clogged exhaust could to it. Mine is a PVC that comes out the side of the house and is a two in one pipe, combination inlet and exhaust in a nested pipe. I did notice steam coming out of the exhaust today before I noticed the problem. I noticed cold air leaking out of some seams that were not already closed with the steel tape, which I have 1/2 a roll of so I sealed those. The video says it open switch could be a cracked or hole in the heat exchanger though which has me spooked due to possible CO backup into the house. The CO detectors did not go off. Surely if I wake up tomorrow in the same situation I will not reset it but it's a holiday weekend and I am getting heat now. Can this be a freak thing or a bad switch itself? Furnace is 7-8 years old. The other side did not trip (there are two of them left- and right. Also is some sort of furnace cleaning in order? Any advice welcome. Thanks in advance.
 

JC1

Messages
5,999
Location
Oshawa, Ontario Canada
Other possible issues that I know about are to do with (1) The flame rod (I think that's the name) that gets a caked on white substance that causes the furnace to not come on. It's a good practice to remove it and clean it at least every second year with steel wool or a scotch brite. (2) Igniter dies after a number of years. I need to get myself a backup igniter.
 
Messages
360
Location
Virginia, USA
If you are mechanically proficient, I'd be tempted to de-energize the unit and swap the left and right side limit switches if they are identical parts. If the other side begins to trip, I'd suspect a bad switch. If the same side continues to trip with the switches swapped, you have an actual problem with the flame overheating - possible due to some exhaust flow blockage.
 
Messages
602
Location
The ATL
Clean the flame sensor rally good and check to make sure the flue pipe is not blocked. Even a partial blockage can cause problems. There are videos on YouTube on how to clean the flame sensor.
 
Messages
239
Location
NY
Some things i have heard of when flame is lost is dirty flame sensor, bad flame sensor, bad flame sensor aim, high furnace pressure (blocked flue).
 

wdn

Thread starter
Messages
1,639
Location
NH
The actual switch that popped on mine is the flame rollout sensor switch not the reular flame sensor (which turns off the if the flame does not light). The rollout switch(es) detects the flames backing up past the burner box instead of going up the flue. It detects if that part of the furnace is too hot. Thanks for the tips this is valuable stuff keep it coming.
 
Messages
602
Location
The ATL
Usually if the one of the flame rollout sensors is tripping that means your heat exchanger is plugged or cracked. But first check your flue pipe.
 
Messages
2
Location
Seattle
Make sure your furnace is level or pitched forward. You don't want it pitched back or you may have issues and or shortened equipment life.
 

wdn

Thread starter
Messages
1,639
Location
NH
Thanks to all who responded. The heat exchanger was in fact clogged and probably also cracked. The inside of burner box was extremely sooted and to them it smelled strongly of gas fouling. The heat exchanger has a lifetime warranty but not for soot blockage. The problem is you are expected to take it apart and flush out the soot and also check for cracks. Flushing ‘might' fix it. If there's a crack it is warranted. By the time you do that labor it is cheaper to just buy a new furnace. If it is not cracked it's out of pocket and you still have the problem that caused it to soot up. If it is cracked you paid to clean it so they will warrant it and you wait for a part. The reason it was clogged, the HVAC guy and who talked to York tech support concluded, is the original install did not do the natural gas to LPG conversion properly. The furnace lacked a low pressure cutoff switch and lacked stainless steel burner tubes which were part of LPG conversion kit. My original contractor filed for bankruptcy in 2017. It was running rich and super hot and over time the heat exchanger clogged. General consensus on top of everything else is York furnaces are cheap junk. I got 12 years out of it. Also the condensate tank had cracks and that was leaking so there was rust and that would need replacing. It also needed a new gas valve and to be properly (re-) converted to propane. The 95.5% York was running at 72.5% efficient and super rich, and hot. The furnace was red tagged yesterday by the HVAC guy. I got some quotes and am having it replaced tomorrow with a new Rheem, which is one of the top 3 brands in Consumer Reports. Rated Very Good. It's 96% efficient, 100,000 BTU, two stage furnace with variable speed fan, which is what I spec'ed for all of them. I got quotes on Lennox, Goodman and Rheem from 3 different HVAC dealers.
 
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