My furnace just blows cold air. I checked on thermostat. There is nothing wrong. I took off the panel on furnace and pushed white button. The two green Led Indication Lights are on with(DS1-- Slow flash, a flash/per second, DS2--on). According to the label on the panel it means "primary/secondary limit switch open". I have no clue what it means and what the problem is. I notice there is no gas coming out and the electronic ignition does not turn red. But It blows cold air.
My furnace model is Lennox Merit Series G40UH-69D-155-01. It was installed Apr. 2001.
Here are my questions.
1) What is the possible problem?
2) What does "primary/secondary limit switch open" mean?
3) Is it still under warranty? The Lennox website says "5-Year Limited Warranty on covered components."
The primary/secontary limit switch is open. Could be something else though.
It is not working. Closed mean it is making contact or "on" while open means "off".
Yes, if it is a covered component. No if it is not a covered component.
Simplified, the thermostat senses the need for heat. It closes a swithch that starts the furnace operation. The igniter glows, then the gas valve opens to alow gas to flow to the burners. Once it reaches a pre-set temperature, the blower kicks in and distgributes the heated air. When the thermostat says "it is warm enough", the burner shuts down, but the blower will run until a low limit switch says the heat exchanger is cooled off enough. Then the blower stops. If the controls think the heat exchanger is still hot due to a faulty switch/sensor, the blower will continue to run.
I'm not a technician so I can't give you more details than that. It is a bit more comple than my simplified version and I'm not qualified to give specifics. Call the heater guy.
warm the house with the kitchen stove temporarily with the window open an inch or 2. i looked for your mfg on google. i might try to register at mfg website and check there for possible tips and manuals at:
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you have 5 to 20 year warranty on these
i dont know your furnace, maybe updraft flue blower tube off motor needs cleaning with a paperclip is a common problem on a couple of mine. which means if you pull off the hose from the flue blower and sucked on it would the switch close and allow furnace to operate.
Well, IF the switch is bad, yes, if its not, no. Have to ask...when was the last time you got it serviced? If its been a while, its telling you something.
Also, Lennox is one of the WORST for bad boards. The Surelight board in particular. White Rodgers really screwed the pootch with those...I have one that got replaced today as a matter of fact sitting here for a warranty card... Looks fine on the front...roll it over and its a mess. 24V side, the side that your limits are on, looks like someone took a torch to it..and amazingly enough, it still threw a code.
I can make a suggestion, but anything you do is at your own risk, since if it IS an open limit, and you reset it, and there is a reason it tripped..they dont just trip... What you are looking for is a dime sized disk, with two orange wires to it..you should have two..one near the burner assembly and one over at the blower somewhere..depending on your serial number, you might have up to 3 around the burner assembly. There will be a red button on at least one of them. With the POWER OFF, you can press each one in slightly, and if you hear one go CLICK, check the rest, and retry the furnace. If it still does not fire up, DO NOT EVEN THINK about bypassing one..you bypass one, have an overfire condition, its overheating the unit, and your home burns down....well, guess what? The furnace will be one of the first things they check and the fire marshal can tell you bypassed it...and badda bing...you or someone in the home might be dead, and you got no insurance... If you cant find it, dont feel safe doing this, or just dont want to, CALL AN INSURED TECH out...if he fucks something up, you are covered. Expect to pay even if its under warranty. Warranty parts are covered, labor is not. If its the board, and its a POS SureLight, let them find it, cover it, and even with labor, you come out much cheaper.
I also strongly suggest that if you DO find a limit tripped, that you STILL get a company out there, because I cant stat it enough..those are one of the first lines of warning you get...they are there for a reason. FIND OUT WHY IT TRIPPED. Believe it or not, those parts in the furnace dont talk about you all night and take suggestions from other parts.. hey man...lets freeze this asshole today....OK...who is gonna trip first? Ya know?
If you havent had it serviced since you got it, shame on the installers, and shame on you. This might be what we call a nusance trip that could have been prevented with a simple service.
" I also strongly suggest that if you DO find a limit tripped, that you STILL get a company out there, because I cant stat it enough..those are one of the first lines of warning you get...they are there for a reason. FIND OUT WHY IT TRIPPED."
Even if the part that is bad IS under warranty, that does not mean everything is free. You are STILL going to have to pay for a service call. I can almost bet you that will be more than what the part will cost.....
It turns out that a camstat switch sensor is broken. The switch is not reset-able. The switch has maximum temperature setting 210. Replaced with a reset-able switch it is running fine now. Lennox furnace (G40) may have a design flaw. It happens quite often.
You don't replace a non resetable limit switch with a resetable one. That's done to alert you to a potential serious problem (IE. cracked heat exchanger). If design was easy, we'd all be engineers. Bubba
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