Lennox G40UH furnace barely 3 years old is going through phases of working fine and not working. Both LED's flash normal when unit is working.
When not working I'm getting the LED#1 Flash and LED#2 ON, which indicates the primary or secondary limit switch open.

I read a little bit on internet and found that Filtrete filters could be the cause. Last night I switched the filter back. We had been using Filtrete since June.

Powered the system back on and it worked fine, but konked out again sometime over the night. Woke up to 61 degrees. But unit again came back on with an flick of the on/off switch and heated 1850 sq ft home back to 66 before throwing the same code.

I guess I'm wondering where to go from here.
1. Did Filtrete filter cause some kind of an issue?
2. Why are they not recommended?
3. Would 61 degrees indicate total failure overnight or did it come on 1 more time? The low last night was low 30s.
4. Any idea as to what I can try to rectify this problem?

 

These filter can be bad if they are undersized. What MERV is the filter (or packaged color)

How big is your furnace? Filter size?

Limit can be caused by a few things.

-Dirty air filter
-Poor air flow
-blower motor running too slow
-Blocked vent
-blocked/lack of return
-dirty blower
-A/C coil dirty
-furnace overfired.

 

Many pleated filters create too much restriction of air flow.
Check the fins on the fan to insure they are clean.
Make sure all vents are open & not blocked.
You may have to increase the fan speed.

 

Thanks for the comments so far. Sorry for the long post, but I wanted to add more information.

We switched to Filtrete filters in June of this year. Not sure MERV spec, but this filter was almost $20 and in a dark blue/purple package. It measures 16x25x1 and says Maxium Allergen on it.
The filters we used to use are NaturalAire Standard with an 8 MERV rating and are much cheaper. This is what I put in last night.

The best I can describe the problem is that it works fine during the day (less calls for heat I'm guessing) then at night when it gets colder the limit switch is locking down the unit. And it may or may not come on one/two more times during the night but never catches back up to the t-stat. Its usually 58-62 in the morning, so I'm not sure if its a total failure during the night or not. (Lows have been in high 20's lately)

I did call the # of the guys who installed it. And they were very quick to say I just need a new limit switch and it would be $124. I'm no expert (obviously) but I didn't really understand how they could diagnose that over the phone. I feel like the limit switch is working fine since its shutting down the unit because it sense a problem?

I also opened the unit for inspection. I removed the filter and checked the area before the filter and its a little dusty but nothing crazy. There was a small plastic tie down in there which I removed. There are small pieces of insulation here and there on the shelves in front and also a few wood shavings. This would indicate to me that work was done in the attic while covers were off when home was built? The only other thing is that the tubes where the flames go have a white residue on them, but it almost looks like it was lightly sprayed on.

The blower which is directly after the filter (at the bottom of the unit) is a little dusty but nothing crazy. Again this unit is only 3 years old and I would say everything was pretty clean. It also worked fine until this year so I don't think its the random piece of insulation or wood shaving.

I mean this could be something as simple as a bird got caught up in vent pipe right? The purpose of the limit switch is to shut the system down when its getting too hot? Or am I not understanding properly?

Thanks in advance.

 

The purple filter are pretty tough on the system. go back to the cheaper MERV8 filters.

Limit is open by temp in the heat exchanger is too hot. It could be a bad limit. but won't know till a temp rise is taken. (before the air filter, and above the A/C coil)

 

You didnt say if you have AC on this furnace or not.
There are small pieces of insulation here and there on the shelves in front and also a few wood shavings. This would indicate to me that work was done in the attic while covers were off when home was built? The only other thing is that the tubes where the flames go have a white residue on them, but it almost looks like it was lightly sprayed on.
If they ran that furnace as the home was built and you have AC on it. That coil can be dirty. We have been throught this before with the builders. No way will we let them use the furnace for heat or cool anymore . Till the people move in. If they run it the coil and even the burners all have to be cleaned. Is the blower wheel clean? The other thing is we say no filter over a MERV 5 and if you want use a Spraykoat on the new filter. Are go to some thing like a AirBear filter
ED just my .02 cents

 

I switched the filter back to MERV 8 last night and I'm still getting the same result, although I want to say its a little better. But that's probably just me turning off/on (which I probably shouldn't be doing) and having unit reboot itself.

Any chance of this working itself out with the new filter now or has the damage already been done?

Can you explain "temp rise"? Is that just taking a temp of the air coming out of the unit to see if the limit should be shutting the system down?

Sorry for the stupid questions. First home, First furnace, and I have an 8 month pregnant wife that doesn't like waking up to 60 degrees.

 

Yes A/C as well. Now that I think about it there were a few days were it struggled to cool house down this summer, but I always attributed that to Georgia heat.
Blower wheel is definitely covered in a light coat of dust, but you can still see fins clearly. Also unit worked fine for 3 years and filter was replaced regularly. Although I admit to no preventitive maint besides filter replacement. Are you saying PULTE workers could have been running the AC while working on house with no cover and put about 5 years worth of gunk in everything??????

 

Temp rise to see if the heat is getting way from the heat exchanger fast enough.

 

Are you saying PULTE workers could have been running the AC while working on house with no cover and put about 5 years worth of gunk in everything??????

YOU BET WE have made so many no heat or AC calls in new homes After people move in for a year or so .Its always dirt from when the home was built and they ran the unit then

 

I don't think that little bit of dust is going to be the issue.

Are you able to take a temp reading for us, and we can see what the temp rise is.. let the furnace run at least 8 min or so.

 

Is my blower too dirty with even a light coat of dust on it? Should I just go ahead and expect to pay for a good cleaning? What amount should I expect?
Thanks, Jeff

 

Are you able to take a temp reading for us, and we can see what the temp rise is.. let the furnace run at least 8 min or so.

Sure. I could do that tonight. Please let me know what I need (or if I can use my BBQ thermometer) and where to place it for an accurate reading.

 

The BBQ or meat probe will work.

Take a reading at the air filter.

Then above the A/C coil if you have A/C.. Anywhere with in the furnace area. Just don't drill a hole into the A/C coil!

 

Outside temp is 55 degrees

Finally got the unit to come on and set t-stat to 78. (t-stat was showing 69 as inside temp) Let it run about 10 minutes. Put a digital thermometer right in front of the filter and checked it on and off for about 10 more minutes. It ranged from 63-65 degrees.

My units A/C is protected by steel or aluiminum sheeting for almost 2 feet straight above the unit. There is another 2-3 feet of what appears to be a duct, but it actually has metal underneath. It still maintains the rectagular shape and has 4 main ducts extending out. I punctured one of these ducts where the metal stopped with the BBq thermometer. The lowest scale on the BBq is 140 degrees but there is a good inch and 1/2 that needle has to move to make it there. It almost made it to 140 as a high (1/16" away from 140) And a low of 1/8" away from 140. So I would guess 130-135.
But this reading was taken almost 4 feet away from the top of the furnace to avoid drilling into metal.

I also replaced filter from MERV 8 to MERV 4 to see if that makes a difference.

 

YOu got about 70 to 75 degree rise... That's pretty high. Look at your model plate and see what they suggest for temp rise. I think most of them don't want over 60 degree.

Just for kick, turn the t-stat switch to fan ON. and see if it acts up on you.

You make sure there is no blocked return or supply vents closed off?

 

I checked w/ a flashlight best I could inside the unit and didn't see anything. Unit failed again last night but it was 65 degrees this morning when set to 68 and low outside was probably only in 40s. Tonights low is 20!!!!

Are you talking about ceiling vents being closed as well? The home is a raised ranch that has 1 bedroom upstairs that is actually furnace level. (attic is off to the side) There is a valve that is recommened to be closed in the winter and open in summer. Basically it either closes (or restricts) warm air to the upstairs bedroom when closed. If its left open it gets really hot in that room.

All ceiling vents should be open but I will double check. I'll also call my wife and have her turn the fan to on instead of auto.

On another note, the service company called this morning and said they could make it out today.(instead of monday) I have to be a work today but the wife is home and she accepted the offer. I'm pretty sure they are going to just change the limit switch for $124. Does that sound appropriate at this point?

 

It could very well be a weak limit. but I don't know what your's is rated at.. when you said your temp was as high as 70 degrees rise, that is kinda high, but you will have to look at the model/info plate in the furnace and see what they rated for heat rise. I think mine says 35 to 60 degree rise. (Trane)

 

Busy day at work today so I wasn't able to meet the tech. Talked to him on the phone and seemed like a very knowledable fella. Diagnosed the system as follows:

Limit switch ok
Problem was a bad capacitor in the fan. This caused the fan to not run properly and eventually the heat inside the unit rose high enough to trigger the limit. Once that happens the unit goes into lockdown for an hour at a time before it retries.

So does that makes sense to you guys? The unit would go through about 2-3 cycles working fine with lockdown in between. So can the capacitor work ok sometimes and fail others. Or is it possible for the unit to run ok without the fan when its a lighter load? OR is it a fan speed issue, like capacitor works ok for 1 speed and doesn't allow the fan to increase speed when needed.

 

OR is it a fan speed issue, like capacitor works ok for 1 speed and doesn't allow the fan to increase speed when needed.
If the capacitor is out or dont work then the blower motor is out in any speed you try to run it on

 

If the capacitor is out or dont work then the blower motor is out in any speed you try to run it on

So can the unit run w/o blower?

 

It worked fine last night. Thanks for all the help. Have a good weekend.

If anyone wants to explain why the fan capacitor fixed the problem that would be wonderful.

Thanks.

 

A capacitor gives the motor a little more ump to run faster and more efficent.. and if the capacitor is not working, the fan will run slower. Since the air flow was slower, the temp rise was higher, and tripped out on the limit.

http://electronics.howstuffworks.com/capacitor1.htm

 

Thank-You. Makes perfect sense. I appreciate the help.

 

This sounds very feasable. You probbably have a direct drive blower (The motor is mounted inside the blower housing). I have encountered a similar problem with belt driven models where the belt was glazed over and stretched due to normal wear and tear causing it to slip and spin the blower at a lower RPM. In that case, the filters were pretty dirty too. I know you changed your filters regularly, but let this be a lesson for others. Before you call the service company in the middle of the night, change the filters (pleated are reccomended) and turn the unit off, then back on after a minute (to reset the controls) and see if it starts back up. If you have diagnostic lights, make a note of what the code was and when it happened.