High Eff. Lennox G26Q3-75 - installed in 1997.

I am struggling with getting my house up to temperature. It has something to do with the pressure sensor i believe.
Seems to be 2 symptoms at the moment. I let the house go down to about 60F during the night, then, when the thermostat calls for heat:
1) if i just let the furnace take intake air from in the house (I only do this temporarily for testing) it will act normally for a few cycles (burner will stay on for 10-15 minutes before shutting off). Then the burner will only run for about 3 or 4 minuers at a shot. The blower motor pretty much never shuts off unless i lower the thermostat enougth to shut down the call for heat.
2) On quite a few occasions, with the intake line all in place, taking air from outside, the inducer motor will take 3 or 4 tries to start.
I have checked the exhaust and intake lines. All free and clear.
Have cleaned out the AC evaporator, but cleaning it didn't change anything.

Thanks in advance for any advice folks.

 

Welcome to the forums.

Make sure the small hoses that connect the pressure switch are clean. Clean the ports where the hose connect at the opposite ends from the pressure switch. When you pull the hose off.... there will be a tiny hole at the bottom of the fitting. That must be clean.

It almost sounds like that furnace is overheating. Have you checked/changed the air filter ?

That furnace has a control board.
There should be a sight glass in the blower door where you can see the fault LED(s).
The codes are on the back of the blower door.
Opening blower door clears faults. Same as turning power off and back on.

G26Q3 manual - pdf

 

Thanks
The filter was the first thing I checked, have run with new filter since this started, and have tested with no filter.
The control board has no fault lights, nor is there a windo to observe, i sincerely wish there was.
ICM289
Not sure what you mean by a "tiny hole at the bottom of the fitting" - what fitting? The one in the burner box assembly?
I did a test where I waited till the furnace was blowing with the burner on, and then jumped the pressure switch terminals. THe burnder shut off after 4 minutes or so anyway.
I did one earlier where i bypassed the temperature limit sensor, the burner shut off anyway.
I have noticed that when i lower the thermostat down and shut off the call for heat for a time, the 1st cycle seems to last longer - I can get the house warmer by turning the thermostat down for 10 minutes, then leave it up for a 1/2 hours.

 

Just noticed too, the drain at the bottom of the inducer motor appears to be solidly closed - this obviously is not the way it should be, but I don't see how it could cause the problem i am seeing.
Anyway, I am digging into it soon, but I think it has been this way for some time, as it doesn't seem like a debris plug, more like it was never open to begin with.

 

I'm having trouble finding a 1997 furnace with your model number.
The 2000 model has diagnostic LED's.

I can bring up that control board and I see no diagnostic LED's.
It is strange that it says it controls an inducer but I see no inducer power connection.
Possibly CAB.


The inducer drain is usually tied into the pressure switch circuit.
That is likely in your gas valve control wire circuit.
I would say that the drain could be the cause.
I would service that drain and still clean the nipple that your pressure switch tubing connects to.

 

I've been staring at that schematic for a few days, and yes, the "motor damper" is actually the inducer motor. That was a bit confusing, but i checked some time ago, and the motor is hooked to the "cab" connection.
I will likely tear into the motor tomorrow. I'm a bit afraid of just jamming a screwdriver in there and making my situatin worse.
All the pressure switch tubes seem to be clear and moving air just fine.
How is the drain tied to the pressure switch circuit? It doesn't apper to be tied to anything. Internal to the fan housing?
Full model number is lennox g26q3-75-2. I have downloaded a few guides for the model, running into some issues attaching them to this post.

 

Can't currently directly load pdf's to the board. Update in progress.
The -2 is the version of your furnace.

The white hose at the top is the pressure switch.
The clear hose at the bottom is the drain.
Are you saying where the drain connects is plugged ?
Don't remove the inducer. Take the clear hose off and use a small screwdriver to clean the hole out.
Turn the screwdriver will inserting. Don't hammer in.
The inducer blower is pretty strong. Just cleaning the drain won't hurt it.

I saw your problem description online.... you were describing an overheat problem.
The ICM-289 acknowledges open limits. It will shut the burner down and just run the blower.
If you have a voltmeter... connect to R and C. Monitor it for 24vac. Should be there at all times.
If it shuts down when burner does.... you have an open limit.

 

Pulled the inducer, because in putting a screwdriver in the drain hole, it did not feel like gunk, it felt solid. I also wanted to see what kind of build up was inside, just to rule that out. Also wanted to see what the outlet burner chamber looked like - I still have to do this. Indeed, the drain is actually covered solid -there is no hole. Perhaps it was intended to be drilled out upon installation? Has anyone heard of this before(I cant find a manual for it)? I don't want to drill anything till i see something official.
Inducer motor is: Fasco 7021-9450
67k0401
In any case, I don't really see how this is related to the problem:
-It has been like this for years with the furnace functioning properly
-It is clean and mostly dry inside, some residual moisture in the outlet. Perhaps the intent is that it removes all moisture through exhaust - the exhaust flue has it's own water collector and drain.
-There is no sensor that is involved in this.

Pete, I agree that this sounds like a temp limit issue. I had previously timed the burner with the limit switch jumped, and the burner shut down anyway. Have checked the limit switch right after shutdown several times, and it has always been closed. It's a L260-30F.
I will meter the control inputs as you suggest, but what is the logic here? R is 24v, c is neutral. Does the control board shut down R if it sees a an open limit?
Anyway, your input is greatly appreciated.

 

Hooked the meter up to "LImit" and "c" and yes, the burner and the limit shut down at the same time.
If then, the temp limit switch was getting hot, figured I would try and improve airflow. I went ahead and pulled the blower just to see what kind of crap was on the heat exchanger fins - they weren't particularly bad. I cleaned them off as best as i could with vacuum brush and compressed air. Also blew off the turbine fins, but I wouldn't guess that I made a big difference in overall airflow.
Also, I pulled the temp limit switch, it basically looked ok, but don't know how to test it to see if it's bad for sure. I could just order one and install, but I am usually prettly slow to blame a sensor. Usually, they are doing what they are supposed to do. How do bad temp limit switches usually act? Do they just start tripping at lower temperatures as they age, or do they go all at once?
I also remove one of the end caps on the AC evaporator to allow for some bypass.
At this point, the system is working better than it has lately, but I think something is still wrong - it previously never struggled to get the house up to temperature, so i should be able to run as before. Might be easiest to put a new limit switch in just to rule it out. Diagnosis by replacement.

 

Solved!!!

Well, as it turns out, it's the little things that matter.
There is a small rubber grommet (about the size of a quarter) under the pilot assmbly in the burner that is kind of a pain to get in place just right. It has slots and holes for the pilot gas line, spark wires, and flame sensor wires. Getting everything in place and fitting right takes some fiddling around.
As it turns out, having this grommet completely sealing of the hole as it's meant to do is critical. When the grommet is not there or incorrectly installed there will be a flow of air through the hole that blows vertically up and over the pilot at a fairly high velocity as soon as the inducer motor comes on. I had just been kinda jamming it in place, not realizing how important it was. This makes it hard for the furnace to light, and blows the pilot and burner flames away from the flame sensor just often enougth to create random shut downs.
For a couple of days now I have all covers in place with the grommet correctly installed and the furnace is operatiing as good or better than ever. It lights every time and the furnace stays on. I checked and tested lots of things, bought a few parts, before it occurred to me how important that small part is. So, just as a re-cap, here is all the stuff i tried over several weeks, when just sitting back and observing and thinking would have probably got me to the solustion sooner:
-New flame sensor (unnecessary)
-New limit switch (unnecessary)
-Re-plumbed the intake line (unnecessary)
-Cleaned (somewhat) the A-coil (will do this better before summer, it desparately needed this anyway)
-Remove A-coil end cap to improve blower air flow (unnecessary, esp after a better A-coil cleaning)
-Cleaned blower motor turbine and heat exchanger where accesisble. (Meh, probably a good idea anyway)
-Tested the limit switch with a homemade gizmoe involving a heat gun, pvc pipe, a meat thermometer and a multi meter. (good excersise, but ultimmately unnecessary)
-Used a thermocouple to test the temperature by the limit switch ((good excersise, but ultimmately unnecessary)
-Many other multimeter tests to check various circuitry, etc., lots of reading literature and schematics, reading forums like this one.
---------Made a homemade manometer to check the intake pressures. This, in cunjunction with the fact that the furnace ran normally with the burner box cover off, is the operation that got me thinking in the correct direction. I had always noticed the difficulty the furnace had getting the pilot lit, and then i saw no vaccuum at all on the intake when I had expected to see at least a little, due to the pilot lighting ssues. This got me thinking about the burner box chamber and how it sealed, and then it hit me. I put the grommet in place (takes some fiddling, it is really not well-designed), re-checked the vacuum (saw may .2" of wc) and have been warm and happy since.

 

Has the primary heat exchanger (the secondary is right above the blower with fins) been visually inspected?

In these furnaces, they split open right at the top.
Considering your furnace is from '97, it is at the end of its life and most likely has a cracked heat exchanger.

Cracks pose a carbon monoxide risk and render the furnace unsafe to use.