Trane XR80


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Old 12-01-09, 04:53 AM
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Trane XR80

I have a Trane XR80 gas furnace that is approximately 4 to 5 years old.

Recently I heard it doing something that didn't seem quite right. The thermostat calls for heat, the electric glow plug will ignite the gas, and the furnace will start up.

After about 10 minutes, or so, I can hear the gas flame go out. But, the blower motor is still running. Then, after another 30 seconds or so, I can hear the glow plug click back on and the burners will ignite again.

Based on what I've read, on this forum, the problem appears to be a dirty "flame sensor" or dirty "Ignitor/Flame assembly".

Any ideas on where this part would be located on my Trane XR80?

Thanks.
 
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Old 12-01-09, 05:10 AM
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The flame sensor is usually on the opposite side of the furnace from the hot surface ignitor. It's usually a single round rod sticking up into the flames of the burner, with a wire coming out the bottom that goes back to the circuit board or ignition control module.

Cleaning that with a wire brush or fine sandpaper might be worth a try.

I'd also be suspicious that the furnace is overheating and shutting off on a limit switch due to a dirty filter, return air vent covered over or too many warm air vents closed.

Another possibility is that the thermostat is shutting off the furnace too early. If you have a no digital thermostat, many have a "heat anticipator" that heats up the temperature sensor to cause the thermostat to react more quickly. The heat anticipator is adjustable and can be set too high, causing the furnace to shut off too early, then turn on again when it cools off as you describe.

If you have that kind of thermostat, you can take the cover off to see if the blob of mercury or magnetic contact is opening and shutting off the furnace under the conditions your describe.

This is your 5:09 AM furnace repair advice columnist, on duty!
 
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Old 12-01-09, 05:22 AM
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Nice to "meet" you, 5:09am Furnace Repair Advice Columnist.

I do have a digital thermostat. It is programmed to start/stop with a plus/minus 1 degree temperature difference. And, it has a built in 500 hour filter timer. I replace the filter every 500 hours of run time. It's never very dirty, but it gets replaced and the timer is reset.

Now that I have more information of where the "flame sensor" is located I do recall seeing this part on the older previous furnace that I had. I just wanted to make sure I knew exactly what it was.

I'll have to take a look at my Trane and see where this part is located. Then give it a good gentle cleaning to see if it fixes the problem.
 
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Old 12-01-09, 05:43 AM
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With a seventeen minute turnaround on providing advice, I'd say we are meeting out standards!


Good that you can rule out a couple of likely causes for the problem you describe. Cleaning the flame sensor is good routine maintenance to do once/year in any case, and perhaps it will solve your problem.
 
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Old 12-01-09, 05:56 AM
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Absolutely.

Yeah, I'll have to add "clean flame sensor" to my list of annual maintenance chores.

What about the electric glow plug/ignitor? I would think that it would be just as susceptible to becoming as dirty as the flame sensor.

But, I also think that it would be more delicate and would probably be damaged, or broken, if touched with fine grit sandpaper, emory cloth, scotchbrite cloth, etc.

Can the glow plug be cleaned by using a cotton ball, or Q-tip, and some rubbing alcohol?
 
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Old 12-01-09, 06:23 AM
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I wouldn't clean the HSI. They are very fragile.

The problem with the flame sensor is that they tend to develop an invisible coating of oxides which prevent the flame from touching the sensor. The circuit then can't verify that the burners have lit and shuts off the gas.

That's just not a problem for hot surface ignitors, especially those with separate flame sensors.
 
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Old 12-01-09, 06:38 AM
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I figured that the ignitor would be too delicate, but I wanted to make sure.

I'll clean the flame sensor and see if the problems is resolved.

Thanks for the advice.
 
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Old 12-01-09, 02:53 PM
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I decided to watch the diagnostic light when the issue occurred again. It was flashing 4 times indicating an "open limit device".

I'm still going to clean the flame sensor.

But, any ideas as to what is causing the "open limit device" error?

I have a clean filter and I don't see any blockage of any kind.

Thoughts?
 
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Old 12-01-09, 03:03 PM
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Originally Posted by ron1999
I decided to watch the diagnostic light when the issue occurred again. It was flashing 4 times indicating an "open limit device".

I'm still going to clean the flame sensor.

But, any ideas as to what is causing the "open limit device" error?

I have a clean filter and I don't see any blockage of any kind.

Thoughts?


I'd identify the limit switch and use an AC voltmeter to verify that it's opening and shutting off the burners.

More than likely, it is.

Usually the limit switch has the temperature at which it open on it "L170" would identify a limit switch that opens at 170 degrees F.

You can stick a suitable thermometer in the warm air plenum and measure the temperature of the air when the limit switch opens to verify it's operation.
 
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Old 12-01-09, 03:49 PM
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If filters were neglected, even left out, in the past (by even another owner of the house), the secondary heat exchanger fins may be clogged with dust. They are directly above where the blower has to blow air through. And if that gets choked down the heat exchanger temp will rise.

Also, since you are in OK, you likely have central air? If so, for same reasons stated, the evaporator coils above the furnace, in the plenum just above, can get choked with moist dust, and restrict the flow out of the top of the furnace, causing it to limit. Both secondary and evaporator coil can be accessed and cleaned.

To do the secondary, you have to remove the blower assembly(a relatively easy job usually involving only a couple screws and maybe moving the circuit board box out of the way). To access the evaporator coils you have to open up the upper plenum the easiest way possible, at the seams. Some can be easy and some harder to do.
 
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Old 12-01-09, 04:59 PM
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I removed, and cleaned, the flame sensor. It didn't look that bad, but it looks brand new now.

I studied the wiring schematic, on the removable panel, and found what I think is the limiting sensor. I removed it and cleaned the sensor pad.

I took a flashlight and shined it in all the open spaces I could find without removing any plenum panels or the blower motor. I can do all that, but since the unit is only about 5 years old I didn't want to void any potential warranty.

I didn't find anything that appeared to be clogged. As for the filter it has been replaced every 500 hours of operation since the unit was installed. So, I really don't think the coils are dirty.

The furnace is back in operation, but the house is warm and there hasn't been a need for heat yet. When that time comes, as I'm sure it will in about an hour or so, I'll watch and listen for the unit to see if the burner shuts off and restarts again.
 
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Old 12-01-09, 05:53 PM
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Maybe you do not even have a problem. 10 minutes run time on a 1 degree swing temp might be right. You can also easily get a new call for heat overlap while the blower is still running from the previous call for heat. Try bumping the swing temp up another degree or two to see if that works out better for you.

Often the layout of where the stat is in relation to heat registers and cold air return(s), say in a hall where heat registers pour heat out of bedrooms and bathrooms, and drift past the thermostat on it's way say to a big cold air return, can augment your scenario. I've seen such cases where the thermostat shuts off in seconds, from such a scenario.

On one apt., last year, I had to shut down their bathroom register because of this.
 
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Old 12-01-09, 06:30 PM
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You don't have to wait around for the thermostat to operate the furnace.


Turn the thermostat up to 80 degrees or whatever and let it run continuously for awhile. If it's overheating, the limit switch will open when it's run long enough.

A lot of intermittent problems can be diagnosed by just letting the equipment run long enough to allow it to fail, or to turn it on enough to get it to fail.

If you need to turn it on 200 times to get it to fail, do so. The idea that "I can't fix it because it's operating OK right now is usually bogus.
 
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Old 12-02-09, 04:28 AM
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Nothing has changed, on the furnace setup and any room registers, since the furnace was installed 5 years ago. There is definitely something wrong going on.

The furnace has never acted like this before. Previously, when the furnace started up, it would run until the set thermostat temperature was reached plus one degree.

But, now, after the furnace has been running for a few minutes, I can hear the burners shut down. The blower motor is still running. After about 20 to 30 seconds the ignitor will light up and the burners will fire back up.

This cycle is repeated a couple of times until the house reaches the temperature on the thermostat plus one degree.

So, the only thing that has changed, from the previous years until now, is that the burners are cutting out for some reason, and the furnace is giving the number 4 code.

I know that I don't have to wait for the thermostat to send the signal for heat, and that I can always just set it up higher. But, I wanted all the variables to remain the same to see if the issue could be duplicated after I had cleaned the flame sensor and the temperature limiter sensor pad.

The issue is still occurring this morning. Maybe it's time to call in the Pro's instead of just swapping parts.
 
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Old 12-02-09, 05:00 AM
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It may seem perfectly logical to you that the furnace isn't overheating, but that doesn't cut it as a diagnostic method.

If you want to call a service agency to check on the limit switch for you, help yourself. Among the first things they will do is to check on the furnace limiting out.
 
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Old 12-02-09, 05:22 AM
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Originally Posted by SeattlePioneer
It may seem perfectly logical to you that the furnace isn't overheating, but that doesn't cut it as a diagnostic method.

If you want to call a service agency to check on the limit switch for you, help yourself. Among the first things they will do is to check on the furnace limiting out.

Well, I don't want to call a service company. But, when it comes to actually diagnosing this furnace, I don't have the knowledge to do it correctly.

I can remove, and replace, parts all day long. That's no problem.

But, when it comes to diagnosing, my skills are limited. You can bet that, if I do call a service company, that I will be watching the Tech closely and asking all kinds of questions. That's the only way I learn when it comes to equipment that I'm not familiar with.
 
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Old 12-02-09, 06:04 AM
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My post of 3:03 PM explained how you can check to see if the limit switch is opening, and if it's doing so for a reason.

Frankly, you've wasted more than a day telling us that you don't have a problem rather than checking to see if you do. That's a big waste of time all around.
 
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Old 12-02-09, 06:27 AM
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I don't like your bad attitude.

So, in the future, I'd appreciate it if you'd refrain from replying to any of my posts.

Thanks.
 
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Old 12-02-09, 06:57 AM
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Your problem is not my attitude, believe me. You lack the competence to analyze the problem with your furnace or to take direction from those who do have that competence.

Why you come to this board and then ignore the help you are offered is a mystery.
 
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Old 12-02-09, 07:00 AM
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I requested that you refrain from replying to my posts.
 
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Old 12-02-09, 08:13 AM
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Ron,

Okay, thanks on the backfeed from your 5:28 am post. Do you own a volt-ohm meter, also known as a multi-meter, to test with? If not, they can be bought quite cheap and you can save hundreds and hundreds of dollars in DIY'ing stuff yourself, with one. I buy lots of them on sale for only a few dollars(literally) at one of the national chain tool stores. I give them away to people I know as stocking stuffers. I gave one to my brother-in-law and then a couple months later he called me long distance and by it's use and my help over the phone he changed out parts himself in his fridge (and he works at a grocery store and is no mechanic!)

It is also nice to go to say the wel known chain electronics store and buy an alligator clip jumper wire for testing. Then we can tell you to clip between 2 terminals to see if your problem goes away.

Can you identify the pressure switch and limit switch on your furnace? When either of these go out within the call for heat sequence, this can replicate your problem. Alligator clip jumper wire across either of these(for test purposes only!)wil determine if either of these is tripping out on you.
 
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Old 12-02-09, 08:31 AM
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ecman51,

Thanks for the reply.

Yes, I do have a voltmeter. And, I have a jumper wire with alligator clips. Troubleshooting appliances is nothing new to me.

However, I've never dealt with a furnace. So, this is still a learning experience for me.

I have the issue narrowed down to the temperature limit switch. But, I don't think that means that there is something wrong with the switch itself. However, after 5 years of use, it very well could be a defective limit switch.

Those switches aren't expensive, but I don't want to buy a new one until I can verify the switch is bad.

So, do I simply disconnect the two wires that are attached to the limit switch, and then touch the terminals of the voltmeter to the wires while they are disconnected?

Or, do I leave them connected to the limit switch when I test it?

What should the voltage read before, and during, testing?

Do I test the pressure switch the same way?
 
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Old 12-02-09, 10:14 AM
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The odds are low that this is a limit switch problem.

More than likely the limit switch is opening because THE FURNACE IS OVERHEATING.

I described how to test for that condition in my 3:03 PM post yesterday.

Once you verify that the limit switch is opening and is responsible for turning the furnace off, and determine whether the limit switch is shutting off because the furnace is overheating, you will then need to determine why it's overheating.
 
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Old 12-02-09, 11:21 AM
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ecman51,

Okay, I tested all three limiter switches and the pressure switch.

I Ohm'd my voltmeter to .000 and started with 1 on the display.

Here's what I found:


Limiter switch:

With wires connected I started with 1 and tested to .000

With wires disconnected I started with 1 and tested to .000

Testing only the wires themselves I started with 1 and tested to .000


Pressure switch:

With wires connected I started with 1 and it moved to 1.3

With wires disconnected I started with 1 and it stayed on 1.

Testing only the wires themselves I started with 1 and it tested to 1.3.


So, does this mean that the pressure switch is defective? Would a faulty pressure switch cause a heat build up in the plenum that would trigger the limiter switch?


Also the pressure switch has an "eye" in the center of it that has kind of a sticky coating on it. The outside of the eye is clean, but the inside looks like little grains of fine sand, or fine dust, are stuck to it.
 
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Old 12-02-09, 12:19 PM
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I think I found the problem.

When I removed the pressure switch I blew some air into the rubber hose and into all the orifices on the switch.

When I went to retest the limit switch, and pressure switch, with the unit running I started with the thermostat at 63 degrees. I turned the thermostat up and let it run. Everything worked as it should on the furnace.

I let it run for about 20 minutes and it didn't cut out one single time.

There are wires running from the limit switch to the pressure switch. So, one controls the other. So, my guess, is that the pressure switch was dirty on the inside which caused the limit switch to trigger and shut down the gas flow.

I'll watch it for a few more days and see if the issue returns. If it does then I'll order a new pressure switch.
 
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Old 12-02-09, 04:08 PM
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I believe the limit switch is usually first in the series. But it really does not matter, as any outage in the 24 volt safety system will cause the burner to go out. Pressure switch circuit is often the culprit becasue there are several causes that can make the plunger inside it close marginally. On some, you can actually watch the plunger oscillate in and out. And if it oscillates outward just enough too much, it can shut down the burner, and cause the furnace (ignition control module or board)to have to go back to the beginning and try to reignite all over again.

The only ohms test on the pressure switch that means anything is the one where you tested with the wires off. You got an open reading (a non-movement 1), which is correct. Testing the pressure switch for volts, when the inducer runs, is good at finding out if current stays, or gets lost, exiting the pressure switch. You run the furnace/inducer and check for voltage from the incoming PS terminal/wire to ground, and then do the same for the other exiting terminal/wire. As long as the inducer runs, you should have about 24 volts.

Come back and let us know if all stays well. Great to see how you thought your way through this.
 
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Old 12-03-09, 06:28 PM
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I found the problem to my furnace.

Yesterday, at 2:19, I posted that I had cleaned the pressure switch. Then turned the furnace on and let it run for a while. During that time the furnace burners never cut off.

However, during that particular test, I had both front panels removed. One that covers the burners and the other that covers the blower motor on the bottom of the furnace.

When I replaced both panels the issue eventually returned. You know where I'm going with this already don't you?

So, today, I removed the two front panels again and turned on the furnace. Yup, you guessed correctly, it ran perfectly.

I'm standing there thinking, "Okay, what does removing both panels do to the furnace that isn't being done when the panels are installed?"

Then it finally hit me. When the panel over the blower motor is removed then the blower motor is using the air from the room and is bypassing the filter. When the panels are reinstalled the blower motor is using the air from the return register that must pass through the filter.

So, I removed the filter and looked at it. It didn't look very dirty since I had installed it less than a month ago. I opened a new filter (different brand) and held both of them up to the sunlight coming in through the window.

The new filter was clean and white. The other filter showed plenty of sunlight through it, but it looked noticeably browner than the new one. That is to be expected since the "old" filter had been used for a few weeks.

I installed the brand new filter, reinstalled the panels, and fired up the furnace. It's been running perfectly ever since.

Even though the "old" filter wasn't that dirty it was apparently made of inferior filter material. It didn't take much to clog that old filter and that was causing the blower motor to not allow enough air to pass through into the furnace. Which caused the plenum to heat up too much and trigger the limit switch.

I bought that crappy filter at WalMart. It was a PureFilter 2000brand. I will never make that mistake again.

I had replinshed my supply of filters from Lowe's. Lucky for me that I did. And, that filter is working just like it should. Trapping the dust with a Merv rating of 8. It is the brand of filter that I had always used. I just bought that crappy one at WalMart because I happened to be there one day and happened to remember that I needed an air filter.

So, as much physical pain that it causes me to say this, the filter was the problem the entire time.

NEVER get your filters at WalMart. Or, if you do, do NOT get the PureFilter 2000 brand.

Lesson learned.
 
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Old 12-03-09, 06:51 PM
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Originally Posted by SeattlePioneer

I'd also be suspicious that the furnace is overheating and shutting off on a limit switch due to a dirty filter, return air vent covered over or too many warm air vents closed.

!

Sigh. This was the first reply to the initial post. Twenty odd posts later the filter finally gets properly checked.

As a furnace repairman, one of the very early things that gets checked when a furnace shuts off is the limit switch, pressure switch and such, to identify what it shutting that sucker off.

Repeatedly the Ron was directed to check the limit switch --- even the diagnostic light was pointing to that.


This isn't intended to be critical of Ron personally. It's just a weakness DIYers tend to have --- going off on various wild zebra chases rather than looking at the horses that are the usual suspects and problems.

And I'm sorry if I irritated Ron. Perhaps he can understand that it's frustrating to be trying to help make repairs remotely but not being able to get things tested and checked in a systematic way that makes sense to someone who knows how to track down problems.

I'm sure it can be irritating for the DIYer as well as the advice giver.

I oughta write a book.
 
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Old 12-03-09, 07:22 PM
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One other thing, Ron,


As I read through this thread again, it occurs to me that you may not have known how to do the checks on the pressure switch and limit switch that were recommended.

When you did check the limit switch, you checked for continuity through the switch, while what we were looking for was a test of whether and when 24 VAC was present on both sides of the switch (limit switch closed), or 24 VAC present on only one side of the switch (limit switch opened, shutting off the burners).

It's not easy knowing how much instruction needs to be given on how to do a test or task. Give too much, and it rapidly becomes confusing. Give too little and people perhaps don't know what to do and may be reluctant to ask.

And I do appreciate your coming back to explain what you found. As I describe above, it gives me an opportunity to try to figure out better ways to try to help people.
 
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Old 12-03-09, 07:30 PM
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Originally Posted by ron1999
I bought that crappy filter at WalMart. It was a PureFilter 2000brand. I will never make that mistake again.
Well? It filtered obviously. It clogged up some because it filtered.

The only thing that can cause a filter, made like this, to clog, even though you think it is not very clogged, is the fact they must not have had very much pleating to it. The more pleating, the more surface area it has. The same amount of particulate mannor per time period can be scattered across more surface area.

And per chance there really is not much compartively wrong with it's pleating design, then the dirty house has to be blamed, and not the filter. But, if it does not look al that clogged, it does appear that the paragraoph above applies.

Then the bottom line becomes if one buys such filters, that were in there, you have to just change them more often. Then weigh out if you truly save anything.
 
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Old 12-04-09, 05:03 AM
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SeattlePioneer,

It wasn't your advice that I had a problem with. It was your attitude.

Yes, you mentioned checking the filter. And, I did that. But, just by looking at it it looked fine. Still looked "white". Only by holding it up to the sunlight, and comparing it side by side with a brand new one, could I actually see the difference.

How is a novice, like me, suppose to tell if a filter (that looks clean) really isn't? Nobody steered me in that direction. So, it would have been more helpful if you, or anybody else, would have suggested comparing the filter to a brand new one. Or, using a different brand to see what happens.

As far as troubleshooting you'll recall that I mentioned that I had no experience troubleshooting a furnace. Working on furnaces might be old news to you, but it was a new learning experience for me.

I didn't need attitude. I needed assistance. And, that's why I posted on this site.



ecman51,

Yes, the Walmart filter worked fine. But, only for a few weeks. Usually I replace the filters every 250 operating hours. I think, in an earlier post, I said 500 operating hours. But, I checked the programmable thermostat and it only goes to 250.

So, if you figure 4 hours of actual run time per day, that means the filter gets changed every 2 months. The filters I normally use are never all that dirty by looking at them. And, they never caused the problem I was having.

The Walmart filter did it's job, but I think that it "clogged" too quickly. No, my house isn't dirty. It gets dusted and vacuumed every week. And, the return air register gets cleaned as well.

But, you're correct. The filters should be changed more often than every 2 months. From now on I won't go by the operating hours on the thermostat. I'll just change the filter at the first of each month and be done with it.
 
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Old 12-04-09, 07:21 AM
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Originally Posted by ron1999
So, it would have been more helpful if you, or anybody else, would have suggested comparing the filter to a brand new one. Or, using a different brand to see what happens.
It may be even overlooked by pros, sometimes. One can even start concentrating, before even arriving at the address, pondering the various mechanical failures that might cause it, and forget the filter, as is it something so mundane, that...well, you might just forget.

I have seen shows on tv where even doctors do not test for some quite common things(a disease we have all heard of), and the patient sees like 5 doctors before their symptom gets diagnosed! And then we the viewer learn what the disease was, and go, 'You mean to tell me that none of those doctors would have thought of that?"

No, my house isn't dirty. It gets dusted and vacuumed every week. And, the return air register gets cleaned as well.
It does not have to mean you live like a pig. Maybe you have an inordinate amount of fabrics in the house. Maybe you have lots of kids (people even shed skin you know), or you do a lot of vacuuming of carpeting that is breaking down, when the furnace or a/c(also sucks though furnace) is running, or have not given thought to shaking out clothes and blankets in the house, figuring you will be vacumming, or pets, or lots of hair brushing - drawing it to the cold air returns and filter - and other such stuff.
 
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Old 12-04-09, 07:43 AM
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Originally Posted by ecman51`
It does not have to mean you live like a pig. Maybe you have an inordinate amount of fabrics in the house. Maybe you have lots of kids (people even shed skin you know), or you do a lot of vacuuming of carpeting that is breaking down, when the furnace or a/c(also sucks though furnace) is running, or have not given thought to shaking out clothes and blankets in the house, figuring you will be vacumming, or pets, or lots of hair brushing - drawing it to the cold air returns and filter - and other such stuff.

All very true. I thought about that last night. Trying to determine if I actually had changed anything in my routine. Normally it's just me, and my daughter, who are in the house.

I couldn't think of anything that has changed other than me installing that Walmart filter. Sure, the filter worked. But, it just seems like it became quickly clogged enough... to restrict the airflow... that caused the plenum to heat up... that caused the limiter to open.

The overall design of the so called "clogged" filter is identical to the brand that I normally buy. And, it was a couple dollars cheaper. But, apparently the filter material wasn't the same.

However, the more I think about it, the more I might wonder if that might actually be a good thing. Maybe the filter material, of the Walmart filter, traps more fine dust particles than the other brand that I normally use.

This would explain why it became clogged so quickly, and would therefore require more frequent changing (i.e. monthly instead of every two months). Even though it hadn't been a month since I installed that Walmart filter.

The good part about all of this is that I learned ALOT about my furnace. I know what each part does and what to look for.

So, even though the fix turned out to be embarrassingly simple, I now have a good troubleshooting technique to use in case someone I know starts having similar problems.

Point being: If the filter looks okay it is a wise idea to compare it, in strong light, side by side with a brand new filter. If it looks "okay" then remove the blower motor panel. If the issue goes away return the blower motor panel. If the issue returns then the it's a good bet that the filter is the culprit.

All other variables being equal of course.
 
 

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