Troubleshooting high limit switch problems on furnace

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Old 11-13-16, 08:59 PM
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Troubleshooting high limit switch problems on furnace

Hello, I have been looking around the forums for a solution to this problem but so far no luck.

I have a luxaire gas furnace. Model g1fa060s21g. I recently installed an ecobee thermostat however i don't think that is the source of the problem. I have the ecobee set to allow the hvac to control the fan.

So the problem: when heat is called, the furnace turns on, then the electric ignition lights the flame and the burners come on. Then the fan blower turns on and circulates warm air. After 3-5 mins the burners turn off and the furnace flashes a 4 red signal which means a high limit switch was set off. However the fan stays on. After a few more minutes the furnace ignites again and the cycle repeats. After about 30-40mins of this cycle, the whole furnace including the fan turns off.

I have the set temperature high so the thermostat is still calling for heat, but after this cycle I need to power cycle the furnace to get it to start again.

The first thing I did before anything else was replace the filter, which was very dirty. I just purchased this home and we just started using the heat about 2 weeks ago. So I am not exactly sure how long the furnace was running with a dirty filter. I also checked all the registers and they are open. Looked at the blower fans and it looks clean. My return registers are also clean and unobstructed.

Next I checked the switches. It seems when I am in the cycle and the burners are turning on and off it's the primary limit switch that being tripped. I bypassed this switch for a few minutes and the burners did not turn off.

However at the end of the 30-40min cycle when everything turns off and cannot be restarted without a power cycle then it is one of the secondary limit switches that has tripped. My furnace has two secondary switches, one on each side of the blower and it seems like it's the one on the right side further from the air return that gets tripped.

So after all that, the question. I am guessing the furnace should not be hitting the limit switch to cycle the burners off so why is it doing that in the first place. Second, what is causing the secondary limit switch to go off? What should I check next?

Thanks in advance!

Neil
 
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Old 11-13-16, 09:04 PM
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Correct model number gm8s115c20uh11b
 
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Old 11-14-16, 05:31 AM
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Never jump a safety control. There are better ways to diagnose issues.
What air filter is being used, and what size it it? How many BTU is the furnace?
 
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Old 11-14-16, 08:12 AM
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The air filter is a home depot filter (16x25x1) grade 4. As high flowing as I could find. I am not sure how many BTU. I did not see it on any label and I can't find it by looking up the model number.

Thanks for the response
 
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Old 11-14-16, 06:10 PM
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<<So after all that, the question. I am guessing the furnace should not be hitting the limit switch to cycle the burners off so why is it doing that in the first place.>>


That's right, the furnace should not be cycling on the high limit switch.


I'm guessing that you have a counterflow furnace that blows warm air down, rather than upm through the furnace. Counterflow furnaces often have a manual reset limit switch in the fan compartment near the top of the furnace, to shut off the burners if the fan fails, which might allow the fan compartment to overheat.

Very likely, something is restricting the air flow through the furnace which strips away the heat produced by the burners. wITH REDUCED AIR FLOW, THE FURNACE GRADUALLY OVERHEATS until the limit opens.

Check to be sure that the warm air registers are open and that the return air vent(s) aren't covered over.

Check to be sure that the fan wheel isn't saturated with dust and that the air conditioning coil in the furnace (if any) isn't plugged with dust and dirt.

You can have insualtion fron the ductwork that comes loose and plugs things up.

I'm presuming that the fan motor is coming up to speed ---also check to be sure that the fan isn't loose or broken on the motor shaft.
 
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Old 11-14-16, 07:34 PM
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Thanks for the input.

The furnace is set up so that the blower fan is at the bottom and the burners are in the middle and the AC coil is on top. I dont think its a counterflow. However, there are 3 limit switches and 4 roll out switches. One limit is a self resetting limit that is near the burners, the other two limit switches are manual reset that are on either side of the blower fan on the bottom of the furnace.

It seems that the manual reset limit switch is the one which shuts down the whole cycle. However, I can run my fan without the heat and it never trips the switch, so I don't think the fan motor itself is overheating. Is that the only cause that would trip the manual reset limit switch?

All the registers in the house are open except for 3 in the basement. Thats a total of 11/14 open. There are two return registers that are also open and clear. I opened one of the registers on the 1st floor (furnace is in basement) to look into the ductwork and I did not see any obstruction. I have checked the fan blades and they have only minimal dust on their surface. The fan spins freely and it looks like its getting up to normal speed. When the fan is on, there is good airflow coming out of the vents.

The only thing I haven't checked is the AC coils. I'll do that tomorrow after I buy some foil tape.

Last thing, how do I check all the ductwork in the house? Do i need to buy a special camera to go into the ducts?

Thanks again!
 
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Old 11-15-16, 03:48 PM
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https://www.hamiltonhomeproducts.com...S-DN-Parts.pdf


The specifications above list your furnace for use as a counterflow or horizontal furnace. You should check the illustration to see if the furnace is installed properly.

The illustration inn the specifications show the furnace set up for use as a counterflow furnace, with the fan on top and the burners on the bottom.

The fan blows air DOWN, across the heat exchangers which are between the fan and the burners.

Model GiFAR is for the air conditioning coil:

https://www.master.ca/documents/regr...rParts0803.pdf


You should check the make and model number which will be found on the rating plate in the burner compartment of the furnace. I'm guessing we still don't have the correct model number.

Heh, heh! Third Time's the charm!
 
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Old 11-17-16, 10:49 AM
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I just checked again and the model number is correct. That was on the label inside the burner housing area.

If the unit is installed incorrectly then maybe that is the issue?
 
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Old 11-17-16, 11:30 AM
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If you have the model number correct and your description of the way the furnace is installed is correct, the furnace is installed upside down and that could easily cause the furnace to operate improperly.

The illustration in the mmanual illustrates how this counterflow furnace should look when it is installed as a counterflow furnace. If your furnace looks like it's upside down compared with that illustration, it probably is.

Sorta hard to believe, though, isn't it?
 
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Old 11-17-16, 11:33 AM
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Actually i have found this: https://www.hamiltonhomeproducts.com...S-UP-Parts.pdf

which suggests that there is an upflow model with the model number i gave.

gm8s115c20uh11b

The downflow model is GM8S115C20DN11B

Highlighted is the difference. So I think it is installed correctly.

On another note, how should the burners cycle in a normally working furnace, is there another switch that gets tripped other than the high limit switch?
 
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Old 11-17-16, 11:38 AM
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That furnace is listed for Upflow/Horizontal use.

Climasure/pdffileredirect/344764-lss-a-0807.pdf
 
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Old 11-17-16, 11:40 AM
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I'm glad you solved that mystery! Unfortunately, you can encounter almost anything out there is "the field"!

<<Correct model number gm8s115c20uh11b >>


I was lazy.


I looked up the furnace using "gm8s" as the model number. Most manufacturers would distinguish between upflow and downflow models in that part of the model number, and use subsequent parts to identify BTU input of different furnaces and such.

Searching with the complete model number can make finding installation manuals difficult, so I used the shorter part of that, and gave you misinformation because of that.

I apologize for that, and you get three gold stars for discovering the correct information! ***!
 
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Old 11-17-16, 11:44 AM
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Is this furnace in a manufactured home ?
For some reason the model number links directly to Coleman.

Manual...
Manuals/Parts List/80AFUE/Furnace/GM8s-UPFLW/GM8S-UP-Install.pdf

After about 30-40mins of this cycle, the whole furnace including the fan turns off.
The board locks down as it see this as the flame being interrupted three or more time in one call for heat.

It seems that the manual reset limit switch is the one which shuts down the whole cycle.
A manual reset limit is going to be a flame rollout sensor. The limits that monitor the plenum are all automatic reset type which is what you mentioned earlier.
 
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Old 11-17-16, 01:20 PM
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Thanks for all the input. The furnace is not in a manufactured home.

So during a call for heat on a normal working furnace, the burners stay on the whole time?
 
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Old 11-17-16, 03:04 PM
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Yes... the burners remain running all during the call for heat.
 
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Old 11-17-16, 05:07 PM
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I checked the AC coils and they look clean.

And it seems you were right about the board. When the furnace shuts off completely I just have to reset the power to the board and it turns on again. So after 4 or 5 burner lighting cycles the circuit board shuts off everything.

So back to square one, what's causing the limit sensor to cycle. I have checked the AC coils, all the registers, the filter and the blower fan and everything looks good.

Do I need to increase the blower fan speed?
 
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Old 11-17-16, 08:50 PM
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Alright, tested it on the highest blower fan speed and burners still went out after about 5 minutes.

I also tested the fan capacitor and it seems fine.

I am out of ideas.
 
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Old 11-17-16, 08:58 PM
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How many CFM is the unit moving?
 
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Old 11-18-16, 06:52 AM
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The CFMS are listed at the last two pages of this:
https://www.hamiltonhomeproducts.com...UP-Install.pdf

Is there a way to know which setting my furance is set to?
 
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Old 11-18-16, 08:34 AM
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See page 25 of the manual
 
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Old 11-18-16, 11:11 AM
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I repeat my diagnosis and advice:

Very likely, something is restricting the air flow through the furnace which strips away the heat produced by the burners. wITH REDUCED AIR FLOW, THE FURNACE GRADUALLY OVERHEATS until the limit opens.

Check to be sure that the warm air registers are open and that the return air vent(s) aren't covered over.

Check to be sure that the fan wheel isn't saturated with dust and that the air conditioning coil in the furnace (if any) isn't plugged with dust and dirt.

You can have insualtion fron the ductwork that comes loose and plugs things up.

I'm presuming that the fan motor is coming up to speed ---also check to be sure that the fan isn't loose or broken on the motor shaft.



And here's a prediction:


You will discover that the colder the outdoor temperature, the worse this problem will get.


When it gets cold outdoors, the furnace needs to operate for longer and longer cycles. The limit switch opening will keep the furnace off, so the house will get increasingly cold.

So you need to find and correct the problem.

You need to go through each listed possible problem and see if that's an issue.

If you don't have the time or skill to do that, hire a repairman to do it for you before it gets cold.

A SKILLED repairman would use a manometer to test varying parts of the duct work and furnace to identify where there is a large drop in air pressure, which would indicate an area that needs to be examined for a possible problem. But systematically examining the various parts of the system for obvious problems will usually identify the problem.
 
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Old 11-18-16, 12:10 PM
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Thanks again... I agree that the problem is going to get worse as it gets colder. My wife thinks I am crazy obsessing over something that its not really even effecting the heat in the house. I even got an HVAC guy to come by and he said he found no problem (though he saw that the burners were cycling off after 4 minutes).

So I think need to check all the ductwork because everything else seems to be in good working order.

Registers are all open, return vents are clear, fan is running up to speed and even runs fine on the highest setting, no significant buildup on the fan blades, Fan seems to be smooth spinning and balanced, AC coils are clean, filter is new.

If I were to get a manometer, what should be the expected pressure, or should it just be the same at all ducts?
 
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Old 11-18-16, 01:47 PM
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The high limit switch is not intended nor designed to be a continuously cycled operating control. Which means it will fail completely and leave you without heat until a new one can be installed.
Post a picture of the furnace and associated duct.
 
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Old 11-20-16, 09:50 AM
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Here are photos of the furnace and wiring diagram.

I am getting a fiberoptic camera to go into the vents and look for obstruction. I checked the airflow from each vent and they seem to be roughly even by my hand manometer.
 
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Old 11-21-16, 07:18 AM
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From the manual:

ROLLOUT SWITCH CONTROLS
These controls are mounted on the burner box assembly. If the temperature
in the burner box exceeds its set point
, the ignition control and the
gas valve are de-energized. These are manual reset controls that must be reset before operation can continue.
==========================

These things can wear out over time and need to be replaced. Although my furnace is much simpler than yours, I had the same happen. I cleaned the blower, checked the registers etc and the darn thing kept shutting off the burner before it should. When it was really cold I had to remove the door to keep it from tripping.

I finally replaced the limit switch and no problems. Be sure to get the correct temperature as you have now.
 
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Old 11-21-16, 10:24 AM
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Thanks for the input, but what is happening is the primary high limit switch (which has an auto reset) is being tripped. The rollout switched and the secondary limit switches (which are manual reset) are not being triggered.

I have also removed my high limit switch and tested it and it opens and closes appropriately. Opens at 150 and closes at 115 F.

I have a 2nd HVAC guy coming to look at it today... so hopefully we can get an answer.
 
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Old 11-21-16, 03:21 PM
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Latest HVAC technician said he thinks its either the blower motor or the exhaust fan motor that's causing the problem.
 
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Old 11-21-16, 07:04 PM
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Looking at my furnace specifications, the maximum outlet air temperature is 170F but my limit switch is a 150F switch. Is this the cause of the problem?
 
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Old 11-22-16, 02:17 PM
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<<Looking at my furnace specifications, the maximum outlet air temperature is 170F but my limit switch is a 150F switch. Is this the cause of the problem? >>


Interesting question. I looked up part replacements, but they didn't specify the limit switch temperature.

Try checking the circuit diagram on the furnace, it might list the temperature.

You might have to call the manufacturer to get the correct temperature for the limit switch.
 
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Old 11-22-16, 07:17 PM
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It's not on the circuit diagram. I will call the manufacturer. However, when I look up the part for replacements, it looks like the limit switch is a 140F or 150F limit.

I took another look at the AC coils, it seems that I was looking at the wrong side of the coils (the outside) and the inside is the area that gets dirty. I was able to get a view of the inside from below, and it looks like there is a lot of build up that is the source of the problem.

But now I have to figure out how to get the cover off the coils to clean it. I have attached a picture... but all the copper tubing seems to be in the way. Can that be bent out of the way to take the triangular metal piece off?

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Old 11-23-16, 09:21 AM
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Ok. I got the coils cleaned.... and it hasn't fixed the problem.

I have checked the upflow ductwork and there is no obstruction within 5 feet from the furnace. I checked the flue and the chimney and no obstruction there. I have not checked the pressure at all the room registers but there is air flow at all of them.

I am again stumped. I think the next thing to replace will be the blower motor, or I can change the limit switch to a 170F.

Any other ideas?
 
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Old 11-23-16, 12:50 PM
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If anyone is wondering you can get to the inside of the coils by just taking off the screws holding the triangular piece and pushing that piece forward and down toward the heat exchanger below the evaporator. It can't be removed through the copper tubing of the evaporator.
 
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Old 11-23-16, 01:19 PM
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I'd examine the fan wheel blades to see if they are plugged with dust and dirt. Check for missing or damaged fan blade. Verify that the motor is turning fan in the proper direction.

Fans can get loose of the blower motor shaft. Be sure yours is tight.

Satisfy yourself that the motor is coming up it's proper operating speed. Lack of lubrication can cause he motor run slow.

The rating plate in the furnace list the maximum static pressure that the furnace should develop when the fan is operating. You should have a competent repairman use a manometer to test the static pressure at various locations in the furnace and ductwork which is the scientific way to determine if the ductwork is of adequate design and in good condition.

You really need a competent repairman to diagnose this problem. Replacing parts based on guesses is not recommended.
 
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Old 11-23-16, 01:30 PM
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https://www.youtube.com/watch?v=GLCqi2-Ywmk

Here's a u tube video on checking static air pressure.

It's really the scientific way to check out the operation of the ductwork, filter, fan and such and to locate any defect or problems.
 
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Old 11-23-16, 01:30 PM
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Thanks,

I have had two HVAC repairmen come by and one suggested to replace the blower motor and another suggested to replace the whole furnace. I mentioned to both of them about testing the ductwork and both dismissed the idea.

As far as the fan condition, it has a little bit of dust but nothing impressive like I have seen on the web as examples of dirty fans. It is secure on the shaft and spins freely and there are no fan blades missing. The motor seems to run easily and it not making any weird noises. Is there a way to check what speed its getting up to (RPM)?

I am going to have a 3rd HVAC guy come by next week... but if anyone has recommendations for another contractor please share.
 
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Old 11-23-16, 01:42 PM
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<<I have had two HVAC repairmen come by and one suggested to replace the blower motor and another suggested to replace the whole furnace. >>


These were non repair repairman.


You need a COMPETENT repairman who owns and understands how to use a manometer to diagnose air flow problems.

I'd start by calling back one or both of the outfits you've had out earlier, and talking "turkey" to a manager or supervisor.

Lots of repair services have marginally qualified people up front trying to diagnose problems. There is usually a competent repairman available to come out and diagnose problems that the first guy can't handle.

You want a repairman who 1) has a manometer and the fittings and hoses needed to use it and 2) understands how to use a manometer to diagnose furnace and ductwork air flow problems.

In the video, the static air pressure across the air filter is measured. The difference is slight, which indicates that the air filter is good.

If the static air pressure across the entire furnace is measured, it should be equal to or less than the static air pressure listed on the furnace rating plate. That will tell you if there is a problem in the furnace itself (fan wheel, motor or whatever) If you measure the static air pressure across the AC coil, you can verify whether that is clean or a problem.

In short, you can take reading across individual parts of the furnace or across the entire furnace system to determine whether and where a problem exists.

Or, if you aren't properly equipped and competent, you can recommend the furnace be replaced and earn yourself a $100 commission if someone takes your advice.

Sad, isn't it?
 
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Old 11-23-16, 01:48 PM
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https://www.youtube.com/watch?v=GLCqi2-Ywmk


Here's a u tube video showing how a manometer is used to diagnose furnace air flow problems.


There are lots of videos and explanations of how this is done if you Google furnace static air pressure problems.
 
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Old 11-26-16, 03:34 PM
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So I did another experiment to test if it was an obstruction in the ducts.

I took the access panel for the evaporator off and ran the heat. This way, if there was an obstruction in the duct system, since there was a way for the air to escape it would not build up pressure and decrease flow and trip the limit switch.

The furnace ran for about a minute longer... but still tripped the limit switch.

Then I took the blower cover off and ran the system and it did not trip the limit switch. So I guess this means the obstruction is in the return duct system.
 
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Old 11-26-16, 04:43 PM
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Also, if I take the filter out of the circuit and run the heat the limit switch does not trip. This is with the evaptorator cover and blower cover in place.

The filter I bought was a MERV 4 filter from home depot. It was the lowest number I could find. Should I be using a MERV 2?
 
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Old 11-26-16, 07:15 PM
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N,
Something that is overlooked sometime is the importance of return air. You are only going to get as much air out as you have return air coming back. If you block your returns and run your furnace you will see that no air will get to the registers and your furnace will cycle on limit like now. It's like filling a straw with water and covering the top. It's like pulling a vacuum. Release the top and the water releases. It's the same principle. I mention this because you may just need more return air in your system. By opening the blower door all you did was add more return air. When you removed the filter you removed a possible restriction to restore proper operation because your return duct sizing is that close to being too small. Maybe even just adding a little return air would solve the problem.
Hope this helps.
 
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