Troubleshooting Limit Circuit Fault
Hello! I'm hoping someone can help me troubleshoot and pinpoint the cause of a limit circuit fault on my heater please.
About 6 months ago I moved into a house with an 80% Single Stage Gas Furnace (ICP N8MSN0902120A) which was installed 8/2017. The house was built in 1928. Recently, the heater started going through one or two heating cycles normally but then began continuously blowing cold air out the vents. After investigating, I found it to be a limit circuit lockout, initiated by a limit circuit fault. Reading about the status code, I found this info:
LIMIT CIRCUIT LOCKOUT - Lockout
occurs if the limit, draft safeguard, flame
rollout, or blocked vent switch (if used) is
open longer than 3 minutes.
LIMIT CIRCUIT FAULT - Indicates the
limit, draft safeguard, flame rollout, or
blocked vent switch (if used) is open.
Blower will run for 4 min. or until open
switch remakes whichever is longer. If
open longer than 3 min, code changes to
lockout #7. If open less than 3 min. status
code #4 continues to flash until blower
shuts off. Flame rollout and blocked vent
switch require manual reset. Check for:
- Restricted vent
- Loose blower wheel
- Defective switch or connections
- Proper vent sizing
- Dirty filter or restricted duct system
- Excessive wind
- Inadequate combustion air supply (Flame rollout switch open)
- Defective blower motor or capacitor
According to the info above, lockout occurs if one of the switches is open longer than 3 minutes, which it was. So, going through the list of possible causes, I checked the blower wheel, and it does not feel loose. I also changed the air filter and made sure all of the vents were open. I think I can safely eliminate any issue with proper vent sizing because they are the same vents that have been in the house for decades, and the ducts are large (I'm guessing at least 12"). There is no blockage of the return vents either. I don't think there is a problem with the blower motor as it seems to run normally, but I don't know if there's a way to test it to be sure. Based on the given list, this narrows it down to either a blockage inside the ducts (the flue is not listed but could this be a cause too?), a defective switch or connection, inadequate combustion air supply, or (less likely) a defective blower motor or capacitor.
I would like to know
- Are there any other causes not listed that could also be the culprit?
- How to check the switches (I have a multimeter) to determine which switch is the cause and check if the switch itself is bad.
- How to check for inadequate combustion air supply? Does a manometer test for this?
- I don't see a draft safeguard or flame rollout switch. Any idea where these might be located? The only switches I see on the heater diagram are the blocked vent switch, and the manual reset and main limit switches
- Do I need to test the blower motor and capacitor further? If so, how do I do this?
- How do I check for a blockage inside the ducts (and flue if relevant)? I don't think I can safely get up on the roof to look down. Would a manometer be able to tell me if something is blocked? NOTE: I do have a central air conditioner which has been running all summer with no issues. If there were a blockage inside the vents, would this have also stopped the A/C from running? Meaning, can I assume the vents are not blocked?
If someone would be willing to help me figure out the problem, I would be so grateful. I have a multimeter and can get a manometer if needed. Cold weather is coming quickly here where I live in Northeast Ohio! If any more info is needed, please let me know.
This is from the furnace manual:
And here's a picture of the actual furnace:
Comments
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Do you own this house or are you a tenant?
Can you ask the previous people who were living there last winter if they had the same problem.
My first go to would be the high limit behind the gas valve. You could put meter leads directly on the 2 terminals of the limit. Leave all the wires connected to the limits.
With the furnace running normally you should have no voltage showing across those two leads.
If the limit opens then you will see 24 volts. This test works on any simple 2 wire limit switch.
0 -
Call a professional familiar with your brand. Safety first. Know what you don't know and know to ask a question.
John Cockerill Exquisite Heat www.exqheat.com Precisions boiler control from indoor reset.0 -
Gotta check static pressure and temperature rise. Gas pressure measurements and combustion analysis need to be performed.
Its probably oversized equipment installed on undersized ductwork.
2 -
The rollout switches are labeled as "manual reset limit switch". You need to use the meter to figure out which safety is opening, once you figure that out then you can figure out the cause.
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If one of the rollout switches opened then chances are good that your heat exchanger is shot or the vent is blocked
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Thanks for your comment! I do own the house. I was living here at the end of last winter and used the heater a good bit before it warmed up and it wasn't having any problems. I did ask the previous owners and they say it was working find for them.
If I test the switch like you instructed and get different results than you said, would that mean the switch itself is bad?
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Probably not. it is much more likely that the safety is opening because the condition it protects against is occurring than the switch is bad.
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Thanks for your comment! I will get a manometer and check the pressure. I can test the temp rise too.
I really don't think the issue is an oversized furnace or undersized ducts though. Input is 88,000 BTUs and Output is 71,000 BTUs which is the right size for my ~1500 square foot home. The vents are 25" in circumference. I had a hard time figuring out how to do the calculations to check this but I really don't think this is the cause of the issue.
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Safety first: I appreciate the concern but I'm certainly comfortable testing switches, checking pressures and temperatures, and any other light diagnostics I have a background in mechanics and also do my own repairs around the house.
Know how to ask a question: This is the reason I posted here for help 😉
2 -
who said that’s the right size furnace?
71,000 out here would be oversized.
I agree a safety is doing its job.
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Thank you for that! I will check it and see. @JUGHNE suggested I should also check the high limit behind the gas valve too. I'm assuming the rollout switches should also read "0" if they're closed and "24" if they're open? How would I know if one of the switches is faulty, i.e. incorrectly detecting flame rollout?
Also, I do not know when the furnace was last maintenanced. Could cleaning the burners possibly help?
Thank you again for your help!
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Really hoping it's not the heat exchanger but I do have a warranty in case it is! Wouldn't there be other signs if it cracked though? I checked for CO and didn't detect any, the flames are blue (not orange), and I don't hear any strange noises or smell weird smells. I suspect (hope) it's a faulty switch or a blockage. A couple of people here told me how to check the switches, so I'll do that but I'm still not clear on how to check for a blockage.
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Because the blower puts the hx under positive pressure and listed CO detectors don't alarm until there are huge concentrations of CO it is unlikely that the amount of CO that would leak out of a bad HX would set off a CO detector. A better at least quick check for the integrity of the HX is to watch the flame when the blower starts. There should be no change to the flame when the blower starts, it shouldn't move or change shape or color. If the blower starting changes the flame that means that it is pushing air in through some sort of a hole in the HX.
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A digital combustion analyzer is a great tool for checking bad heat exchangers, blockage in the flue or the secondary heat exchanger in high efficiency furnaces. A good tech like myself would have all the tools to figure this out quickly and it would only cost you a diagnostic fee. The diagnostic fee is probably less than 1/10 of what the tools to do the diagnostic work costs. I know you want to attempt this yourself but you could really benefit from the knowledge of an experienced tech who has the tools to do the job.
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The meter test will tell you which limit opens. It is not necessarily a bad switch.
You are simply looking for which limit shuts down the burner. The roll out switches are on either side of the burner and are manual reset type. That is you have to push the small red button to reset them back to being closed. The other limit is on the inducer fan housing, it may be or not be manual reset. If you have never had to reset any of those 3 limits then the most likely limit is the main limit above the burners behind the gas valve.
Do you know if you have a variable speed blower (ECM) or an old school 3-4 speed blower motor?
With the old type you could put the tstat fan switch to on and it might run on high speed (AC speed).
That would provide more air flow thru the furnace and perhaps keep it from overheating.
I had a problem with a replacement furnace that the owner insisted to oversize. I did so and was still smaller than the original unit. The main limit eventually failed open and had to be replaced. It was cycling the burners by that limit and cool down and then relight. This could have been the case with the original furnace also.
I downfired the gas pressure and had to go to the cheap air filters to keep the furnace from overheating. You can measure the temp rise across the furnace and compare it to the label on the furnace to see if you are overheating.
The 25" circumference you speak of, is that round steel pipe or insulated flex? And how many runs are there?
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47 BTU per sq ft is not the right size. But that's neither here nor there if the ductwork is sized correctly.
You'll see the temperature rise listed on the tag on the blower door. Get accurate supply and return temperatures at the furnace. You'll likely see heat rise is out of range. Do you notice if the furnace makes limit during a heat cycle, where the burners shut off while the blower keeps running? After a few minutes the heat exchanger cools off and the burners fire again?
If it's natural gas, it looks like a typical 3.5" manifold gas valve. So that must be checked as mentioned. I must admit, I've never in my life seen a rear exit on a gas valve, as opposed to left and right in and out.
If it is in fact heat rise out of range, you might be able to increase the fan speed if it's a multi speed PSC motor, which it looks like it is.
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Not to be a tool but have you changed your furnaces air filter ? When confronted w a furnace that is tripping the limit i usually first things first is check the condition of the air filter either at a filter grill or a seperate media filter or worse the cheap cleanable which is in the blower section of the furnace . If you have recently replaced your filter w a higher merv filter try removing and reset the unit and see if it operates . You can also remove the blower compartment door and tap the safety switch so the unit will operate and have your thermostat call for heat . If the unit operates then your have return or blower issues . As for a blocked flue you would disconnect the vent piping and check the chimney for obstructions . I would gather that the inducer blower is running before it goes into lock out or is it firing up and the blower starts running and then locking out if so then it is air flow issue . Before going to nuts are all your supply air vents open and return open also ?
peace and good luck clammy
R.A. Calmbacher L.L.C. HVAC
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating1 -
have we determined which safety is tripping?
I'm betting on a bird / bee nest in the exhaust,
but who knows?
known to beat dead horses0 -
There you go @neilc, off topic again talking about the birds and the bees!
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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