Posted: 11/25/2015 2:49:12 AM EDT
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So I noticed the other day on my nest thermostat log that my upstairs furnace has only been running for a few minutes at a time. We've had fairly mild weather and we only run the upstairs unit at night so I hadn't noticed anything out of the ordinary. Well long story short I was seeing a message saying heating delayed, and then it would kick off for 3 minutes and the cycle would repeat. I swapped thermostats and that wasn't the issue, so I open it up and see an LED indicator light with a list of faults. I fire it up and sure enough I get a fault after it running about 4 minutes.
Code 33. LIMIT CIRCUIT FAULT - Indicates a limit, draft safeguard, flame rollout, or blocked vent switch (if used) is open. Blower will run for 4 minutes or until open switch remakes whichever is longer. If open longer than 3 minutes, code changes to lockout #13. If open less than 3 minutes status code #33 continues to flash until blower shuts off. Flame rollout switch and BVSS require manual reset. Check for: - Restricted vent - Proper vent sizing - Loose blower wheel. - Excessive wind - Dirty filter or restricted duct system. - Defective blower motor or capacitor. - Defective switch or connections. - Inadequate combustion air supply (Flame Roll-out Switch open). Now that all looks like foreign language to me. What am I in for here and how serious is this from a safety point of view? This is a Carrier furnace, mfg date 2009. Luckily this isn't the only house heater, this is an upstairs unit for 3 bedrooms and a bath only. Any suggestions? As of now I plan on calling someone out for a service visit. |
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Quoted:
Plugged filter? Brand new air filter and the one before that was a month old, it also looked brand new when replaced. There are 2 huge vents (returns?) in the ceiling upstairs, they are almost 1.5'x1.5' and a bunch of other vents above the doors which all appear fine, the registers are all in the ceiling for this upstairs unit. I believe the furnaces vent on the side of the house, there are 2 spouts that come out one goes up, one down and 2 larger ones facing down, all are clear on the outside. The only change is I covered both a/c units with tarps, but I don't think that matters for venting purposes. Otherwise everything else has stayed the same. |
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My wife left early from work and a tech came out and inspected the furnace. Said all was good, cleaned the flame sensor and cycled the furnace and it raised temps about 4-5 degrees before he deemed it repaired. I come home, turn it on and the same cycle repeats. Runs for 3-5 minutes normally, fault 33 occurs and it delays for 3 minutes then repeats the cycle.
I checked the limit switch with a multimeter and when running it read 0 and during fault it read 26v. That means it's working normally correct? It is an auto reset, so presumably the temp is failing below the threshold inside of 3 minutes otherwise it would lockout according to the fault sticker on the unit. I see 2 rollout switches and neither are tripped, those are manual reset so I would know if those were tripped correct? The filter is clean, the motor is just barely visible so I can't tell the condition of that and I have no clue how to access the condenser to see if that's dirty. So what could be causing the overheating? It worked fine last year, presumably worked for the previous owners in the 4 years prior to that, I'm at a loss and after having this guy out I don't have a ton of confidence. In 30 minutes of reading it seems I found more things to check easily then he did. He told my wife he was confident the unit was strong and in good repair, but I wasn't here to see what he did. What else should I try before Friday? I spoke to the tech on the phone and he said to let it run tomorrow, it isn't In a dangerous condition and he would come back out Friday. |
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Pull filter and see if it still codes, if it does, Check the mF rating on the blower motor cap and make sure it's 6%+- within it's rating. A bad cap can still allow the blower to run but at a reduced enough speed that it will limit out.
A allergy type filter can also restrict air flow and cause the limit to open unless one kicks the motor up to a higher speed on the heating cycle. |
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Quoted:
Pull filter and see if it still codes, if it does, Check the mF rating on the blower motor cap and make sure it's 6%+- within it's rating. A bad cap can still allow the blower to run but at a reduced enough speed that it will limit out. A allergy type filter can also restrict air flow and cause the limit to open unless one kicks the motor up to a higher speed on the heating cycle. Filter is a low grade, not very restrictive. I ran without a filter, ran it with the door open, it's all about the same, 3 minutes on the low end, 10 on the high end before it faults. That's the part I'm struggling to understand, that seems like small windows in which temps grow to a fault level and fall to a safe level in. Further, it seems by luck you would have at least bigger variation, when it's at 72 in the work area it fails in about the same time span. Is the furnace really that particular on these temp windows and accurate and consistent to boot? Not familiar with the mf test? Could you explain that further. Thanks. |
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Sounds like the limit switch is failing. 0v shows closed, 24v across shows switch is open. May be other issues as to why the switch is opening. You could replace and test if it opens a tech will be needed. This is the part that confuses me the most, what is the desirable reading for a functioning switch and what would one expect to see in a fault scenario? Wouldn't it run at 0 indefinitely assuming it was working and not at the temperature limit? By seeing the 24v during failure, doesn't that indicate the switch responded as it should? Now if it continued to fail at or about the same time, I could understand suspecting it be faulty, but in this scenario it could also be truly showing a fault and my problem is with a temperature danger? It seems like a super easy part to replace, but it's Wednesday night before thanksgiving, so no testing that theory until Friday, and I'm not sure I want to cancel my service call since I at least have it scheduled 
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Quoted:
Filter is a low grade, not very restrictive. I ran without a filter, ran it with the door open, it's all about the same, 3 minutes on the low end, 10 on the high end before it faults. That's the part I'm struggling to understand, that seems like small windows in which temps grow to a fault level and fall to a safe level in. Further, it seems by luck you would have at least bigger variation, when it's at 72 in the work area it fails in about the same time span. Is the furnace really that particular on these temp windows and accurate and consistent to boot? Not familiar with the mf test? Could you explain that further. Thanks. Quoted:
Quoted:
Pull filter and see if it still codes, if it does, Check the mF rating on the blower motor cap and make sure it's 6%+- within it's rating. A bad cap can still allow the blower to run but at a reduced enough speed that it will limit out. A allergy type filter can also restrict air flow and cause the limit to open unless one kicks the motor up to a higher speed on the heating cycle. Filter is a low grade, not very restrictive. I ran without a filter, ran it with the door open, it's all about the same, 3 minutes on the low end, 10 on the high end before it faults. That's the part I'm struggling to understand, that seems like small windows in which temps grow to a fault level and fall to a safe level in. Further, it seems by luck you would have at least bigger variation, when it's at 72 in the work area it fails in about the same time span. Is the furnace really that particular on these temp windows and accurate and consistent to boot? Not familiar with the mf test? Could you explain that further. Thanks. One would need a fluke meter with a micro farad setting. Kill power to unit, drain the cap by crossing the contacts, pull one lead off and read the cap, if the read is lower then 6% of the rating, it's bad. If it's bad the motor can run but at reduced speed which can cause the limit to open. 6 years is quite young for a furnace heat exchanger to have issues. What needs to be done is for someone to stand by the furnace and when it shuts off [burner off on limit], to find out exactly which limit opened. MOST roll outs are a manual reset, high temp limits are usually auto reset which means they will close when they cool down. Your roll outs look to be manual resets so they are likely not the issue. You can have a venting issue, this tends to also cause problems with the pressure switch not closing properly but I have seen pressure switches close when there is an obstruction in the exhaust vent. [birds and small rodents and insect nest seem to be the biggest culprits. |