Odd Coincidence with Carrier Furnace Code 33

[Qs Laptop]

I want to run this past the brain trust here on ER.ORG. The breadth of knowledge here is a real resource.

I reprogrammed my thermostat last night to have the heat taper off 50 minutes sooner than usual and also set the temperature one degree lower, down to 64 degrees for night time sleeping. We're not staying up much past 11:00 o'clock anymore so there was no need to have the heat setting at 72 degrees up until 10:30 at night anymore.

So this morning I wake up and I notice it's cold in the house. Immediately I'm thinking--is something wrong with the thermostat? After all, I tinkered with it the night before and who knows if it engaged the furnace overnight, as I had been sleeping. The thermostat is in the "Heat On" mode and the temperature readout is 64 degrees. I notice the blower motor on the furnace is running but the air coming out of the vents is not warm.

I look at the furnace diagnostic code light. Carrier Weathermaker 9300 furnace is throwing a code 33.

According to the sticker inside my furnace access door a code 33 means a limit circuit fault caused by:

Dirty and/or clogged air filter. No--I replaced my air filter on less than 3 weeks ago. Air filter is a larger Aprilaire filter and not the common 1" thick fiberglass strand type filter.

Blocked vents or air returns. No.

Loose blower wheel. No

Defective blower wheel. No

Inadequate combustion air supply. Flame Rollout switch is open. Possibility.

Excessive wind. Hmmm...

Another possibility was the limit switch was defective but that wasn't mentioned as being a part of code 33 so I didn't test it.

I reset the flame rollout switch by pressing a button on the backside of the switch. I turn the thermostat back on and observe the furnace operation. I get the flames to ignite, they are shooting directly into the tubes as normal and my furnace is running normally.

So what the heck happened? Was there high winds last night that prevented the intake pipe from drawing combustion air, thus tripping the flame rollout switch? I checked the weather and there might have been wind gusts to 25 mph last night, nothing that we haven't had before and nothing that has turned off the furnace before.

Intermittent flame rollout switch? Could be. I'll be monitoring the furnace over the next day or so.

The weird thing is that I adjusted the thermostat last night, the first time I've done that since setting the programming to "winter" mode back in November. I keep thinking that had to have something to do with it, but what?

 

[frank]

If you reset the flame rollout switch and everything is working right it might be the rollout switch is going bad. although you can reset them it might keep tripping. I would order another one online and have it around or just wait to see if it happens again. the high limit will do what you describe but I think if it goes out it would have to be replaced before the furnace would function properly again.

 

[Qs Laptop]

If you reset the flame rollout switch and everything is working right it might be the rollout switch is going bad. although you can reset them it might keep tripping. I would order another one online and have it around or just wait to see if it happens again. the high limit will do what you describe but I think if it goes out it would have to be replaced before the furnace would function properly again.

Good idea. I will order one today.

I suppose if the high limit switch was bad it would test as being open with an ohmmeter. Worst case scenario, I could run a jumper across the switch terminals to get heat until I replaced it. Or, just order one of those switches and have it on hand as well.

 

[misanman]

I've had a similar problem with our Bryant (Bryant and Carrier are sister companies). In my case at least, it will reset by itself in three hours. If I power cycle the furnace it also resets. Happens only on rare occasions and the service people haven't been able to find the cause.

 

[JoeWras]

I had that happen early on with my previous furnace. It was because I had the garage door open (furnace in garage) and the wind indeed caused a detectable rollout.

Modern furnaces with inducer motors shouldn't get a roll-out from downdraft in the flu. So unless you can figure out why you have air flow near your cabinet, then you need to monitor this or get a new roll out switch. You don't want to ignore it if a true rollout is happening!

How's your inducer motor? Working? Wind can fight the induced draft and cause a roll out.

 

[Qs Laptop]

I had that happen early on with my previous furnace. It was because I had the garage door open (furnace in garage) and the wind indeed caused a detectable rollout.

Modern furnaces with inducer motors shouldn't get a roll-out from downdraft in the flu.

Yes, the furnace is 19 years old and has a forced air exhaust that exits the side of the house and not a flue that goes up through the roof.

So unless you can figure out why you have air flow near your cabinet, then you need to monitor this or get a new roll out switch. You don't want to ignore it if a true rollout is happening!

There isn't any air flow near the furnace. The furnace is in a larger closet ("utility room") that has the door closed 24/7.

How's your inducer motor? Working? Wind can fight the induced draft and cause a roll out.

Yep, inducer motor is working as intended. It spins up and runs 30-45 seconds prior to the ignitor lighting up the burners.

So does everybody that's chimed in on this thread believe it was just an odd coincidence that the same night I fiddled with the programming on the thermostat, the furnace died?

 

[JoeWras]

Yes. Coincidence. Hard to believe a change in time would cause an error code like that. Now if you turned it up to 85 and the furnace was running all the time, it might be different. But what you did was a 1 degree change and actually less time running, so less stress on the furnace.

 

[Sunset]

Seems normal to me, you touch something to adjust it, and soon it breaks

 

[ncbill]

Better than the usual cause for tripping the flame rollout sensor...a clogged or cracked heat exchanger.

 

[Qs Laptop]

Deja Vu. Same thing happened this morning. Furnace ran fine throughout the entire day, cycling on and off as normal. Went to bed last night, woke up today and blower motor is running, house is cold, and code 33 is present again.

Turn thermostat to Off. Turn power to furnace to off. Reset the flame rollout switch. Turn power to furnace to on. Turn thermostat to heat. Furnace comes on. Observe ignition sequence. Blue flames going directly into the burner area(s).

This time it runs for about 2 hours before I notice the sound of it running has changed. (My home office is close to the utility room.) Blower motor is on but no heat coming out of the air vents. Code 33.

Go buy the flame rollout switch or call a HVAC service company?

 

[JoeWras]

Uh oh.

Are you confident enough to check your heat exchanger? Before just buying a switch, you need to do this. Or call the HVAC people.

Actually, before you do anything, make sure you have a working CO (carbon monoxide) detector. Now.

The reason a cracked heat exchanger can cause rollout is by the cool air in the building putting downward backpressure on the system, getting through the cracks, and pushing against the flame.

The other side of a cracked heat exchanger is combustion products getting into the vents into the building.

To check, boroscopes can help. The simple way is to take off a panel, put a bright light against the exchanger, and look in the fire tubes for that light.

 

[Qs Laptop]

Uh oh.

Are you confident enough to check your heat exchanger? Before just buying a switch, you need to do this. Or call the HVAC people.

Actually, before you do anything, make sure you have a working CO (carbon monoxide) detector. Now.

The reason a cracked heat exchanger can cause rollout is by the cool air in the building putting downward backpressure on the system, getting through the cracks, and pushing against the flame.

The other side of a cracked heat exchanger is combustion products getting into the vents into the building.

To check, boroscopes can help. The simple way is to take off a panel, put a bright light against the exchanger, and look in the fire tubes for that light.

Not sure how to check the heat exchanger.

FYI, the burners were replaced about 8-9 years ago.

There is a fairly new CO detector mounted on the ceiling immediately outside the utility room. It's got new batteries and I tested it yesterday. There is one in the hallway outside our bedroom on the 2nd floor that I've tested.

I've watched the burner ignition sequence and that looks good to me.

My fear is if I call for service they might tell me I need a new furnace, when in fact, it could simply be the flame rollout switch.

 

[Qs Laptop]

After the flame rollout switch interrupted the furnace after running 2 hours I noticed it took a while before I could reset it and restart the furnace. I suppose it was 25-30 minutes in total. I tried it right after the furnace quit and it wouldn't reset. Pressing the reset button did nothing. No resistance to pressing the button, no click. Thinking it needed to cool down before it could be reset I tried it again 10 minutes later. No go.

When I tried it another 15-20 minutes later the button offered some resistance and gave a telltale click. Now it was reset and I was able to restart the furnace, which is still running over an hour later.

Regarding the flame rollout switch--how long should it take before it is can be reset? I ask because if it should only take 5 minutes or so, but this one takes 30 minutes, perhaps that is an indication that it is faulty.

 

[JoeWras]

OK. I'm glad you have a CO detector in the sleeping area. A cracked exchanger could cause the CO to go into other parts of the house other than the utility room.

Second: you had the burners replaced. This unit has some time on it. Be prepared, it simply may be time. How old is this unit? More than 25 years?

If you know how to use a multimeter, check out this article.

https://hvactrainingshop.com/furnace-flame-rollout-switch/

If not, then it is time to call. Don't fear, ask the tech to show you why they think there is a problem that requires replacement. You are better off getting a small guy because a big company will always look at replacement for any unit older than 15 years. If it is a cracked heat exchanger, it is a very serious issue that can be dangerous. You need to find out.

 

[Sunset]

OP - If it's below freezing in your area, go outside and look at the exhaust of your furnace. It could be clogged with ice.

I know a person that was concerned about animals going into the pipe, so he put on a screen and it then plugged up with ice in the winter. The HVAC folks he called when the furnace stopped working removed the screen.

 

[freedomatlast]

Qs - I think you're on the right track. Since you know the flame rollout switch is tripping and you can't see any reason for it to do so, and your CO detectors aren't alarming, I'd certainly buy and install a new flame rollout switch. That's probably the first thing any HVAC company would do. Otherwise, calling in an HVAC company, is going to trigger the sales pitch and scare tactics to try to get you replace the 19 year old system. Really no need to replace the system unless the heat exchanger is cracked. Everything else is fairly easily replaced and economically feasible if you do it yourself.

 

[Qs Laptop]

OK. I'm glad you have a CO detector in the sleeping area. A cracked exchanger could cause the CO to go into other parts of the house other than the utility room.

Second: you had the burners replaced. This unit has some time on it. Be prepared, it simply may be time. How old is this unit? More than 25 years?

If you know how to use a multimeter, check out this article.

https://hvactrainingshop.com/furnace-flame-rollout-switch/

If not, then it is time to call. Don't fear, ask the tech to show you why they think there is a problem that requires replacement. You are better off getting a small guy because a big company will always look at replacement for any unit older than 15 years. If it is a cracked heat exchanger, it is a very serious issue that can be dangerous. You need to find out.

Furnace is 19 years old.

I do know how to use a multimeter. Testing the switch is exactly the sort of information I can use.

I'm going to wait until the furnace either gets the house up to thermostat temp, or the switch kicks in and shuts down the furnace. Then I will test the switch.

 

[Qs Laptop]

OP - If it's below freezing in your area, go outside and look at the exhaust of your furnace. It could be clogged with ice.

I know a person that was concerned about animals going into the pipe, so he put on a screen and it then plugged up with ice in the winter. The HVAC folks he called when the furnace stopped working removed the screen.

Checked the exhaust and it's flowing freely.

I just remembered when the burners were replaced years ago, the PVC pipe that brings in cold air was removed at the furnace and a dead, desiccated mouse slid out of there! I will check that PVC intake pipe.

 

[Qs Laptop]

Qs - I think you're on the right track. Since you know the flame rollout switch is tripping and you can't see any reason for it to do so, and your CO detectors aren't alarming, I'd certainly buy and install the flame rollout switch, That's probably the first thing any HVAC company would do. Otherwise, calling in an HVAC company, is going to trigger the sales pitch to replace the 19 year old system. Really no need to replace the system unless the heat exchanger is cracked. Everything else is fairly easily replaced and economically feasible if you do it yourself.

This seems reasonable to me. If a new flame rollout switch continues to trip, I will definitely call an HVAC pro.

 

[JoeWras]

If the switch isn't too expensive (most are less than $50), it may be worth you replacing it. Sounds like you can do it. Then move from there.

Of course, check your exhaust vents and be save with the CO detectors and all that at the same time.

 

[ncbill]

Short of a cracked or clogged heat exchanger a gas-fired furnace is worth repairing.

Heck, my OEM even sells a universal control board for all their gas furnaces 2005 & up.

 

[Qs Laptop]

[Edited to add: I checked the resistance of the flame rollout switch and it was 0.4 ohms.]

Before I installed the new switch I measured the resistance and it was 0 ohms.

Now that it is out of the furnace I checked the resistance of the old switch. It danced around between 0.4 ohms and up to 38 ohms, finally settling at 0.6 ohms.

I'm posting this pic because I know I will get raves for my antique VOM.

 

[freedomatlast]

So, did the new switch fix the problem?

 

[Qs Laptop]

So, did the new switch fix the problem?

So far, about 24 hours with no interruption in furnace operation. Looks like it did the trick.

 

[JoeWras]

And you got a nice tour of the city and you checked your CO detectors. All good, right?