# Help- stumped! Control board, limit switch, or something else??



## RMSTU (Dec 3, 2018)

I have Carrier furnace model: 58STA090---14116. It’s about 7 yrs old and never was registered, so out if warranty. I’ve been working on this for about a month. A month and a half ago my furnace went out in the middle of the night a couple different cold nights. The blower continued to work. The furnace gave me a code 13 (Limit Circuit Lockout). Then for about two weeks the furnace quit working/heating in the middle of the night, every night. When I reset it in the morning it worked fine and sometimes after about three hours it resets itself. Based on the error code I have checked the vents and air returns. All are clean, clear and open. 2” filter is clean and new- I change them regularly. The error code and online information, then, made me think to look at the limit switches. I ran continuity tests and all switches were fine. Checked ohms and all was good there, too. I cleaned the flame sensor and burners, just in case. 

The more days passed and I tried things, the more frequently, it stopped working and threw code 13. There was an infrequent code 33 (related to 13), that I saw and twice through it all, I saw a 14 and 34. Every time I flip the power switch off and back on, the furnace resets itself and works for some time.

Finally had someone come in and look at it. He found the same things I did and measured the various temperatures. Nothing irregular about the temperatures. He bypassed the high limit switch to see if there was anything weird w the switch. Code 13 in the middle of the night. Next day he bypassed the switch on a vent going out. That worked for 4 days before before 2 days of throwing code 13 again. Then it worked for 2 more days before the repairman felt he had to remove the bypass for his liability reasons. The only other thing that the repairman can think of is that there is something funky going on with the control board. The problem is, he can’t be sure and I’m supposed to spend $400 plus his time on a guess. One last ditch effort, I bypassed both of the switches that had been pie past previously, but I bypass them at the same time. I had no issues for nine days when I decided to replace the limit switches. Within hours after replacing the limit switches I had sales and code 13 again. 

What does anyone think? Could the limit switches be sending some kind of data/signals that the board is interpreting it incorrectly? I’d like some other advice before I decide to spend the money on a new control board. Thanks!


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## EdInKentucky (Dec 3, 2018)

I would replace the limit switches first.
They're not expensive.
I think Carrier boards are not cheap, even wholesale.
Could be an online source of a compatible board, not sure though.
If the switches don't solve the problem, my guess is that the board has just gone a little crazy, or has been partially cooked by a powerline surge, or 2, or 50.
Always have a surge protector installed onto any furnace newer than 1990.
So many delicate electronics.
About the warranty, unless it has a manufacturer's warranty for more than 7 years, it's over.
If the Carrier warranty is for maybe 10 years or more, the serial number will probably get you coverage.
I don't think that registration matters much.


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## RMSTU (Dec 7, 2018)

EdInKentucky said:


> I would replace the limit switches first.
> They're not expensive.
> I think Carrier boards are not cheap, even wholesale.
> Could be an online source of a compatible board, not sure though.
> ...



I replaced both the high limit switch (model HH18HA502) and the draft safeguard switch (model HH18HA493). Failed within two hours and 6 times in the next two days. I bypassed them both again and everything seems to work.


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## RMSTU (Dec 7, 2018)

I had a friend recommend checking the temps overnight. I hooked up thermocouplers (engineers had some at work) to measure the max temps last night at the High Limit Switch and the Draft Safeguard Switch/vent switch. The High Limit Switch (Model HH18HA502) maxed out at 129.9 F. The Draft Safeguard Switch (Model HH18HA493) maxed out at 244.5 F. Looking specs up online, the High Limit doesn't seem to be an issue. The Draft Safeguard Switch, though, is supposed to open at 200 F and close at 101 F. (Just to check- does that sound correct?) I assume there's room for a +/- 200F, but 45 F over??? That seems WAY to high- am I right?

What do I check next? Vents seemed ok, but do I need to look inside anything? What and how?


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## RMSTU (Dec 7, 2018)

Don't know if it helps to know, but the furnace is located in an open, unfinished basement of about 900 ft2. If the furnace is running the pressure/suction in the basement makes the basement door more difficult to pull open and will suck/slam shut on it's own if it's left slightly open due to suction pressure from the basement.


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## kok328 (Dec 7, 2018)

RMSTU said:


> Don't know if it helps to know, but the furnace is located in an open, unfinished basement of about 900 ft2. If the furnace is running the pressure/suction in the basement makes the basement door more difficult to pull open and will suck/slam shut on it's own if it's left slightly open due to suction pressure from the basement.



Well that's not good.  What happens if you prop the door open so the furnace can breath?


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## WyrTwister (Dec 8, 2018)

The furnace does not have fresh / outside air coming to it ?

     Is it a condensing furnace ( PVC vent ) or " traditional furnace ( metal vent ) .

     Condensing furnaces seem to have more safties .

God bless
Wyr


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## RMSTU (Dec 8, 2018)

It’s a traditional furnace. There’s an open, unblocked vent that brings outside air in from the outside right behind the furnace.


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## RMSTU (Dec 8, 2018)

I assume there’s some give on a 200 F limit switch (that they don’t all open at exactly 200).  What might be the +/- on a 200 F limit switch?


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## RMSTU (Dec 8, 2018)

I got of the roof to check. The chimney/exhaust vent is completely clear, but it’s still overheating at the Draft Safeguard Switch- up to 245 F on the 200 F limit switch. Any chance there could be some issue w the inducer motor? Maybe not running fast enough to blow the heat out of there??


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## WyrTwister (Dec 8, 2018)

If you have an outside air vent , I am wondering why you are having a suction trying to affect the door .

     OK , you looked and the vent path is clear .

     Does it have a combustion air fan ?  Did someone mention that .  I thought only condensing furnaces had combustion air fans ?  Confused .   :-(

     Is there a way to throttle back the gas flow ?  Or increase the draft or combustion air flow ?

God bless
Wyr


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## EdInKentucky (Dec 10, 2018)

A medium-efficiency (around 80%) furnace also has an inducer motor/blower (combustion air fan), although those furnaces are not condensing furnaces.

Normally, the flue exhaust gases will be warmer than a condensing furnace ... about 220 degrees F.
This is enough to trip the temperature sensors.
It should be venting out through a 3" or 4" metal flue pipe, probably double-wall.

There is a LOT of suction (negative pressure) pulling the basement door shut.
With that much negative pressure, there should be a strong flow of air coming in through that unblocked vent, maybe even enough to make a whistling sound.

I cannot stress enough how serious this negative pressure situation is.
SOMETHING is sucking a lot of air out of that space.

Even if that unblocked vent is flowing well, this much negative pressure can overwhelm the draft inducer and pull combustion exhaust back into the burner area.

Even at full flow adjustment, an inducer doesn't flow much.

If the flames are not burning efficiently (likely if the inducer is being overwhelmed), some carbon monoxide may be pulled into the basement.

It's possible that the furnace's return duct is pulling a lot of air out of the basement, and that air isn't being replaced (made up) by a supply duct.
This could happen if there's an air intake register in the return duct, or if the return duct is just seriously leaky.

Has the supply ductwork been changed or otherwise worked on recently ?
And is there anything else sucking air out of the space where the furnace is ?


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## WyrTwister (Dec 10, 2018)

Like an exhaust fan ?

God bless
Wyr


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## kok328 (Dec 10, 2018)

RMSTU said:


> I got of the roof to check. The chimney/exhaust vent is completely clear, but it’s still overheating at the Draft Safeguard Switch- up to 245 F on the 200 F limit switch. Any chance there could be some issue w the inducer motor? Maybe not running fast enough to blow the heat out of there??



I have seen one instance where the inducer motor would run but, not fast enough to "make" the pressure switch.
Maybe some return air registers are blocked?
Maybe looking at a cracked heat exchanger?


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## WyrTwister (Dec 11, 2018)

I have had the air pressure switch / sensor go bad , a LONG time ago .  Picked up a replacement at Johnstone Supply .  Then replaced it .  

     Replaced a Hot Surface igniter about 3 weeks ago .  

     Best of luck too you ,   

Wyr
God bless


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## RMSTU (Dec 16, 2018)

Replaced the inducer motor assembly today. Failed 3 hrs later w another code 13 (limit switch). The  200° vent limit switch up to 230.1°. 10 min after a restart of the furnace and trying to get back to temp- from 63 to 70, the inducer motor is EXTREMELY hot to the touch. So is the exhaust vent elbow. (4” exhaust vent and the elbow, to which the vent limit switch is attached, looks probably double walled.)

All the home vents are open and the ductwork hasn’t been worked on, so not sure it could be that. The negative pressure doesn’t make sense- don’t know if it’s been there, but I’ve had no issues w the furnace for 7 years and the vent to the outside hasn’t changed, in fact I cut some plants away 2 months ago. I had a HVAC guy tell me the negative pressure is a completely separate issue. 

I hope it’s not a cracked exchange like kok328 mentioned. But, heck- if it is, I have the hole furnace replaced and that’ll take care of the issue! I’m so done w this frustration.


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## RMSTU (Dec 16, 2018)

EdInKentucky said:


> A medium-efficiency (around 80%) furnace also has an inducer motor/blower (combustion air fan), although those furnaces are not condensing furnaces.
> 
> Normally, the flue exhaust gases will be warmer than a condensing furnace ... about 220 degrees F.
> This is enough to trip the temperature sensors.
> ...




All the home vents are open and the ductwork hasn’t been worked on, so not sure it could be that. The negative pressure doesn’t make sense- don’t know if it’s been there, but I’ve had no issues w the furnace for 7 years and the vent to the outside hasn’t changed, in fact I cut some plants away 2 months ago. I had a HVAC guy tell me the negative pressure is a completely separate issue.


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## RMSTU (Dec 17, 2018)

Thank you everybody for all your input! Here's where I'm at today.

New inducer motor didn't due the trick, so I increased airflow in the basement by opening a window about 3". 2.5 days now with no issue. The temp at the exhaust draft (vent) switch only gets to 136 F. It had been opening and hitting temps of 230-240 F. I still notice some negative pressure when the blower is on as the upstairs door to my basement now starts pulling shut around 3" open, but does not slam or completely close the door as it did (w closed window) from 6" open.

Their is a hole (is it called a combustion air vent) above the furnace (but behind the exhaust vent) that is covered w small mesh and grate on the outside of the house. This hole is on the same wall as the furnace, directly behind the exhaust vent of the furnace. No piping from the hole- just open from the outside. The basement is competely open (no walls) with a little bit of racking for some storage around the outer walls. This hasn't changed in 7 years. For the last 6 winters the hole behind the exhaust vent has been supplying sufficient air to keep the furnace running without issue. I can't think of a thing that has changed.

So... extra airflow keeps things running smooth. Air flow from outside (w/ window CLOSED) hasn't changed in 7 years. Could there be something wrong in the furnace? Where else could I look and what could be causing this?


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## WyrTwister (Dec 18, 2018)

Increase the size of the fresh air passage ?

Wyr
God bless


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## billshack (Dec 18, 2018)

I had one case where a bird had built a bird nest in the fresh air intake . go and check .


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## EdInKentucky (Dec 18, 2018)

The furnace working ok for several days after you opened the basement window 3" is good information.

But .. sounds like there is still negative pressure in the basement, even with that window open 3".
2 possibilities come to my mind ..
1. the return duct in the basement, is somehow pulling a lot of air out of the basement.
2. there is a "chimney effect" flowing air between the basement and the attic, possibly through walls

And it is clear that something changed, to cause this negative pressure in the basement, since the furnace did fine for 7 years.

Possible negative pressure in the space where the furnace is ... is the main reason why furnaces are often installed as "closed combustion" ... using 2 pipes to the outside, one to supply combustion air, the other to conduct exhaust gases to the outside, and the combustion area being sealed.

If this problem persists, it's possible your furnace can be converted to "closed combustion" without a lot of expense.

If that's not cheap and easy to do, you might consider a new high-efficiency furnace.
Your existing furnace is probably 80% efficient max, since it's a non-condensing type.
And it has 7 years of use on it ... not a lot, but it's not new.

A new condensing furnace will increase efficiency to the high 90's, noticeably reducing your gas bill.
Get a good USA made, known-brand furnace, avoid the exotic brands with lots of proprietary parts.
Consider Trane, Carrier, etc.

A new two-pipe furnace, properly installed, won't be affected by strong negative pressures in the basement.
It might even qualify for a tax credit.


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## WyrTwister (Dec 18, 2018)

These all sound like reasonable ideas .

God bless
Wyr


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## RMSTU (Dec 24, 2018)

Thanks, EdInKentucky. Those are ideas worth considering. I’ve got a request in (waiting for a return call) for a complete Home Energy Audit from the gas company. Maybe they can track down the source of the negative air pressure. 

I’ve also done a temp rise test and it seems to be within the manufacturer’s range.  An HVAC guy told me to check that during a phone conversation. He was completely stumped over the phone. 

What about the possibility of a cracked heat exchanger? Any chance that may be causing the issue? If there’s a chance, how do I check for it?

Is there a chance that  the vents or the combustion hole or a combination could have been slightly undersized from the beginning (7 yrs ago), and after 7 years, it’s staring to show?


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## WyrTwister (Dec 24, 2018)

I would , in the mean time , plug in a carbon monoxide detector , near the furnace .

     I would really look into the present outside air / combustion air situation .

     Also , while you are waiting on the gas company to do a report , you might be pricing a furnace change out , as a worst case scenario .  We installed a 90% or 92% , two stage Goodman gas furnace , maybe 20 years ago & have been happy with it .

     To get enough CFM of air movement for 4 ton of A/C , the furnace is oversize .  That is no problem .  with the 2 stage tstat , it never has to go into 2nd stage .  Unless we turn the tstat way down when we go out of town .  Upon return , move the tstat up to " normal " and it kicks into high flame and REALLY puts out the heat .

     It is a condensing furnace and the venting is PVC , easy to work with and inexpensive .  But it has to have a drain for both the A-Coil and the furnace condensation .

Best of luck to you and Merry Christmas .

God bless
Wyr


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## EdInKentucky (Dec 28, 2018)

I agree, definitely install a carbon monoxide detector in the basement near the furnace.

And ... since the temp rise is right, a cracked heat exchanger is unlikely ... although always possible.

But ... a cracked heat exchanger would not explain the negative pressure in the basement.

That's the problem to solve.

If the gas company does an energy audit using the "blower door" method, that might be able to quickly find the source of the negative pressure.

Really though, with gas prices being high now, and probably for quite a while .... you should seriously consider a 90+ efficiency replacement furnace, with closed combustion.

One that has a 2-stage burner and variable blower speed .. you will like it.


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## timnuroomremodeling (Jan 27, 2019)

Realize this is an older post, but I had a similar code thrown on a Bryant system. Solved the issue by cleaning the flame sensor probe with #0000 steel wool. I'm pretty sure that's one of the code 1-4 solutions. Simple cheap maintenance fix that's worth trying.


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## pjones (Feb 4, 2019)

Have you recently rearranged furniture, or moved items, possibly blocking a return air grill somewhere? 

Board is a possible cause. You could pull it out and check for damage. You can’t always see the damage but often you can see indications of the issues so it’s worth a look. 

I’m not on site to see the unit myself but from what you describe it leads me to disagree with your HVAC technician, I believe the negative pressure is an issue worth resolving and may be related to this issue that you are having. Sometimes it will work ok for years then one small thing changes and escalates an issue just enough to make it start causing problems. 

Make sure that combustion air grill is clean and unblocked. With the negative pressure you describe you should feel a strong draft coming through it. Your furnace should not be in a negative pressure room for many reasons. There is usually bug screen installed in these that gets plugged up. 

Get a CO monitor installed now if you don’t have one installed yet. Don’t wait to do this. In your situation this needs to be done.


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## pjones (Feb 4, 2019)

What are the other error code descriptions for the 14,33,34 that you mentioned? (Did I get those numbers right? I’m going off memory here)


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## pjones (Feb 4, 2019)

pjones said:


> What are the other error code descriptions for the 14,33,34 that you mentioned? (Did I get those numbers right? I’m going off memory here)



Searched them... also check that the pressure switch port is clear and not obstructed.


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