# Need help diagnosing a carrier code 33 limit switch issue



## kevang (Nov 11, 2016)

Hello, I have a carrier furnance 58sta070 and it is throwing error code 13 then throwing error code 33 after being on for three minutes. At that time it turns off the burners and just runs the blower. The manual says this is the limit switch, so I bought an replaced that. Others online said it was my Nest thermostat so I replaced my nest with an older battery powered thermostat that didn't fix the issue either. I also freed up all the vents and removed my filter. 

With the new limit switch whenever I put my voltmeter to the switch it always has 0 ohms and is passing current. I took the board off slightly and found quite a bit of rust on the bottom 1/10 of the board but otherwise it seemed old yet decent. 

Do you guys have any clue what the problem may be? I think I would be able to replace the board if I tried, but I really do not want to bring someone in for thousands of dollars if I can help it. Thank you very much!


----------



## user_12345a (Nov 23, 2014)

You have to check the resistance of the limit when the furnace stops working, then figure why it's tripping.

with a limit trip, the fan will continue to run but the burners and inducer will shut off.

You can't check ohms with the limit switch connected btw - it will backfeed through the board. You can check voltage drop when the furnace is operating though, it will be 24v +/- with the switch open and near zero when closed.

if the limit is tripping there's a reason. Check the filter, blower wheel for dust buildup. open all vents. got a/c? could be a plugged coil. the secondary heat exchanger on carrier furnaces doesn't have fins so dust passes right through. furnace could also be over-fired.



The most reliable source of info will come from the schematic on the power access panel, as there can be several revisions of one furnace over a 10+ year span.


----------



## beenthere (Oct 11, 2008)

Measure the return and supply air temp.


----------



## kevang (Nov 11, 2016)

Thank you guys very much! Tried a lot of these things, with no luck. About to check/clean the evaporator coil then I honestly am out of ideas.


----------



## user_12345a (Nov 23, 2014)

have u verified that it's the limit which is tripping?

is the blower wheel clean?

have u done a temperature rise measurement?

i also suggest that u clock the gas meter. ask google how.


----------



## supers05 (May 23, 2015)

If you've done the temp rise, we'd like to know what it is. Along with what you're furnace range specifies. (its on the name plate. Post a picture of it if you can) 

Cheers!


----------



## kevang (Nov 11, 2016)

Just did the check on the temperature rise and the panel says it should be between 30-60 F and it was 68 to 140f so pretty far above that. About to try and do a clocking of the amount of gas used. 

The blower wheel is very clean and the I can not verify that the limit switch is tripping or which is. About to try each individual switch but it seems to be overheating on the burn to me? 

Thanks so much for the help!


----------



## beenthere (Oct 11, 2008)

Your furnace should move a min of 1018CFM(60 degree temp rise), but by your posted temp rise, its only moving 848 CFM. Sounds like the high limit is tripping, as it should.


----------



## kevang (Nov 11, 2016)

Just clocked the burn rate at roughly 67000 btu and it is supposed to be 66000. So I am burning too cold or warm?

About to try and check the other side of the evaporator coils to see if it is clogged. Any thoughts on how to fix my issue? All vents are open.


----------



## kevang (Nov 11, 2016)

By camera and mirror the evaporator coils look very clean. No easily detectable blockages. 

Moving on to try and find any blockage in any of the systems. Gonna check the exhaust and return systems first.


----------



## user_12345a (Nov 23, 2014)

The firing rate is correct.

Has it done this since day 1 or when did the problem develop?

Are all the vents open? Are the returns clear?

What kind of filter are you using? size and type? avoid anything labelled allergen reduction or above merv 8. 

Check the temp rise with no filter.

If you can't identify an obvious cause, raise the fan speed.

check the schematic and wiring to see what speed is wired for heating and cooling.

The heating speed can be raised to second highest speed in most cases; if you have a smaller a/c like 1.5 or 2 tons the heating speed should be higher than the cooling speed in your case.

Really, you want a temp rise of 50f ideally but it will be difficult to attain with undersized ducts. Each speed tap increase may only drop the supply temp by 2-3f and the law of diminishing returns applies. the static pressure increases exponentially as airflow increases.


----------



## user_12345a (Nov 23, 2014)

1000 btu/hr with well within the margin of error for meter clocking. the heat content of the gas can vary slightly day to day, timing it isn't 100% accurate either.

meant to say that you may be able to use high speed for heat. furnaces come wired from the factory for high speed a/c, in your case that would only be for a 2.5 or 3 ton unit.


----------



## user_12345a (Nov 23, 2014)

edit:

can you give us the full model number?

can u also give us the a/c model number to tell what size it is.

i just pulled up the specs and carrier has a 58sta which has a 800cfm blower and a 40 to 70f temp rise. for the others reading this, it's a mid efficiency furnace. the cfm doesn't have to be all that high, it only has a roughly 52000 btu/hr output.

the cfm numbers beenthere posted don't like up with that.

I certainly hope you don't have this version of the furnace; better to have the extra fan capacity.

if u do the temp rise isn't out of line. it's still a little high.


----------



## beenthere (Oct 11, 2008)

kevang said:


> By camera and mirror the evaporator coils look very clean. No easily detectable blockages.
> 
> Moving on to try and find any blockage in any of the systems. Gonna check the exhaust and return systems first.


So you actually were able to see teh bottom of the coil, the part of it toward the furnace?


----------



## kevang (Nov 11, 2016)

To beenthere yes i was able to see the coils. In other news I seem to have now blown a fuse and I am working on fixing that. Was measuring one of the limits and blew the secondary voltage fuse.

Here are the pictures of the information for both. I really am starting to not like this furnance.









Also just a link to the picture: http://imgur.com/a/niSkC


----------



## user_12345a (Nov 23, 2014)

okay, yah so you have the 3 ton drive furnace.

would need the model of the outdoor unit to see the size. the coil is 2.5 ton but may be matched to a 1.5 or 2 ton condenser, that's done to get a higher seer rating.

the reason for asking this is to see what kind of airflow the a/c needs. there's a way to run both on high speed, but no need to resort to that if the a/c only needs 800 cfm or so.

which colour wire is being used for the heating and cool taps? the schematic will tell u which speeds they correspond to.

has the furnace always had this problem?


----------



## kevang (Nov 11, 2016)

The furnance has always been a little weird, but never fully not working. blue wire is attached to the heat, and black to the cool side. 

Fuse is back in so back to checking everything possible.


----------



## user_12345a (Nov 23, 2014)

You need to check the schematic and see which speed is blue.

manual says it's medium and it's a 3 speed motor but it may depend on the reivison..

i suggest temporarily connecting the black to the heat tap and re-checking the temperature rise. if this is a 4 speed motor try yellow first, which is medium-high. the thermometer in the supply duct can't be in direct line with the heat exchanger or u will get an artificially high reading.

If it stops tripping you can jumper the heating and cooling connections on the board and wire black to that. 

Need wire, spade connectors, crimper and a space connector splitter.

I very much suspect that you have a plugged indoor coil (did u check the under-side?), a restrictive filter and/or undersized ducts.


Sounds like the limit has been tripping since day one.

In that case the heat exchanger is suspect and the blower should be pulled even if you get the furnace running right. the heat exchanger should be inspected for cracks and popped crimp rings - constant overheating will cause damage.

The furnace isn't the problem, it's what's attached to it that is.


----------



## kevang (Nov 11, 2016)

Thank you guys so much for the help, I have tried everything up and down the sun to fix this issue myself. After 16 hours on it I think i will have to call in an expert. 

Tried removing the filter and changing the speeds. Is there any chance my nest messed up the circuit board? Or that it just went bad after 12 years? 

Thanks


----------



## user_12345a (Nov 23, 2014)

If the limit switch is opening for real, the board is good.

i suggested checking the voltage drop across the limit when it trips.

if it's tripping the board is fine.

you can rule out the thermostat by jumping r and w at the board. disconnect w.

limit trips in your case can be attributed to low airflow. low airflow caused by undersized ducts, filter, plugged coil, etc.

If you proceed in a logical way you can rule out everything in the furnace itself without the expert.

Based on everything you've said, like i said before this unit has probably had low airflow since day 1.

The nest thermostat you put in may be cycling the furnace on for longer periods of time, so the problem may have been there all along but hidden. some electronic t-stats cycle 6+ times per hour so the furnace doesn't have time to overheat a few times and lock out.

i do wonder what kind of space this is heating and how many vents there are.


----------



## carmon (May 8, 2010)

T.D is whats important here...


----------



## supers05 (May 23, 2015)

Check all the vents in the house. Make sure that they are ALL open. 

No couches against any return vents, no cardboard blocking of a vent somewhere, etc. 

Cheers!


----------

