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  1. #1
    Join Date
    Feb 2011
    Posts
    11

    Bryant 90% Furnace

    Model # 340mav036080ahka
    Serial # 1303A12514

    2.5 ton AC compressor

    Furnace tripped on high limit Code 33 in Dec. has ran fine for 8 yrs.

    Changed High Limit-Same issue

    Return Air: CFM around 1150 but probably not real accurate measurement
    Just for fun cut in a register grill in duct board to give more return air SAME ISSUES.

    SO, when furnace trips on limit, if I change filter I have no issue for 7-10days.
    High Limit will trip, change filter NO ISSUES FOR 7-9 days

    NOTE: It appears when honeywell Chronatherm Prg.Stat calls for a 2* rise from 70-72 is when it is tripping on limit as the stat shows "Recovery" I don't think this is an issue as the stat is just calling for heat to reach set point by "X" time the furnace I don't believe has a "high fire" which would call for more BTU's

    This has been going on for 2 mos now. I am using a Flanders pleated filter 16"x25"x1" after removing a 3M tight pleated filter.

    Temp Rise is a problem 40-70* running on the 80* side but again if I put a new filter in no problems until the filter gets a little grey/white matter on filter.

    SIDE NOTE: I have a Honeywell bypass humidifier which appears to me is blowing a large amount of CFM from supply through the pad and into the return at high temps then mixes with 72* return air even with damper in "summer" which is closed not a tight seal

    I can't figure out why 1) if I remove filter temp rise drops dramatically. Clean/NEW filter no issues until 7 to 9 days change filter and all is fine.

    Any ideas why slight filter load would cause issue, the filter does not in any way look like it needs to be cleaned, kicking around the humidifier (new pad installed) but how would that cause filter issues?

    Thanks I am at the end of my rope and 2 service techs are just burning my good money by saying "change the filter" which to me every week is not an acceptable solution.

    I am in Indianapolis/Brownsburg IN.

    Thanks need suggestions...

  2. #2
    Join Date
    Dec 2001
    Location
    Ocean Pines, MD
    Posts
    6,984
    You need some airside diagnostics from the sound of it. Sounds like these techs aren't up to it.
    One of the members here is in your area, maybe he'll chime in.

  3. #3
    Join Date
    Jul 2010
    Location
    wisconsin
    Posts
    270
    Try a fiberglass filter instead of your fme. Insist that a good tech check static pressure, pull blower motor, check secondary heat exchanger, clean blower wheel,and verify proper gas pressure.open any and all registers in the house. I can't tell you how many customers of mine close all but a few of their registers,and then wonder why the furnace can't get rid of the heat.m

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,089
    This is the main limit tripping, not one you have to push to reset?

    Are you using the pleated filters? If so, I'd try tiger's suggestion to use a fiberglass. These pleated, especially if they get a little bit of dust on them, can get very restrictive. Actually they are very restrictive clean! Why now after 8 years? I can't answer that.

  5. #5
    Join Date
    Feb 2011
    Posts
    11
    Yes, it is tripping the High Limit Code 33, I say that it is tripping on limit because the fan keeps running, gas re-fires and runs out to temp. I have never had a hard lock out and once it reaches set point it will run all day at the set temp without a problem. The weird thing is I have only caught it tripping on limit when it drops the 24V power to the Stat. As well it appears that the only time this has tripped on limit is in "recovery" mode on the stat while bringing the temp. up 2* from 70-72* which makes no sense I know. Is it possible I am not tripping the limit switch? I don't know for sure as it is so hit or miss when this happens and by the time I see the Stat drop power it has cooled down and re-firing so don't have the meter on the switch.

    The static readings I have taken are a little jumpy but it appears with the register I cut into the return opening the static drops off to -.009 with the register closed it is around -.10 which isn't quit right but again same symptoms with limit 7-9 days filter barely dirty trips on limit with a 2* rise on stat in recovery mode. Clean filter it will run temp up 5* with no problems. Really weird I know.....As for putting in a fiberglass filter, I was a bit concerned as the filter housing/cardboard is pretty flemsy and it appears the blower is pulling pretty hard on the pleated filter, so I thought it might suck in the fiberglass style and these are the cheap filters Lowe's carries, if there are other filters out there that are bit more rigid box please let me know and I can purchase that type.

    I originally thought short on return air but velocity to cfm calc indicates while I may be on the edge I am getting 1150ish on return and a bit higher with the register grill that was cut in 5"x9" opening actual square size. But even after giving it more return I still tripped on limit and played the filter game over again with no issues. The thing I can't figure out is 8 yrs with 3M very tight pleat no issues, now $3 flanders pleated filter still having issues. Any thoughts on the humidifier being the problem although it hasn't been running at all due to 0-10* outside temps here so unless the pad is wicking media off the into the air stream and plugging filter I am getting lost as to why in 7 days I trip limit and change to new filter and it runs with no issues.

    I have checked the A coil it appears clean. The blower wheel does have some white residual looking stuff on it but certainly not filling U shaped blades so I haven't pulled it to clean. The secondary heat exchanger I have not checked and I have been told this is a very expensive job to clean or replace if that is the issue.

    Thanks, JN

  6. #6
    Join Date
    Feb 2011
    Posts
    11
    Another thought which doesn't make any sense since the furnace is throwing a code 33. Which is high limit, flame roll out, dirty filter/restriction. Could the the condustion blower not removing enough heat from the heat exchanger throw a 33 code. Just grasping at straws. The intake and exhaust are 2" pvc 1 90* ell coming out of furnace, vertical 4', 90* ell and horizontal 4' out wall and to a rolled 45* ell to vent to atmosphere. Same set up on the intake and exhaust, the manual from sizing perspective says this is sized correctly. Again, just grasping at straws here. The furnace is 85000 BTU's with a 250F/40F limit and gas pressure has been checked and we have been messing with that a bit to try and lower the temp. rise as well. Which throws another cog in the wheel that even with BTU's reduced and in the temp. rise range of 40-70 it still tripped or threw the code 33 which keeps pointing me back to a filter that is 7 days old........LOL driving me nuts.

  7. #7
    Join Date
    Jan 2007
    Posts
    51
    Maybe check for a plugged evaporator.

  8. #8
    Join Date
    Feb 2011
    Location
    Paige, TX
    Posts
    11
    I ran into a very similar situation a little while back. The furnace was flashing 33 and the t-stat was losing power intermittently. Luckily I was able to catch it while it was messed up. I was getting 24V to the control board from the transformer, but there was no 24V coming out of the board. I changed the board and have had no issues since.

  9. #9
    Join Date
    Feb 2011
    Posts
    11
    Any chance I have a bad blower motor? If so, how would you check to verify w/out just changing it for the heck of it. The reason I ask is, with no filter in furnace the temp. rise drops dramatically, put the pleated filter in and temp rise goes back up. It appears my problem is when a slightly dirt filter kicks a code 33 which I think is high limit. Change the filter and I have no problems for 7-9 days. Could the fan/blower motor be under strain and not moving enough air.

    Also, A coil looks clean, fan is not overly dirty at all.

  10. #10
    Join Date
    Feb 2011
    Posts
    11
    One other thought, I have not checked the secondary coil for cleanliness, I don't know if this furnace was part of the class action lawsuit with the coil material dropping off/rusting etc. I am being told to check carbon dioxide in inducer motor/box and if it is high bad coil. Could a plugged secondary coil cause temp rise issues and trip limit/ aka code 33? I just don't think I am short on return air, as I have temp cut the duct board to add more return but still getting random trip when filter gets slightly loaded after 7-9 days. Again change the filter I have no issues. Which is why I posted the other question on blower fan etc.

  11. #11
    Join Date
    Feb 2011
    Location
    Paige, TX
    Posts
    11
    What stuck out to me is that you said you caught it tripping when you lost 24v to t-stat. I'm pretty sure no matter what safety is tripped you won't loose power to t-stat. The loss of power stopped the fan which opened the high limit since it was unable to dissipate the heat that was left. If power loss to t-stat always occurs prior the 33 code it may still be the board. Just my 2 cents. Hope it helps.

  12. #12
    Join Date
    Dec 2010
    Location
    Central US
    Posts
    417
    If this is a 90+% i would guess it has 2 stages of heat. Their should be a low fire gas pressure setting and a high fire gas pressure setting(That explains while it is close to setpoint it runs fine (LOW Fire) and if its away from setpoint it uses high fire until closer to setpoint), that should be set per the factory literature or on nameplate. Its probably ok but needs to be confirmed. Have you confirmed? Also im with the other guys, A-coil must be clean, secondary heat exchanger coil must be clean, as well as blower wheel.

    My personal opinion on the pleated filters are remove it a put a fiberglass filter in and change more often. If circumstances require better air quality, allergies etc.. Install an electronic air cleaner. The pleated filters create way to much static pressure drop on a furnace especially on ductwork that may or may not be sized for the static drop.

  13. #13
    Join Date
    Feb 2011
    Posts
    11
    A coil is clean....

    Secondary HX haven't checked.

    Fan looks like fine.

    The 24V power dropping out when it hits limit has me intrigued. I do not think the fan stops running when the stats goes blank. Need to watch that. Or look at wiring.

    As for control board not sure how you check if that is bad.

    I am trying to avoid the fiberglass filters as they don't really stop anything dust wise. But if that is the right answer so be it. JUST DON'T UNDERSTAND WHY AFTER 8 YRS OF RUNNING 3M 1500 PLEATED FILTER I AM HAVING ISSUES.

    Need to check gas pressures.

    Any chance Fan is bad? How would you check this?

    Remember I don't think I am short on return air as I have cut a 10" x 5" register in my return to add some air on a temp. basis.

    THE ONE INTERESTING THING IS STATIC PRESSURE ON THE RETURN SIDE DROPPED DRAMATICALLY FROM -.15 TO -.009 WITH A CLEAN/NEW FILTER.

    ALSO, PULLING FILTER COMPLETELY OUT OF SYSTEM MY TEMP RISE DROPS DRAMATICALLY AS WELL WHICH TELLS ME I AM GETTING ENOUGH RETURN AIR AND THE FILTER IS THE ISSUE ONCE IT IS IN THE SYSTEM FOR A WEEK WHICH MAKES NO SENSE THE FILTER LOOKS CLEAN.

    SO I DON'T KNOW HAVING TECH OUT ON TUES. BUT NOT SURE WHAT TO TELL THEM TO LOOK AT AS THE SYSTEM IS WORKING FINE AS WE SPEAK.

    CHECK AIR FLOWS I WOULD GUESS, TEMP RISE. GAS PRESSURE. LOOK AT SECONDARY COIL, FAN WHEEL FOR CLEANLINESS. ANY OTHER IDEAS?

    CONTROL BOARD BEING BAD NOT SURE HOW TO CHECK OR SIMULATE THE CODE 33

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