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Old Bryant Furnace - Heating Cycle Question

Blower55
Blower55 Member Posts: 24
Hello all - you guys previously helped me solve a mystery with my AC unit, so I am returning to ask a question about my furnace. The unit is a nearly 40 year-old Bryant gas furnace, model #397HAW036080.

I thought it would be helpful to describe the sequence of operation once there is a call for heat from the t-stat:
  1. Inducer motor comes on
  2. Pilot igniter activates
  3. Pilot comes on
  4. Burners come on
  5. Fan motor comes on
  6. Burners go off before temperature setting is reached, but inducer motor and blower stay on
  7. The igniter activates again, pilot comes on, and burners come on for anywhere from 3 to 30 seconds, then burners shut off while blower and inducer remain on. Sometimes the igniter clicks for quite a while before pilot ignites (maybe 30-45 seconds?)
  8. The cycle in #7 repeats a number of times until t-stat temperature is reached, then unit shuts down normally

    I checked the parts diagram and looked around the burners and I am reasonably certain this furnace does not have a flame sensor, so I am wondering what the most likely causes of this issue might be. There are times when the furnace cycles normally, i.e, call for heat, normal cycle occurs, unit shuts down when t-stat temp reached, so I am not sure why the odd cycle in #7 would occur intermittently. One last thing is that the odd cycle tends to occur more if I need to raise the temperature by a few degrees at one time.

    Thanks for any guidance you are able to provide.

Comments

  • bburd
    bburd Member Posts: 576
    edited October 2022
    This might be due to restricted airflow through the unit, causing the high limit temperature switch to shut down the burner while the fans continue to run.

    Is the air filter clean? Return air grilles unobstructed? All supply dampers open? Blower working normally?

    Bburd
  • Blower55
    Blower55 Member Posts: 24
    @bburd Thanks for your response. The air filter was just changed and there are no issues with the return and supply grilles. Blower is working fine.
  • pedmec
    pedmec Member Posts: 712
    Is this heat only or do you have a coil on top? Like @bburd said it sounds like the high limit is tripping out. Air flow being the #1 culprit. Any work done to the blower motor?
  • mattmia2
    mattmia2 Member Posts: 7,148
    If it is shutting the burner down on high limit it shouldn't take a prolonged period to relight.

    A thermometer in the plenum would tell you if it is an airflow problem.
    bburd
  • Blower55
    Blower55 Member Posts: 24
    To answer @pedmec's question, the AC coil is on top. There has been no work done to the blower motor, as it was working perfectly throughout the cooling season. I have the ability to measure temperature in the plenum. Will check and post results as soon as I can get to it.
  • mattmia2
    mattmia2 Member Posts: 7,148
    Check the gas pressure. Is this natural gas or propane?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,919
    edited October 2022
    Lights then runs for a little the goes off. Then the trial for ignition starts again and lights runs for a short time then goes out. That will work in this weather. Not so much when it is colder. Did you check the flame sensor circuit? Bad ground or dirty flame rod can offer a weak signal. Enough to turn on the main burner. Then after things heat up (like cherry red heat on the flame rod) the marginal signal fails and the system shuts down. Once it cools down from post purge, the cycle goes again.

    Try an alligator clip jumper from the control ground to a metal part of the pilot burner that is not rusted and see if it works. Then you know it is a poor ground circuit. If that does not work, then try cleaning the flame rod. Both of those fixes are free. Once you have eliminated that, you can look at the more expensive parts.
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • pedmec
    pedmec Member Posts: 712
    If you do have alligator clips use them to jump out the high limit too. If it stays running with the high limit jumped out then you have restricted air flow and you will most likely have to chemically clean the evaporator coil. This is provided that nothing else has changed, like the return is block, dampers adjusted closed or failed closed (zone system) etc.
  • JUGHNE
    JUGHNE Member Posts: 10,278
    Not only clean the coil, but I would look at the blower wheel.
    Filters are not perfect and the blower wheel will pick up dust/dirt and slam that onto the wheel.
    Can make a lot of difference in air moving.
  • Blower55
    Blower55 Member Posts: 24
    Hello everyone - I had a little time to test some of your suggestions and have some feedback:

    The following temperatures were taken inside the plenum that sits right on top of the furnace and contains the AC coils.

    Plenum temperature checks - Normal high limit wire connection

    • Normal call for heat and normal heating cycle
    • Burner shuts off between 120-125F, while blower and inducer remained on
    • Igniter clicked at 114-118F
    • Burner came on again at 101-105F
    • Burner shut off between 120-125F
    • Etc...

    Plenum temperature check - High limit wires disconnected and jumped

    • Normal call for heat and normal heating cycle
    • Burner shut off at 140F, while blower and inducer remained on
    • Did not check to see when burner would come back on - would take too long - can check if this is important

    Additional notes

    • Cleaned the blower fan and had just changed the filter a few days ago
    • Had also just replaced the high limit switch with ratings that match the old one - 170-40F. (Tested continuity to be sure it was good before installing)
    • Tested temperature in area where limit switch is mounted and the reading was 135-137F
    • Tested plenum temperature one more time and got a reading of 135-138
    • Air temp at supply registers = 125-127F
    • Not sure what would explain the burner consistently shutting off between 120-125F (with limit switch wired normally.)
    • The plenum and area where switch is mounted never reach anywhere close to 170F
    • If the limit switch was opening, why would the burner come on again at around 105?
    • Furnace is heating the house just fine, but I'd like to get to the bottom of this issue
    I hope this helps and thanks to all of you for the input.
  • pecmsg
    pecmsg Member Posts: 3,535
    What’s the temperature at the closest return and closest supply when it shuts down?
  • mattmia2
    mattmia2 Member Posts: 7,148
    Does the flame stay steady when the blower comes on?
  • mattmia2
    mattmia2 Member Posts: 7,148
    Do you have working co detectors?
  • pecmsg
    pecmsg Member Posts: 3,535
    mattmia2 said:
    Do you have working co detectors?
    Low Level CO detectors?
  • mattmia2
    mattmia2 Member Posts: 7,148
    edited October 2022
    pecmsg said:


    mattmia2 said:

    Do you have working co detectors?


    Low Level CO detectors?
    Ideally.

    I'm thinking perhaps the heat exchanger is compromised and products of combustion are contacting the limit switch and causing it to open. The excess air from the blower would also explain why it won't relight when the limit switch closes.

    If I'm right you need to stop using the furnace immediately and shut off the power and gas to it before someone is killed.

    At very least it is time to shut off the power and gas and call a competent professional.
  • pedmec
    pedmec Member Posts: 712
    looks like you might have an issue with the sail switch. reading your sequence of operation if the high limit would restart the whole ignition sequence with inducer shutting down. but that is not the case. if the sail switch is faulty it just restarts the spark ignition and pilot assembly again but inducer stays running. check to make sure sail switch is clean and can move freely
  • Blower55
    Blower55 Member Posts: 24
    Any recommendations on a low level CO monitor? I am looking at the Defender LL6170 which measures as low as 5 ppm or more and there is a much less expensive Kidde Ultra-Sensitive model that will show 10 ppm or more.

    I do have one hardwired unit on my second floor, but it is due to be replaced. Fortunately, it is warm enough right now where I do not need the heat, so I'd like to grab a good monitor and be sure I'm absolutely safe. THANK YOU for the suggestion!
  • mattmia2
    mattmia2 Member Posts: 7,148
    edited October 2022
    If you look at the flame when the blower starts it shouldn't change at all. If the hx is compromised the air from the blower usually will cause the flame to change or move.

    you can put a voltmeter across each safety to figure out which one is opening, it will be 0v when closed and will jump to 24vac if the safety opens. but either the limit not working properly or the hx being compromised could kill you. whoever works on it has to be absolutely certain they found and fixed the problem.
  • JUGHNE
    JUGHNE Member Posts: 10,278
    Pedmec, where is the "sail switch" in this scenario?

    Also could this a "Pick and Hold" gas valve that Carrier/Bryant used years ago?

    I don't know much about them other than they can get finickily.
  • Blower55
    Blower55 Member Posts: 24
    @JUGHNE - I am pretty sure this is the same as the pressure switch.


  • Blower55
    Blower55 Member Posts: 24
    FYI - I just ordered one of the CO Experts low level monitors. Will report back shortly.

    Also, @mattmia2 - I watched the flame when the blower comes on and it did not move or look any different than when the burners first come on.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,101
    Sounds like the flame sensor is finicky. And the pilot is slow to light.

    Look at how the high limit is wired. Also there must be a centrifugal switch or an air pressure switch for the inducer. Need to check those
  • Blower55
    Blower55 Member Posts: 24
    @EBEBRATT-Ed - This furnace does not have a flame sensor like you would normally see. I think it may be integrated with the igniter unit. There is a pressure switch, so I'll definitely check that out. Thanks for the feedback!


  • Blower55
    Blower55 Member Posts: 24
    Good afternoon folks - while I await the arrival of my low level CO monitor, I wanted to get your input on the level of CO you would typically encounter in a house.

    Devices that use gas in my home are the furnace, hot water heater, and stove. I park my cars outside, so there would never be any exhaust in the attached garage. I also have a wood burning fireplace but won't be using it until it gets much colder.

    For those of you that use low level testing equipment, do you ever see homes with 0 ppm? If not, what range would you normally expect in a smaller home (<2000 sq ft) with the devices I listed? My home was built in the mid 1940s and has the original shingles, insulation, etc., so it would not be considered energy efficient by today's standards.

    There is quite a bit of information on the web and the EPA site, for example, says "Average levels in homes without gas stoves vary from 0.5 to 5 parts per million (ppm). Levels near properly adjusted gas stoves are often 5 to 15 ppm and those near poorly adjusted stoves may be 30 ppm or higher."

    My first test will be to test for CO without the furnace or stove being used and then check it with the furnace on. If no change in readings, I'll turn the oven on as a second test. I'm assuming the monitor (CO Experts 2016-10) adjusts quickly to changes in CO levels, but I am not sure how long it takes for CO levels to build up. In other words, if I use the furnace and it does in fact have a bad heat exchanger, how long after I turn it on will I see the CO levels start to rise?

    If you have any other suggestions for the testing process, please let me know.

    Thanks again for your help!
  • mattmia2
    mattmia2 Member Posts: 7,148
    it should be 0. maybe something <10 ppm briefly right at the draft hood of an appliance that doesn't initially draft properly until the vent warms up but it really should be 0 if everything is working properly.
  • Tim McElwain
    Tim McElwain Member Posts: 4,553
    Do you have the manual for this furnace? I have the service manual for the Bryant 375A and the 397H models. I can make you a copy and mail it to you if you are interested, Contact me at tim[email protected] or call me at 401-437-0557. it also includes a trouble analysis chart.
  • pedmec
    pedmec Member Posts: 712
    Sail switch is located i believe on the firewall under the drafthood. And it does serve the same purpose as a pressure switch. Would prove that the blower is operating. Thats where it was located in the manual.

    Yes @JUGHNE it is a pick and hold gas valve