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Circulating Blower Motor Cutting Out Mid-Cycle

Hello!

I have a particularly odd behavior that a horizontal gas furnace is exhibiting, and I’m trying to determine the root cause and the faulty component.

The Symptoms and Observations
When the thermostat calls for heat, the system will behave normally for the first 5-10 minutes, but then the circulating blower relay will click off in the middle of the cycle. The system overheats since the burner and induction fan are still operating, the primary limit sensor is tripped, and the system shuts down. The circulating blower comes back on until the system is below the threshold temperature plus running through the safety timing, and the cycle repeats until the house is to temperature.

However, if the thermostat calls for fan AND heat, the system behaves normally even for extended periods of time.

Current Diagnoses
The air filter is brand new and completely clean – the issue continues even if the filter is completely removed from the system, so this is not the root cause.

The thermostat was then expected at fault, but the issue continues with the thermostat completely removed and the terminals jumped directly at the control board. When R and W are jumped, the circulating blower will turn off mid-cycle. But when R, W, and G are jumped, the system will behave normally for extended periods of time.

The control board was next expected at fault, but has since been replaced with a brand new unit and the issue continues exactly the same.

The circulating blower and capacitor are not expected to be at fault, because the system functions normally when the fan is called for in addition to heat, operating with no faults for up to 3 hours. Per the manual for the controller board, the call for heat and the call for fan both use the heat speed on the circulating blower, so I believe the fan operates at the same speed in both cases.

At this point, the only thing I’ve found that might indicate a component issue is on the SmartValve. The burner and system seems to behave normally, but the SmartValve will randomly “click” during operation. The flame does not seem to be affected when the valve is making this noise, but the noise does seem to happen less frequently when the fan and heat are both called for. I’ve taken a video of an entire cycle, from cold start up through the circulating blower relay clicking off and the control board throwing a “primary limit reached” error. The clicking noise the SmartValve is making can be heard starting at 1:24 in this video. https://www.dropbox.com/s/fc1ssvtrxdkuujn/CI_Furnace_Cycle_09NOV2018.mov?dl=0

That said, I don’t yet understand if or how a bad SmartValve could cause the circulating blower relay to switch off. I've theorized that it could be faulty and requiring excessive current, the clicking noise is the valve being unable to get sufficient current, and eventually the control board (which powers the smart valve) is sending enough current to the SmartValve that there isn't enough to keep the circulating blower relay energized. When both the fan and heat are called for, additional power is sent to the control board (since both W and G are now powered) that could mitigate this. But I have no clue if this is even realistic.

System Specifications
Furnace: Consolidated Industries MBA 080 NH3R, 1994
Control Board: Honeywell ST9141A1002, Replaced with Honeywell ST9120U1011
Smart Valve: Honeywell SV9500M8667, Replacement Part Honeywell SV9501M8129 (Not Yet Replaced)

I appreciate any feedback or help that anyone can provide!

Thank you!

Comments

  • JUGHNEJUGHNE Posts: 5,725Member
    You might have 1 or 2 other motor speeds parked on the board.
    I would swap the heat speed motor connector with one of these and park the existing heating connector. See if the same results show up.
    It would be rare but you could have that heating speed motor wire opening.
    When it goes into overheating, usually the high, black AC speed in kicked in so it seems the fan is working....
  • HVACNUTHVACNUT Posts: 2,264Member
    With the burner circuit opening on limit, the board will default to high speed fan in order to quickly cool the HX. It's not the gas valve unless its really screwed up.
    Switching the fan to on at the thermostat also runs high speed fan.
    How's the air filter?
    Is it zoned?
    Need to take a TD reading at the supply and return at the furnace and compare it to the allowable heat rise of the furnace.
    Is this a recent problem?
    If so, switching to a faster fan speed isn't a fix. The limit is telling you something is wrong.
  • JUGHNEJUGHNE Posts: 5,725Member
    Yea, the limit tells you the fan quit running.
    Switching speeds to see if the problem follows a different speed or the board just drops the heating relay out regardless of the chosen speed is what I am talking about.
  • AeroEngineerAeroEngineer Posts: 10Member
    JUGHNE said:

    You might have 1 or 2 other motor speeds parked on the board.

    I do! The heat currently runs the Med-High lead, and cool is on the High lead, while the Med-Low and Low speeds are on the "unused motor terminals". I'll switch to the Med-Low lead and see if it's any different this evening.

    I wouldn't initially expect this to make a difference though, since the Honeywell documentation for the control board specifically states that when the thermostat calls for fan, the heat speed is used, and the high speed is only used when cool is called for. But it never hurts to try!

    Page 11 talks about which fan speed the controller uses for which terminal: https://forwardthinking.honeywellhome.com/related_links/combustion/st9120u/install/69_0644.pdf

  • AeroEngineerAeroEngineer Posts: 10Member
    edited November 2018
    HVACNUT said:


    Switching the fan to on at the thermostat also runs high speed fan.

    I'll certainly test this, but the controller board manual states specifically the heat speed (med-high) is used with the G terminal. Only a call for cool is supposed to use the high speed. Wouldn't be the first time a manual is wrong though!
    HVACNUT said:


    How's the air filter?

    Is it zoned?

    The filter is brand new, and the issue happens even with no filter installed. No zones at all.
    HVACNUT said:


    Is this a recent problem?

    Yes, this is a brand new problem. Haven't had problems in years with heating or cooling. The temperature rise is well under the limit until the circulating blower turns off in the middle of the cycle. Only when the blower turns off while the burner is still running does the temp rise and trip the primary limit.

  • HVACNUTHVACNUT Posts: 2,264Member
    You'll need to check the amp draw on the heat motor speed. Seems like thermal overload or windings.
    Check the capacitor?
    It's not an ECM motor is it?
  • HVACNUTHVACNUT Posts: 2,264Member
    > @JUGHNE said:
    > Yea, the limit tells you the fan quit running.
    > Switching speeds to see if the problem follows a different speed or the board just drops the heating relay out regardless of the chosen speed is what I am talking about.

    I understood and agree.
    I think it's open windings.
  • JUGHNEJUGHNE Posts: 5,725Member
    edited November 2018
    I am thinking that if the OL or cap opened then the high speed of AC/open limit cool down function would not work.
    Does the OL open the neutral/common wire?

    If windings...only the heating speed opens for whatever reason.
    Not the OL opening or no cooling speed.
  • AeroEngineerAeroEngineer Posts: 10Member
    Ok, I did some motor lead testing as suggested.

    The motor diagram calls out: H is high speed, MH is medium-high speed, ML is medium-low speed, and L is low speed for the circulating blower.

    I connected the ML motor lead to the heat speed on the control board where the MH motor lead was connected. The system behaved the same as I described originally; it works just fine if I call for fan and heat, but if I only call for heat the relay on the control board turns the circulating blower off around 5 minutes into the cycle while everything else stays running. You can actually hear the relay clicking the blower off.

    I reconnected the MH motor lead to the heat speed, and disconnected the H motor lead from the cool speed entirely (H, ML, and L motor leads were all disconnected, and nothing was connected to the cool speed on the control board). The system behaved the same as I described originally. I then disconnected the MH motor lead from the heat speed, and connected the H motor lead to the cool speed (MH, ML, and L motor leads were all disconnected, and nothing was connected to the heat speed on the control board). Neither a call for fan nor a call for heat would turn on the circulating blower. This means that the control board does use the heat speed for both fan and heat, and that both a call for just fan and a call for just heat are using the same windings in the motor. Since a call for just fan works for hours, a call for fan and heat works for hours, but a call for just heat stops working a few minutes in, I don't believe the fault is in the motor, otherwise I'd expect the call for fan and heat to also stop working a few minutes in since it's using the same speed/windings as when the fan is called for.

    I also tried connecting the H motor lead to heat speed and the ML motor lead to cool speed, but a call for heat never activated the circulating blower at all. I think the heat speed relay just isn't rated for the power that the H motor lead winding requires.
  • unclejohnunclejohn Posts: 1,374Member
    can you post a picture of the schematic
  • AeroEngineerAeroEngineer Posts: 10Member
    unclejohn said:

    can you post a picture of the schematic

    Sure thing!

    The original furnace schematic:


    The schematic of the updated control board:


    The schematic of the SmartValve:

  • unclejohnunclejohn Posts: 1,374Member
    On the original schematic both the blower relay and the speed control relay has to be energized for hi speed fan. To get low speed or heat operation fan, only the blower relay need be energized. So the board must do that. What tells it to is most likely the gas valve. And if your replacement board is the right part then I would say the valve is the culprit. IMO the only thing "smart" about thoses valves is they are not used anymore.
  • JUGHNEJUGHNE Posts: 5,725Member
    Are you thinking that the "clicking" of the Smart valve is dropping the signal to the fan board for heat fan relay?
    Then after hi limit event the high speed relay is engaged?
  • AeroEngineerAeroEngineer Posts: 10Member
    unclejohn said:

    On the original schematic both the blower relay and the speed control relay has to be energized for hi speed fan. To get low speed or heat operation fan, only the blower relay need be energized. So the board must do that. What tells it to is most likely the gas valve. And if your replacement board is the right part then I would say the valve is the culprit. IMO the only thing "smart" about thoses valves is they are not used anymore.

    I won't pretend to never make mistakes, but I do feel pretty confident this is the correct replacement board. I even mapped out the wiring diagrams between the original and replacement to make sure everything was the same, and even matched the neutral terminals, even though that usually doesn't matter since a neutral is a neutral.

    I am very curious if you'd be willing to elaborate on the "smart" gas valve's interaction with the system. I'm not very good at reading the upper schematic in the original furnace diagram, so I'm ignorant on how the gas valve affects the blower motor.
  • unclejohnunclejohn Posts: 1,374Member
    On the original schematic there is a terminal labeled FT. I don't know for sure what the is but all the rest are pretty straight forward. 24v com theremo then the pilot. Some how the board has to know that the burners are lit. My guess is the FT terminal. So burner on power FT. Time delay start fan. No FT no time delay no fan.
  • unclejohnunclejohn Posts: 1,374Member
    On the new sche. that terminal is marked eft and is a output.
  • JUGHNEJUGHNE Posts: 5,725Member
    edited November 2018
    This has been very educational. The most I deal with Smart valves is replace them when they have obvious issues......skipping the pilot ignition....lighting only the main burner first....this snuffs out the flame...goes for relight several times and then may stay on.

    But now studying it, my guess is, going from the top:

    T-stat starts inducer motor
    inducer motor closes pressure switch
    pressure switch energizes HSI
    warm up time passes
    pilot gas valve opens (Usually)
    FS proves and main valve opens
    EFT output (electronic fan timer?) is energized and starts heat fan on delay timing.

    But if Smart valve drops the EFT signal the board thinks the fire is out and counts down to time delay off for fan (heat) relay.
    The limit switch(s) sense over temp and kick the fan on high speed and also run the inducer motor to cool down unit.

    If you set the delay off for fan relay up to 180 sec it might run longer before tripping after you hear the clicking of valve.
    My best guess.
    Please let us know if new valve fixed this.
    I have about 20 of these that are nearing 20+ years old.

    And does anyone have a good sub for these HW SV's?
  • AeroEngineerAeroEngineer Posts: 10Member
    That all seems to make good sense. I was originally thinking all timing was done on the controller board, but you're right - the SmartValve has to indicate via the EFT to the controller that the flame is good and the timer even needs to start in the first place. I'm curious if the primary limit wasn't tripped first, if the SmartValve ends up bringing power back and the blower would end up turning back on, but the heat on delay is long enough that the primary always trips first.

    I'd love to change the heat off timer to see if that ends up letting the blower run longer, but the SmartValve clicks sporadically and frequently enough that I wouldn't be able to determine which "click" actually corresponds to the loss of power to the EFT lead. I might see if I can measure the EFT lead output on the controller during a cycle, but since it's in the 6-pin header that might be hard to get to.

    For the order of operations, Honeywell provides this in the SmartValve documentation:


    Honeywell lists the replacement for the SV9500M8667 on the system currently as the SV9501M8129. I was able to get it off Amazon for ~$200. The full replacement chart is on page 15 of https://forwardthinking.honeywellhome.com/related_links/combustion/universal_smart_valve/install/69_1270.pdf

    I'll hopefully be able to get the valve replaced tomorrow evening, as long as UPS holds my package like they should, and a friend who's a licensed HVAC guy shows up. I can do electrical and bolted stuff all day long - but I'd rather let the pros handle anything to do with natural gas.

    Thanks for the help everyone! I'll update with what ends up happening.
  • JUGHNEJUGHNE Posts: 5,725Member
    Only HW would attempt to correct what many think was a flawed product by adding more numbers to the "Smartvalve Family".
    By a substitute I meant maybe something simpler from some Brand X.

    I will have to say that their HSI have had a pretty good track record being 20 years ago that was a somewhat shaky technology. It is the only 24VAC HSI I have seen.
  • AeroEngineerAeroEngineer Posts: 10Member
    edited November 2018
    Update with results and a new issue!

    Before replacing the SmartValve, we watched an entire "fail" cycle again. During this we noticed that the valve, in addition to "clicking" was cycling the ignitor even though the flame was constantly on.

    I had the new SmartValve installed, and the blower motor no longer cuts off in the middle of the cycle. As long as the call for heat remains, and the temperature limits aren't tripped, the blower remains on constantly.

    However, the burner is now cycling. It varies in duration, but will remain on between 1 minute and 10 minutes, then the flame cuts out and then re-ignites. After the flame cuts out 5 times, the valve goes into a 5-minute retry delay, and then repeats. This follows as expected from the flow diagram. The control board never throws any error codes throughout any of this.



    I'm guessing this is just a dirty or worn out flame sensor? Would the new valve be pickier about the current it deems "acceptable" from the flame sensor, and that's why this issue wasn't present with the old valve? Any reason to suspect it's anything else?

    I'll probably just replace the flame sensor assembly at this point unless there are other concerns here, it's a pain to get to in this furnace and don't want to try cleaning it only to have to replace it anyway.
  • SuperTechSuperTech Posts: 938Member
    It sure sounds like a flame sensor issue. I'm not sure why it wouldn't happen with the original valve. I always install a new igniter and flame sensor when replacing the smart valve, to ensure reliability.

    I'm glad they no longer use those damn valves anymore.
  • AeroEngineerAeroEngineer Posts: 10Member
    edited November 2018
    Update #2 - Everything appears to be fixed!

    I replaced the original igniter and flame sensor assembly Q3400A1008 with a Q3400A1024 (the replacement part, just with a 30" lead instead of 18").

    Ran the furnace (with just a call for heat) for about 25 minutes (67F up to 71F), with no faults or issues.

    Tidied up the wiring inside, and ran another test (just calling for heat again) to ensure that moving wiring around hadn't mucked anything up. This test ran about 23 minutes (71F up to 74F), and again no faults or issues.

    So, in summary:
    • Controller board was replaced, but likely wasn't necessary. Doesn't hurt though given the age of the original board, might not have been long for this earth anyway.
    • SmartValve was replaced, this corrected the issue where the circulating blower motor would turn off in the middle of a heat cycle.
    • Igniter and flame sensor was replaced, this corrected the issue only seen with the new valve where the burner would cycle on/off during a heat cycle.
    • Total part cost: $485
    I'll check in on it again in a few days just to make sure it's still cycling correctly. Thank you everyone for your insights and helpful discussion!
  • SuperTechSuperTech Posts: 938Member
    Thank you for keeping us updated. As a service technician I enjoy reading about these types of issues that I haven't encountered and how they are resolved. I'm glad you have it operating normally again!
  • AeroEngineerAeroEngineer Posts: 10Member
    Final Update - All is Well

    The furnace has been running unattended for about a week now, and I ran another long cycle this morning. All systems still operating as expected. I'm going to consider this closed.

    Thanks again everyone!
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