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!