dnewton3
Staff member
I think it would be beneficial for folks to understand the basic operational steps a furnace goes through to run successfully.
This is a generic list; it's fairly common among all brands and models. Many of these steps necessitate being done in sequence; only a few can be "concurrently" run. Don't nitpick the nuances; this covers the basics and not your brand-specific application of a unit from 50 years ago. Also, I'm describing the basics of a single-stage system; a two-stage unit is VERY similar, and only varies in that they will fire in high heat mode and then drop to a lower burn state in the lower stage(s), using the additional safety checks unique to a multi-stage unit. And, some of the very new designs have amp draw measuring capability to verify some conditions, but that's rare and so I'm going to ignore it for now.
Here are the general sequence steps:
1- thermostat calls for heat
2 - the call is acknowledged by furnace control board logic and starts the system pre-ignition checks.
3a - control logic checks to see if the pressure switch is open (makes sure that it's not stuck in the closed circuit position; that would make downstream check results invalid)
*NOTE= if p/s circuit is not open, the board sets fault code and stops the sequence
3b - concurrently, the logic is also checking to see that the permissive circuit is closed (the high-temp limit switches and flame roll-out switches are in the desired state)
*NOTE= if the limit circuit is open, system sets a code and stops the sequence
3c - concurrently, the logic will check to see there is no flame being sensed; this assures the flame sense circuit is not defaulted to flame-present condition
*NOTE = if the flame sensor circuit is not open at initial check, the system will set a code and abort sequence
3d - concurrently, there may be logic present which will check to see if the gas valve is not energized via a micro-voltage feedback loop (not on all models, but some)
*NOTE = if the gas valve check is present and not shown in the desired state, the system will set a code and abort the sequence
4a - If the above pre-ignition tests above are passed, the control logic then starts the combustion air draft inducer motor. The draft inducer's purpose is to purge the combustion chambers of any residual gas prior to start up, as well as establish a good draft flow of available combustion air during operation.
4b - At the same time the control logic starts a delay count timer (typically 5-20 seconds depending on brand and model of furnace)
5 - system logic then checks the p/s circuit to see if it's now closed after that waiting period above is satisfied, thereby confirming that HX purge is good and stack vent is not blocked, meaning combustion air is moving properly
*NOTE = if p/s circuit is not confirmed as closed, system sets a fault code and stops sequence
6 - if p/s is now closed as desired, system controls now energize the ignitor circuit (typically a hot surface ignition coil)
6b - system starts a 5-7 delay count timer; this is to allow the HSI to fully warm up (glowing red hot);
*NOTE = this HSI type ignitor is by far the most common system. While rare, you may come across a spark-ignitor or standing pilot light. There won't be a counter delay for the warm-up for these alternate methods; it will just move directly to step 7 if HSI is not present.
7 - the control system presumes the ignition heat source is sufficiently present and so it energizes the gas valve circuit (to open the gas valve)
8 - control system continues sequence and initiates a delay count timer of about 3-7 secs to allow the flame to be initiated and the flame front to travel across the burners to the opposite end of the burner rack; (time duration depends upon brand and model, as more burners take more time to light off)
9 - control system will now look for continuity or small micro-voltage shift at the flame sensor
*NOTE = if no flame is sensed, the system sets code and aborts the sequence
10 - If control logic sees successful flame propagation at the flame sensor, then the burn will continue for anywhere from 30-60 seconds to preheat the HX, then the furnace air-mover blower will be commanded on. During this HX preheat time, the high-heat limit switches simply aren't hot enough yet to cause the limit circuit to trip, or they are ignore by the control logic as part of the warm-up protocol only during this warm-up. Warm-up is present in most furnaces, but not all; some will start the blower immediately after flame is proven present.
11 - system runs the furnace until either the thermostat is satisfied (no more call for heat), or the system detects a non-permissible condition (limit switch trips, pressure switch trips, etc)
Anytime during the fire-up sequence something drops out of permissible conditions, the control logic sets a code and the sequence aborts. Further, if the furnace has properly started and is running well and then later in that burn cycle it experiences a non-permitted condition, it would also set a code and abort. There is logic in most systems to retry ignition AFTER a typical 5 minute wait period (for various safety reasons). After three attempts to purge and light-off with no success, the system will set a hard fault and not attempt a start sequence again until the board is powered down and back up. At this point, a service tech should be called or at the very least a truly knowledgeable person should troubleshoot.
Now again, some of these limits, values and steps are a tad different between brands and models; they are not all identical. But the general logic flow path is there in any modern furnace which has a control board, presuming the features described are present on the unit. And in most cases the logic sequence is as stated above; it's that way for safety reasons.
Thus endeth the lesson for today.
This is a generic list; it's fairly common among all brands and models. Many of these steps necessitate being done in sequence; only a few can be "concurrently" run. Don't nitpick the nuances; this covers the basics and not your brand-specific application of a unit from 50 years ago. Also, I'm describing the basics of a single-stage system; a two-stage unit is VERY similar, and only varies in that they will fire in high heat mode and then drop to a lower burn state in the lower stage(s), using the additional safety checks unique to a multi-stage unit. And, some of the very new designs have amp draw measuring capability to verify some conditions, but that's rare and so I'm going to ignore it for now.
Here are the general sequence steps:
1- thermostat calls for heat
2 - the call is acknowledged by furnace control board logic and starts the system pre-ignition checks.
3a - control logic checks to see if the pressure switch is open (makes sure that it's not stuck in the closed circuit position; that would make downstream check results invalid)
*NOTE= if p/s circuit is not open, the board sets fault code and stops the sequence
3b - concurrently, the logic is also checking to see that the permissive circuit is closed (the high-temp limit switches and flame roll-out switches are in the desired state)
*NOTE= if the limit circuit is open, system sets a code and stops the sequence
3c - concurrently, the logic will check to see there is no flame being sensed; this assures the flame sense circuit is not defaulted to flame-present condition
*NOTE = if the flame sensor circuit is not open at initial check, the system will set a code and abort sequence
3d - concurrently, there may be logic present which will check to see if the gas valve is not energized via a micro-voltage feedback loop (not on all models, but some)
*NOTE = if the gas valve check is present and not shown in the desired state, the system will set a code and abort the sequence
4a - If the above pre-ignition tests above are passed, the control logic then starts the combustion air draft inducer motor. The draft inducer's purpose is to purge the combustion chambers of any residual gas prior to start up, as well as establish a good draft flow of available combustion air during operation.
4b - At the same time the control logic starts a delay count timer (typically 5-20 seconds depending on brand and model of furnace)
5 - system logic then checks the p/s circuit to see if it's now closed after that waiting period above is satisfied, thereby confirming that HX purge is good and stack vent is not blocked, meaning combustion air is moving properly
*NOTE = if p/s circuit is not confirmed as closed, system sets a fault code and stops sequence
6 - if p/s is now closed as desired, system controls now energize the ignitor circuit (typically a hot surface ignition coil)
6b - system starts a 5-7 delay count timer; this is to allow the HSI to fully warm up (glowing red hot);
*NOTE = this HSI type ignitor is by far the most common system. While rare, you may come across a spark-ignitor or standing pilot light. There won't be a counter delay for the warm-up for these alternate methods; it will just move directly to step 7 if HSI is not present.
7 - the control system presumes the ignition heat source is sufficiently present and so it energizes the gas valve circuit (to open the gas valve)
8 - control system continues sequence and initiates a delay count timer of about 3-7 secs to allow the flame to be initiated and the flame front to travel across the burners to the opposite end of the burner rack; (time duration depends upon brand and model, as more burners take more time to light off)
9 - control system will now look for continuity or small micro-voltage shift at the flame sensor
*NOTE = if no flame is sensed, the system sets code and aborts the sequence
10 - If control logic sees successful flame propagation at the flame sensor, then the burn will continue for anywhere from 30-60 seconds to preheat the HX, then the furnace air-mover blower will be commanded on. During this HX preheat time, the high-heat limit switches simply aren't hot enough yet to cause the limit circuit to trip, or they are ignore by the control logic as part of the warm-up protocol only during this warm-up. Warm-up is present in most furnaces, but not all; some will start the blower immediately after flame is proven present.
11 - system runs the furnace until either the thermostat is satisfied (no more call for heat), or the system detects a non-permissible condition (limit switch trips, pressure switch trips, etc)
Anytime during the fire-up sequence something drops out of permissible conditions, the control logic sets a code and the sequence aborts. Further, if the furnace has properly started and is running well and then later in that burn cycle it experiences a non-permitted condition, it would also set a code and abort. There is logic in most systems to retry ignition AFTER a typical 5 minute wait period (for various safety reasons). After three attempts to purge and light-off with no success, the system will set a hard fault and not attempt a start sequence again until the board is powered down and back up. At this point, a service tech should be called or at the very least a truly knowledgeable person should troubleshoot.
Now again, some of these limits, values and steps are a tad different between brands and models; they are not all identical. But the general logic flow path is there in any modern furnace which has a control board, presuming the features described are present on the unit. And in most cases the logic sequence is as stated above; it's that way for safety reasons.
Thus endeth the lesson for today.
