Yes, I've encountered similar issues with standby generators and sensitive loads like furnaces. It's frustrating because the standby tests don't switch the house over, so it shouldn't be causing that much of a disturbance. Here's a breakdown of potential causes and troubleshooting steps

Possible Causes:

1. Voltage Fluctuations/Harmonics During Generator Start-up (Even in Exercise Mode)

Even though the transfer switch isn't activating during a test, the generator is starting. The engine startup can cause voltage dips or spikes, especially on a lightly loaded generator. Generators generate harmonics in the current that the furnace might be particularly sensitive to. A slightly "dirty" or unstable sine wave, even if within acceptable voltage ranges, can bother some sensitive electronic components.2.

Inrush Current from Furnace Components

During the generator start-up, even though the main power is still on, the voltage instability could cause the furnace's blower motor, inducer motor, or igniter to try to restart momentarily or react in some way. These can have high inrush currents.3.

Neutral-Ground Bonding Issues



This is a critical area to check. In a properly installed standby generator system, the neutral and ground should be bonded only at the main service panel (or the first point of disconnect). If there's an accidental neutral-ground bond in the generator itself or anywhere else in the system (like a subpanel), it can create circulating currents and voltage imbalances, leading to nuisance tripping. Double check that the neutral is bonded to ground only in the main service panel. This is often a small green bonding screw or wire. Make sure it's securely in place there and that there is no bonding taking place at the generator.4. Weak Breaker (Despite the Upgrade)

Even a new breaker can be defective. It's worth testing the breaker's actual trip current. (See below.)5.

Actual Overload (Rare During Test, but Possible)

Unlikely during a test, but consider what else might be running on that circuit. A small space heater, plugged in and forgotten, could tip the balance.6.

Line Voltage

Verify the line voltage coming from the utility is correct. If the utility has high or low voltage, that could affect the furnace. If the utility has issues that the generator is masking, the furnace issue might be from the utility side.7.

Corrosion

Look for corrosion on the breaker and the connection points.

Troubleshooting Steps


Verify Proper Neutral-Ground Bonding (CRITICAL)



This is the first and most important step. Visually inspect the main service panel and the generator. Ensure the neutral-ground bond is ONLY at the main panel and that there is no bonding at the generator. If you measure continuity between neutral and ground anywhere other than the main panel, you have a problem. Use a multimeter to measure the voltage between neutral and ground at the furnace during the generator test. It should be very close to zero. A significant voltage reading here indicates a neutral-ground issue.2. Monitor Voltage During Generator Start-up (with Meter and Ideally Data Logging)

Use a multimeter or, better yet, a data logging multimeter (that can record min/max/average voltages) to monitor the voltage at the furnace breaker during the generator test cycle. Look for voltage dips or spikes. Check the generator output voltage while running under no-load conditions. Ensure it's within the manufacturer's specifications.3.

Test the Furnace Breaker's Trip Current

Use a breaker analyzer (or a qualified electrician with one) to test the actual trip current of the 20A breaker. It should trip within its specified range. A weak or "lazy" breaker will trip prematurely. A new breaker could also be installed just to rule this out.4.

Clamp Meter on Furnace Circuit

Use a clamp meter to monitor the current draw on the furnace circuit during the generator test cycle. This will help identify any excessive inrush currents or unexpected loads. Do this at the breaker panel.5.

Check Generator's Frequency

Use a multimeter to check the frequency of the generator while running. It should be 60Hz (or very close to it). An unstable frequency can cause problems with sensitive electronics.6.

Inspect Furnace Wiring and Connections

Make sure all connections on the furnace wiring are tight. Check that the furnace is grounded properly. Loose connections can cause voltage drops and heat which would trip the breaker.7.

Consider a Line Conditioner/Surge Suppressor

As a last resort (after you've ruled out other problems), you could try installing a high-quality line conditioner/surge suppressor specifically designed for sensitive electronic equipment on the furnace circuit. This might filter out any voltage fluctuations or harmonics from the generator. However, don't rely on this as a permanent solution – it's just a band-aid if there's an underlying electrical problem.

Important Notes



Safety First

Always disconnect power before working on any electrical components. If you're not comfortable working with electricity, hire a qualified electrician.

Documentation

Keep a detailed record of all your troubleshooting steps and findings.

Consult the Generator Manufacturer

If you're still stumped, contact Generac's technical support for assistance. They may have seen similar issues with specific furnace models or electrical configurations.

Talk to the Furnace Manufacturer

If you've confirmed the generator is working properly, contact the furnace manufacturer to see if they have any recommendations for mitigating nuisance tripping with standby generators. The furnace may need some sort of inrush current limiter.

Why the 20A breaker seemed to help temporarily: A larger breaker can tolerate slightly higher inrush currents or minor voltage fluctuations for a short period without tripping. However, it doesn't fix the underlying problem – it just masks it. It's also potentially unsafe if the wiring isn't rated for 20 amps. The original 15A breaker may have been weak and nearing its end-of-life. A new 20A breaker may have solved the problem temporarily.I hope this helps! Let me know if you have any other questions. Flag for review