HVAC technicians typically focus on diagnosing issues related to mechanical wear, refrigerant leaks, or control problems within the equipment. However, damage can also originate from external sources.

For instance, a failed control board might be attributed to internal shorts or rubouts. Yet, the root cause could be external, such as the voltage spike that can occur after brownout conditions, which can be challenging to diagnose since these voltage irregularities may not be present during an on-site inspection.

What Exactly is a Brownout?

If you've experienced a brownout before, you're already familiar with the frustration of flickering lights and struggling appliances. But for those fortunate enough to have avoided this situation, let's clarify what it entails.

Unlike a complete blackout, a brownout is a temporary, partial reduction in the power grid's voltage. This sag in power typically affects an entire area rather than a single home. These silent voltage drops can severely stress and ultimately damage critical HVAC components, leading to expensive repairs and unhappy homeowners. It is also important to note that the sudden voltage spike that often follows a brownout can cause serious damage.

Brownouts stem from various power grid issues, and unfortunately, there's little an HVAC technician can do to prevent them from reaching a home. However, strategies exist to protect equipment and mitigate damage from voltage irregularities, which we'll explore later in this article. Here are some common causes of brownouts:

Overloaded Power Grids

An overloaded power grid is by far the most common cause of brownouts. Imagine a scorching hot day in a densely populated area where all the homes have air conditioners working double time to keep up with the heat load. This type of situation can put an immense strain on the power grid.

Equipment Failures

Components within the electrical grid, such as transformers, circuit breakers, substations, or even power lines, can fail due to age, wear and tear, environmental factors, or manufacturing defects. Such failures disrupt the consistent flow of electricity and lead to localized or widespread voltage drops.

Severe Weather Conditions

Storms with high winds, lightning, heavy ice, or extreme heat waves can damage power infrastructure (poles, lines, transformers) or significantly increase demand, leading to voltage sags.

What Effect do Brownouts have on HVAC Systems? Why Should a Technician Care?

HVAC systems have several electrical components, and brownouts can contribute to failures in all of them.

Motors 

The impact of brownouts on motors depends significantly on the type of motor.

For standard single-phase motors (PSC, CSC, TR types), a steady state low voltage causes them to become less efficient. They may struggle to start, run inefficiently, or stall. When a single-phase motor experiences sustained low voltage and is still running, it actually draws less current, not more. This is because as the motor's “slip” increases due to the weakened magnetic field, its electrical resistance (impedance) increases, causing current to decrease. 

However, if the voltage drops to a point where the motor stalls (hits “locked rotor”), it will then draw significantly more current, generating excessive heat and potentially leading to winding damage or burnout.

Modern inverter-driven compressors and fan motors (ECMs, VFDs) are particularly vulnerable in a different way. These motors are designed to operate across a range of frequencies and voltages by attempting to maintain a consistent output. During prolonged low-voltage conditions, the inverter's electronics will indeed draw more current to try and maintain the intended horsepower, which can overstress and damage the inverter components due to excessive heat.

Additionally, sustained overheating from any cause can damage motor bearings, resulting in increased friction, noise, and eventual seizure.

Capacitors 

Capacitors are passive devices that store and discharge current. During a brownout, with lower voltage, they simply store and discharge less current. This reduced capacity means they won’t provide the necessary phase shift and current boost required to start the motor. 

Symptoms of a capacitor failing due to the lower voltage can include difficulty starting motors, leading to a completely non-starting unit. While low voltage doesn't directly cause a capacitor to fail, it can be a contributing factor in a chain of events leading to its failure.

Control Boards 

Modern HVAC systems rely on electronic control boards, relays, sensors, and thermostats, all designed to operate within tight voltage tolerances. While voltage sags can lead to erratic behavior, miscommunications between components, and corrupted programming, the most significant damage to control boards often occurs not during the brownout itself, but during the voltage spike that frequently follows a brownout condition when power is restored. This sudden surge can overwhelm and damage sensitive electronic components.

Contactors and Relays 

Low voltage conditions, particularly fluctuating voltage during a brownout, can cause contactors and relays to “chatter” or pull in weakly. This rapid on-off cycling and weak engagement result in arcing and pitting of the contacts, which can lead to intermittent operation or complete failure. Severe chattering can even physically damage the contacts. Similar to control boards, the post-brownout voltage spike can also contribute to the destruction of these components.

Compressor Short Cycling 

Some newer systems have built-in low-voltage protection that will prevent the compressor from starting or force it to shut down if the voltage drops below a safe threshold. While this protects the compressor, frequent cycling due to unstable power still puts strain on the starting components. 

If the system attempts to run under brownout conditions, the compressor may struggle to overcome head pressure, leading to hard starts and increased wear. The on-again, off-again nature of a system in brownout conditions, particularly for single-phase motors, can contribute significantly to contactor chatter and other component stresses.

Some Tips for Diagnosing Damage Caused by Brownouts

It can be difficult to attribute damage directly to brownouts, especially when the voltage reading is perfect while you’re on the scene. It will require some detective work and an open mind to possibilities outside of the equipment cabinet. Keep in mind that brownout damage can look similar to damage caused by other issues.

Capacitor Health 

Visually inspect capacitors for swelling or leakage. While low voltage doesn't usually directly cause a capacitor to fail, it can contribute to a chain of events that leads to its failure.

Contactors/Relays 

Inspect contactor points for pitting or scorching, indicating excessive arcing due to low voltage or rapid cycling (chatter).

Motor Winding Resistance 

Measure motor winding resistance (when de-energized) and compare your reading to the specifications. Unbalanced readings or open circuits can indicate winding damage caused by motor overheating, especially in ECM/VFD motors, or from locked rotor conditions in single-phase motors.

Control Board Status Lights 

Most newer systems can provide diagnostic codes or blinking lights that can indicate voltage issues.

Customer Interview 

Ask the homeowner if they noticed dimming lights, flickering, or other appliances behaving unusually around the time of the HVAC failure. This information will provide some insight into when the brownouts typically occur.

Brownout Protection Strategies

HVAC technicians have a responsibility that extends beyond repairs and installations: educating homeowners. As equipment grows more sophisticated, it's crucial to inform customers about proper usage and the importance of surge and brownout protection.

Whole-Home Surge Protection

A whole-home surge protection system should be at the top of the list for any recommendations concerning power quality. It will protect the HVAC unit and all other sensitive electronic components in the home from voltage spikes that often follow a brownout condition, which are major causes of electronic damage.

Devices that Offer Brownout Protection

Consider upgrading standard contactors with something like the SureSwitch by White-Rodgers with built-in brownout protection. This is a latching contactor that helps prevent the damaging chatter caused by fluctuating low voltage. Unlike whole-home surge protection, contactor upgrades will only protect the HVAC equipment in which they’re installed, but they’re well worth the protection they offer.

Voltage Regulators

Although not as common and usually more expensive, whole-home voltage regulators can be installed to stabilize electricity before it enters the home's utility panel.

Homeowner Education

One of the best protections you can offer a client is to educate them about brownouts and advise them to manually turn off their air conditioner at the thermostat if they notice brownout conditions. Then, they should wait 10–15 minutes after the power stabilizes to turn it back on.

In conclusion, brownouts pose a significant, often invisible threat to HVAC systems. While technicians can't control the power grid, understanding the causes and the specific effects on different components, particularly the damaging voltage spikes that often follow, is crucial for accurate diagnosis and effective mitigation. By combining diligent investigative work, utilizing diagnostic tools, and educating homeowners on protective measures and best practices, HVAC professionals can safeguard equipment, prevent costly repairs, and ensure customer satisfaction in the face of unpredictable power fluctuations.

—JD Kelly