In modern furnaces, the blower motor relies on a chain of discrete components during startup. A malfunction at any point can cause significant problems for your furnace.

The start capacitor is a frequent point of failure in this system that many homeowners aren't familiar with. Here's a guide to your furnace start capacitor to help you recognize capacitor problems in your furnace.

What Is a Furnace Start Capacitor?

The start capacitor is a component that kickstarts the blower motor in a modern furnace. In most HVAC systems, the furnace start capacitor is rated to supply either 370vac or 440vac.

Switching on the heat at your thermostat closes the circuit between the furnace start capacitor and the blower motor. This provides the motor with extra torque to overcome the inertia of the fan.

What Are the Symptoms of a Bad Furnace Capacitor?

As you might expect, a bad capacitor can keep your furnace from starting up at all. In most systems, a faulty capacitor will send no power to the blower motor, so your furnace won't move air at all. A furnace breakdown caused by capacitor failure is often sudden with no noticeable symptoms from your furnace beforehand.

If the furnace does start, the altered impedance of a bad capacitor may disrupt the current flow to the motor. This can cause issues related to erratic operation of the blower motor, like short cycling or overheating. You may also hear a humming or clicking sound when you turn on your furnace as the capacitor tries to engage.

How Is a Furnace Start Capacitor Repaired?

An HVAC technician can easily replace your furnace start capacitor if it's faulty. The three most important factors for finding a replacement capacitor are the voltage output, microfarad capacity, and the size of the housing.

The label on a furnace capacitor indicates whether it is rated for 370vac or 440vac output, as well as its microfarad capacity. Finding a replacement capacitor of the same size will ensure that it fits into the mounting bracket properly.

When you call for professional furnace capacitor repair, your technician will take care of finding the right capacitor for your furnace. If possible, your contractor will order an original part from your furnace manufacturer to ensure maximum compatibility.

The start capacitor is a small but essential component inside your furnace. If your furnace fails to start or runs erratically, call an HVAC contractor service such as Turnbull Heating & Air Conditioning to inspect your furnace capacitor.