Everything You Should Know About Heat Pumps

Does Your Home's Oil Furnace Have A Faulty Photocell?

14 April 2023

Jessica Jimenez

Modern oil furnaces are extremely reliable and are a convenient and cost-effective way to heat almost any home. Unfortunately, they can still occasionally suffer from mechanical problems, just like any other type of furnace. In many cases, the fault lies in one of the smallest and most innocuous components of an oil furnace — the photocell.

What Are Photocells In Oil Furnaces?

The photocell, also known as the cadmium sulfide or cad cell, is an important safety feature in your home's oil-fired furnace. It is a small, ceramic disc, which is coated in a compound that reacts to visible light.

Oil furnaces use these photocells to determine whether there are flames in the combustion chamber. If the furnace is set to activate, but the photocell already detects light from a burning flame, it will automatically stop extra oil from flooding into the chamber and creating an explosion hazard. Similarly, if the flame in the combustion chamber goes out, the photocell will stop the oil supply, preventing the chamber from flooding.

Photocells are robust devices and are usually sealed inside glass ampules for added protection. However, they are still exposed to the intense heat inside your furnace's combustion chamber, which can cause them to degrade and fail over time. They can also lose their effectiveness if they become caked in soot, a problem found in many furnaces using cheap, low-quality heating oil.

What Happens If Your Furnace's Photocell Fails?

In many cases, the first sign of a failed photocell is a furnace that fails to function at all. If the furnace's circuitry does not detect the increased electrical resistance generated by a functioning photocell, it will automatically shut off the power for safety reasons. If your oil furnace has a digital readout screen, it may show an error code indicating that the photocell has failed.

If your furnace's photocell is functional but has been coated in soot or oil deposits, your furnace may start short cycling. The furnace will ignite because the photocell does not detect any visible light — however, the soot will prevent it from detecting light even after the chamber is lit, causing it to shut off the oil supply.

Short cycling causes your furnace to activate and deactivate itself very quickly. It prevents your furnace from generating a useful amount of heat and wastes a significant amount of precious heating oil. It can also cause excessive wear to your furnace's moving components, such as oil valves.

Can Faulty Photocells Be Repaired?

If your oil furnace is showing signs of photocell failure, you should call in a professional furnace repair service to inspect and repair your furnace. These services will check your furnace's photocell for signs of failure, and will also inspect other components (such as the solenoids and control circuits that can cause similar problems to failed photocells.

Because photocells are small, intricate devices sealed in delicate glass ampules, they are virtually impossible to repair. If your furnace's photocell has failed, it will need to be replaced. Fortunately, replacement photocells are usually relatively inexpensive.

If your furnace's photocell is functional but dirty, your service can give it a thorough cleaning and recommend ways to prevent soot build-up and prevent future problems. If the photocell is beyond saving, it can be replaced swiftly, getting your furnace back up and running as soon as possible.

For more information about furnace repair, contact a local company, like Paris Heating and Cooling.

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