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Serving the South Puget Sound and Western Washington Areas Since 1976

Serving the South Puget Sound and Southwest Washington Areas Since 1976

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What Causes Delayed Ignition in a Gas Furnace?


If you’re a frequent reader of our blog, you’ll know that we often discuss alarming sounds in a furnace that require prompt attention. One of these is a booming sound in a gas furnace. That’s because this sound is often made by a gas furnace’s delayed ignition. What you’re hearing is literally a mini explosion when something is impeding the furnace’s ability to ignite when it’s supposed to. 

There are many possible causes for delayed ignition. No matter what the cause is, one thing is for certain: delayed ignition is something that should be addressed ASAP. Delayed ignition can potentially lead to a fire in your home. It can also cause damage to your furnace when it’s subjected to these constant mini-explosions. Let’s go over 4 possible causes.

1. Moisture Buildup

A common cause of delayed ignition is a buildup of moisture in the furnace. After a period of inactivity, moisture can corrode the firebox in your furnace. That corrosion builds up to the point that it starts to block the ports that feed gas into the burners. When these ports get blocked, the burners down the line cannot light immediately. Extra moisture can also lead to excess rust which can also be problematic and interfere with the ignition process.

2. Dirty Burners and Obstructed Gas Lines

While moisture and rust are the most likely cause of delayed ignition, dust, dirt, grime, and even lint can accumulate and delay ignition. Delayed ignition can also be caused by a sulfur buildup caused by the process of burning natural gas.

These are all reasons why delayed ignition is often a problem we see in dirty furnaces that haven’t received the benefit of annual cleaning that occurs on a maintenance visit. The good news is that a thorough cleaning by a professional HVAC technician should remedy the problem of delayed ignition. Of course, you’ll need to continue to schedule routine tune-ups to keep the problem from happening again. 

3. Electronic ignition problems

While old furnaces used standing pilot lights, most furnaces today have an electronic ignition system. When combustion gas accumulates in the chamber, this can cause a delay in ignition. This results in the booming sound you may hear. Today’s furnaces are equipped with safety mechanisms to prevent this in which case your furnace won’t turn on at all. In most cases, the igniter can be replaced by your HVAC technician.

What happens if delayed ignition continues?

Unless the issue is fixed, the small explosions caused by delayed ignition are likely to get bigger and bigger. While we don’t want to scare our readers, this situation is as bad as it sounds. You really don’t want combustion gas explosions to keep getting bigger in your furnace. 

The excess flames caused by delayed ignition can also crack the heat exchanger which is the most expensive component in a furnace. When this happens, we usually recommend replacing the furnace instead of getting a new heat exchanger because a new furnace is often cheaper. 

Count on the team with the most experience. Contact the team at Sunset Air today for furnace services in Olympia, WA.

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