Is your gas furnace making a loud banging or thumping noise when you turn it on? At Minnick’s, we commonly receive calls from homeowners describing startling noises coming from their heating systems. There are several things that can cause your furnace to make a loud banging sound when it turns on, but typically, your furnace is most likely experiencing what is known as a delayed ignition.  In this article, we will cover everything you need to know about delayed ignition and what you need to do to fix it and protect your system and home from damage.

What is Delayed Ignition?

When your thermostat signals for heat from the furnace, a gas valve opens, sending gas to the furnace’s burners for ignition. Delayed ignition occurs when that gas isn’t ignited immediately, hence the name delayed ignition. This delay allows the gas to build up in the furnace until it finally ignites, causing a small explosion inside your furnace. This mini-explosion can cause permanent damage to your furnace and poses a fire hazard to your home.

What Causes Delayed Ignition?

Delayed ignition usually occurs when you first turn on your furnace after a long period of inactivity. When you don’t use your furnace for an extended time, moisture can begin to accumulate and corrode the firebox in your furnace. If too much corrosion builds up inside your furnace, it can start to block the lines that supply gas to the burners. When these lines are blocked, the burners will not have enough gas available to ignite immediately.

Rust and corrosion are common culprits for blocking your furnace gas lines, but they are not the only possible causes. If you don’t have your furnace routinely cleaned, lint, dust, and sulfur buildup can be problematic for your gas lines. Sulfur build up is a natural byproduct from burning natural gas. It will appear as a white powdery substance on the surface of the burners or the pilot light.

Some other possible causes for your furnace delaying igniting are:

  • Too much air mixed with the gas, making it hard to ignite
  • Too little gas at the burners caused by low gas supply pressure
  • Dirty, restricted or weak pilot light (common with older furnaces)
  • Misaligned or dirty burners

Furnace Parts

How to Avoid Delayed Ignition?

Now that you know that delayed furnace ignition is caused by debris blocking your gas line, to avoid it, all you need to do is have your furnace cleaned before turning it on after a long break. An HVAC technician can clean the burners and gas lines to remove any dust, lint, rust or sulfur buildup that might block ignition and cause a delay. Working on a gas furnaces is dangerous for anyone who lacks professional training, so the cleaning is best left to a HVAC technician. It’s also important to note, the longer you wait, the greater the chance that the gas lines will become completely clogged and the furnace will fail to ignite at all.

Other Noises to Be Cautious Of

It’s not uncommon for furnaces to emit strange noises. Normal sounds you may hear are tiny little pops or hums as the furnace runs. However, when your furnace is louder than usual and you start hearing startling noises you have never heard before, then it is time to talk to your HVAC technician about some maintenance and repairs. In addition to the banging and thumping, here are some other furnace noises that you should not ignore.

  • Clicking: A clicking noise is normal when you are turning your furnace on and off. However, if the clicking noise continues this could be an indication that something is faulty with the gas, relay, wiring, ignition or even electrical control.
  • Scraping: Hearing a scraping noise from your furnace means it’s highly likely that the blower wheel is in trouble. The scraping noise can occur because it’s possible that the wheel is loose, fallen or has broken entirely.
  • Rattling: A rattling noise from your furnace could be from something as simple as a loose cover panel or a few loose screws. But rattling noises can also be tricky to diagnose as they can also be an indication of a more significant problem with your furnace such as a malfunction in the furnace’s motor or even a leak in the heat exchanger.
  • Squealing: A squealing sound from your furnace is not a good sign and can be the result of a damaged blower belt or blower motor. Squealing could also be a sign that the bearings need some oil.

Regardless of which of these noises you hear, you never ignore them. It’s always best to contact a professional HVAC technician as soon as you notice them. Minnick’s recommends signing up for an HVAC maintenance plan to make sure you never forget about having your furnace cleaned and inspected.

If you’re a Maryland homeowner concerned about the noises coming from your furnace, Minnick’s offers a thorough 16-point furnace inspection that will uncover any potential issues with your furnace and help keep it running safe and efficient all year long.