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Loud Banging Furnace

The loud bang that you are hearing can be one of two things. Both of these must be repaired as both of them are dangerous to the health of your heating system and one of them is dangerous to you.

Delayed Ignition

The first loud bang problem comes from delayed ignition. It is an explosion of a gas filled firebox igniting all at one time. Most furnaces either have a pilot light or the ignition system lights a pilot light. The gas valve then turns on the main burners by releasing a large amount of gas. The gas ignites the first burner and passes to the second and then the third and so on. There is a bridge between each burner to carry the flames through the furnace heat exchanger. If that bridge doesn’t carry the flame over to the next burner that burner chamber fills up with gas and, when at capacity, spills out into the front area of the furnace. This then covers the bridging flame area that is not working with gas and ignites from the neighboring burner.

This can actually blow the front panel off the furnace and cause fires. It can blow the pilot out and cause the furnace to shut down. This is something that really needs to be resolved before using the furnace again. It is extremely dangerous.

These are some of the problems that can cause a Delayed Ignition Situation

  • The pilot light is too small
  • The pilot light is loose or positioned improperly
  • The gas valve is not letting enough gas through it (Low Pressure or Low Flow)
  • The gas valve is letting too much gas through it (High Pressure)
  • The air fuel mixture is set improperly
  • The furnace blower motor is on and there is a crack in the heat exchanger blowing on the burners
  • The burners are coated with rust not allowing the rear of the burner chamber to ignite

Sheet Metal

The second loud bang problem that we run into is poorly constructed sheet metal plenums. The plenum is that box on the top of your furnace. If the metal is too large, to thin or is poorly constructed it can let out a loud bang when the furnace blower motor turns on.

When the blower motor turns on the pressure inside the ducting increases. Most systems are designed to operate at about 0.5 inches of water column. One inch of water column equals 0.0361273 pounds per square inch. If the sheet metal plenum is 27” x 36” (a common size) it contains 972 square inches of surface for a combined pressure of 35.11 pounds of pressure at 0.5” of water column. If the duct is undersized (very common) we often see pressure in excess of twice this amount.

So picture slapping a 70 weight on a piece of sheet metal. The sheet bows out very rapidly snapping into a different position. This is the bang that you are hearing. It resembles a sheet metal drum. It is annoying and the rapid pressure changes can damage other sections of ducting. The lack of airflow across the heat exchanger due to the undersized ducting can cause it to overheat and prematurely crack.

The Bottom Line…This problem needs to be resolved and should be resolved by someone capable of measuring the airflow through the ducting and determining the correct air fuel mixture at the burners.

Furnace service call costFor Only $39 we will come to your home and diagnose what ever problem you are having with your furnace.

Plus: The service charge is applied towards any work or repair that we do.

Call Today for Fast, Friendly, On Time, Under Budget Furnace Service 1-877-247-6426

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