A Loud Banging Noise Happens When My Furnace Starts Up?

First we need to determine exactly when the furnace starts up as opposed to the fan motor starting up. So is your furnace making a loud banging noise when:

• The Main Burners Ignite
  The Fan Motor Starts Up

The Most Often Causes of a Furnace Banging When Starting is:

Delayed ignition is the most common cause of a banging furnace. Delayed ignition in the furnace is the act of filling the burner chamber or burner chambers with propane or natural gas and then igniting them. This isn’t the nice smooth whoosh of a pilot igniting the burners. The gas rolls out and touches the pilot and ignites in an explosion. This explosion results in flame roll out and can cause the furnace gas valve and other controls to burn up resulting in a large amount of financial damage and could result in a loss of life

What Causes Delayed Ignition

Delayed ignition the furnace burners are caused by:

• 
  Dirty and Rusted Burners
  Improper Air Fuel Mixture
  Misaligned or Plugged Burner Spread Tubes
  Improper Gas Pressure Settings in the Gas Valve

Dirty Furnace Burners

Burners can become plugged with spider webs, rusted, lint and dirt over time. This is one of the many reasons why each and every gas burning furnace (propane or natural gas) needs a yearly checkup and tune up. If the burners become plugged the gas flame does not spread up and down the burners evenly. A small area of skipped flame means that the rest of the burner does not ignite until the gas builds up in the back of chamber and starts to roll out towards the front flames. This results the furnace making a booming sound.

Improper Air Fuel Mixture

In the front of nearly all burners in a setting that can be manually adjusted. Unfortunately it rarely gets addressed. It is an old wives tale that states that the flames of a propane or natural gas burner need to be all blue. They are never all blue and attempting to make them that way will negatively affect the performance of your furnace and could lead to a dangerous situation resulting in the booming noise which is really a small explosion in your furnace.

Misaligned or Plugged Burner Spread Tubes

We see his quite often after the furnace gets a tune up or gets a repair by someone is really isn’t qualified to perform it. Tuning up a furnace each year for the winter is a serious business, sure, most of the time it doesn’t seem like a lot is being done, but that is because most furnaces are now very well made and only need small adjustments to make them work perfectly and, if you know what you are doing, it always seems easy. We have kept the investment amount of our heating and air conditioning Ultimate Maintenance Agreement the same for 20 years. The burner spread tubes, that is the part that allows the flames to skip from one burner to another burners, are not on all furnace designs. If these become misaligned for any reason, or plugged, the flame cannot skip easily from one burner to another resulting in an ignition of a burners or set burners. The result is a delayed ignition explosion.

Improper Gas Pressure Settings in the Gas Valve

A gas valve set at 1,000 feet of elevation is not the same as one set at 2,500 feet of elevation. Occasionally gas valve diagrams can become defective allowing too much or too little gas through them to the main burners. This is why we test the gas valves that we are working on to verify that the working gas pressure exiting the valve is the correct one for the furnace at the elevation that they furnace is located. Too little gas pressure can result in the gas (propane or natural) dribbling out the burners and missing the pilot or ignition system until the natural gas or propane builds up and results in a booming explosion in the furnace. Too much gas can result in the gas bypassing the first part of the burner altogether and coming out the back of the burner in the furnace until the gas builds up resulting in a booming furnace explosion.

The Second Most Often Cause Happens When The Fan Motor Starts Up

If your furnace doesn’t make a booming noise on the startup of your furnace and instead makes that furnace booming noise after the main blower motor turns on you could have any one ofhte following:

• Cracked Heat Exchanger
  Poor Quality and Fitting Sheet Metal Ducting
  Too Much Static Pressure in the Supply Air Ducting
  Too Little Static Pressure in the Return Air Ducting

Cracked Heat Exchanger

If the heat exchanger, firebox, is cracked the actions of the blower pressurizing the other side of the heat exchanger could cause enough air to blow into the furnace burner chambers to actually extinguish your burners or a portion of the burners. Once the flame is extinguished the gas free flows in the furnace chamber until enough builds up to ignite causing an explosion. This is extremely dangerous.

Poor Quality and Fitting Sheet Metal Ducting

Sheet metal for air ducting in home is required to be at least 28 gauge in thickness. There has been a considerable amount of short cutting in the industry and using a thinner gauge sheet metal, 30 gauge, is cheaper and easier to work with. It will work just fined on small pieces of flat sheet metal stock, but when faces with a larger section the metal grows weak over the years and the pressure increase in the ducting from the blower motor constantly stretched the metal. The average standard blower motor is designed to optimally run at 0.5 inches of water column. That means the pressure will move water up a tube only 0.5 inches. Translate this into a 3 foot by 5 foot piece of flat stock sheet metal. That 3 x5 piece of sheet metal has 2160 square inches of space. Multiply that by 0.5 inches of water column and we have pressure that is equivalent to 1080 inches of water column. Converting water column pressure into pounds per square inch (multiply by 0.036127) we find that the blower motor forces 39 pounds of pressure on that piece of sheet metal. It can get pounded like a drum in a high school band.

Too Much Static Pressure in the Supply Air Ducting

Too much static pressure in the supply air ducting, this is the ducting after the furnace that contains the conditioned air, can cause significant problems with a booming noise. I have seen furnaces with 1.5 inches of water column in the supply ducting. That means that 3 x 5 piece of sheet metal has 117 pounds of pressure being exerted against it. The metal can stretch, bind up, and then release resulting in your own little marching band in your attic.

Too Little Static Pressure in the Return Air Ducting

Too little static pressure is what occurs when the return air ducting is too small. Just like the pressure create in the supply air ducting can make a booming noise the vacuum present in the return air ducting can do the same thing. Too much negative air pressure is every bit as bad for your heating and air conditioning system as too much.