Before putting a gas-fired water heater through an extensive series of tests, there is a lot that can be learned about its combustion process by simply studying the burner flame.
Part of being a plumber is installing and servicing gas appliances such as gas-fired water heaters.
Even though there are ways to put a water heater on an extensive series of tests up to and including a full combustion analysis, there is a lot you can learn about the combustion process just by being patient and observing the burner flame.
What is in a flame? When you look into a burning chamber — whether it is a water heater, boiler or furnace — do you know what you are looking at? Most technicians stick with the basics — is it blue or yellow? Yellow equals incomplete combustion and blue means everything’s good, right? Sort of.
What causes a flame to change?
Let’s start with what can cause a flame to change. A flame will change appearance based on the combustion process within the burning chamber. A flame requires oxygen, fuel, and spark ignition (or enough heat to ignite the fuel source).
The amount of combustion air available to burn is a major factor. Think for a moment about an old-school water heater that takes its combustion air from the room that it is in. Now build a 2x2-foot room around the water heater and seal it up so that it has no way to bring in new oxygen to burn. If there is no fresh air available to burn, the flame will start acting crazy and then cease to exist. If it just doesn’t have enough combustion air, it will act a different way.
What does a healthy flame look like?
The flame should have two cones. The first cone closest to the flame source should be the larger of the two and be a light blue flame that seems to have direct contact with the burner. This inner blue flame is unburned gas and air. The outer cone (technically called the outer envelope) should be a darker blue. The inner lighter blue cone should be about 70% of the maximum visible cone height. What changes the height of the inner cone? Oxygen. Allowing more oxygen into the burning mixture will increase the inner cone. Decreasing the amount of combustion air will shorten the inner cone.
A flame that is lifted
Even though you may look into the burning chamber and see a beautiful blue flame, what happens when the flame is raised above the burner and not in contact with it? Even though the flame looks correct, something is wrong. The gas is coming out of the burner so fast that the burner can’t burn it soon enough. It is similar to the circus performer who spits fire — he’s all good as long as the flame is away from his mouth and he blows out at a high velocity. The velocity is so high that the gas is shooting past everything and then finally catching on fire. This usually will happen when an appliance first turns on and then will disappear, but it is still an issue that needs to be corrected by installing a gas regulator at the appliance and making sure you are feeding the appliance whatever gas pressure the manufacturer is calling for.
A dancing flame
If the flame is dancing around all over the place — round and round, side to side, completely unpredictable — this shows you that the flue is having a problem. The flue more than likely is blocked or at least partially blocked. On a traditional tank-type water heater you will occasionally see severely damaged outer shells that appear as if someone torched and destroyed part of the water heater. In these cases, you can poke your finger in the side of the water heater and pick apart the shell very quickly. A dancing flame usually causes this because of a flue blockage, and you need to investigate before putting in another water heater. The fire is burning, but the flue cannot escape the chamber fast enough and is swirling around the burn chamber, causing the flame to act erratically.
Blue flame with yellow tips
This is common — a nice, blue flame but the flame tip is yellow. Before you come to the conclusion that the unit is having an issue, make sure dirt isn’t the culprit. When dust/dirt is in the chamber after you’ve moved some of the controls around, it will burn and cause the flame to appear yellow. If you have verified that it is a consistent yellow tip, you can conclude that the unit is not receiving enough oxygen for the burn.
Flame only when you remove the combustion door
If you happen to be working on a sealed combustion unit, and you can only get the main burner to come on when you open the combustion door, the intake air ports on the side of the water heater may be clogged.