You rely on your furnace to keep your warm all winter long, but these appliances can develop various problems due to age, inadequate maintenance, or bad luck. You know something is wrong when the heat stops, but your furnace may be throwing warning signs your way long before the situation devolves to this level. If you know what to look for, your furnace flame is an excellent diagnostic tool.
Experienced HVAC technicians can easily recognize the signs of trouble, but it's often more challenging for do-it-yourselfers. If you want to give this diagnostic method a shot, find your furnace's sight glass and follow this handy guide to burner flame colors.
Steady, Blue Flame
When your furnace is correctly operating, you should see a steady, blue flame through the sight glass. A flame that burns blue is burning hot and, more importantly, thoroughly burning all of its fuel. In fact, the blue flame that you see through your sight glass is burning at several thousand degrees Fahrenheit. In most cases, this means you have "clean" combustion — you are only burning gas and oxygen.
If you're having issues with your heat despite a consistent blue flame, then it's unlikely that your problem lies with your burner, gas supply, or oxygen supply.
Yellow or Sputtering Flame
Yellow or red flames are much colder than blue flames, and they usually indicate incomplete combustion. A gas or propane furnace requires two elements to burn successfully: fuel and oxygen. Under ideal circumstances, enough oxygen is available in the combustion chamber to thoroughly burn all of the fuel. For natural gas, this means a ratio of about ten parts oxygen to one part fuel.
Yellow flames almost always indicate an imbalance in the fuel-to-air ratio, but the causes can vary. A blocked air shutter may be starving the furnace of oxygen, for example. There may also be an issue allowing too much gas into the chamber, preventing it from burning completely.
Red, Yellow, or Orange "Pops"
You may also notice small sparks within the flame, usually in cooler colors such as red or orange. These little "pops" typically indicate a dirty burner, and they're usually a sign that something is burning other than fuel and oxygen. As with a sputtering flame, this reduces your furnace's efficiency and can ultimately lead to soot buildup in the combustion chamber.
Anything other than a bright blue flame means that your furnace is operating inefficiently. While fully diagnosing combustion problems will usually be beyond the abilities of most do-it-yourselfers, recognizing them can still save you money. Once you spot a problem with your furnace's burner, immediately contacting a furnace repair professional can help you to resolve it before it leads to a costlier issue.
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