Few things are as frustrating as trying to use a furnace whose pilot light keeps going off or can't light up. Below are common issues that can trigger such problems.
Dirt and Corrosion Buildup
The pilot light has a small orifice at its tip. The gas that keeps the pilot light burning flows through this orifice. Over time, debris or corrosion can clog the orifice and reduce its effective diameter. The clogging decreases the volume of gas passing through the orifice, which affects the efficiency of the pilot light.
The thermocouple is a metal rod that acts as a safety device for your furnace's operations. When the pilot light's flame gets hot enough, it heats the thermocouple, which then expands and switches on the gas valve. Thus, the burners can only light up if the pilot light's flame is hot enough.
Unfortunately, a defective thermocouple might prevent gas flow even if the pilot light flame is adequately hot. Such a malfunction might occur if the thermocouple is broken, not properly lined up with the pilot light, or has debris all over it.
Your pilot light will also go off if it doesn't receive an adequate gas supply. Thus, anything that restricts gas supply to the furnace will affect the pilot light's operations. An example is if the gas valve is maladjusted or if your house doesn't have adequate gas pressure.
Strong winds or drafts can blow off the pilot light flame. Strong winds might reach the pilot flame if you have installed a wrong location where it's in a direct wind path. A draft can also affect the pilot light if there are cracks on the wall or something has damaged nearby weatherstripping.
Too much air can affect the pilot light — too little air will also affect it. Combustion, which is how your furnace produces heat, requires oxygen. The oxygen comes from the air around the furnace, which can only get into the furnace if the exhaust or spend gases get out via the vents. Improper venting restricts the volume of oxygen available to the pilot light, so the pilot light stays off.
Cracked Heat exchanger
Lastly, the pilot light can also blow off due to a cracked heat exchanger. The heat exchanger is a series of metallic tubes that transfer heat from the burners to the air without circulating dangerous gases. If the heat exchanger cracks, however, air might blow into the furnace and affect the pilot light.
You might not know what is wrong with your furnace merely by looking at it. Consult a heating repair technician to diagnose the pilot light issue and offer you a solution.