An electric furnace works by heating up air through an element that warms as a current is run through it. Electric parts are also used as back-ups in heat pumps and starters in gas furnaces. The clicking noises in an electric furnace may not be cause for alarm. However, they could also indicate serious problems with the furnace, so if you here persistent clicking, diagnose your furnace for potential issues.
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When an electric furnace first starts, there are switches inside the unit that respond to the movement of air and the thermostat commands to start up the heating element. These typically cause clicking sounds as the unit first starts, setting processes into motion that will generate enough heat for the furnace to do its job and preparing all systems for the job. This is common, expected and should not last long. If the clicking persists, however, it may indicate a serious problem.
If the clicking is persistent, especially while the furnace is running its hardest, this could be a sign that the heat exchanger that passes heat through metal into the air safely has formed a crack. The crack is causing the metal to expand and contract, thereby making the noise. The element itself may also be damaged. Both these problems are serious and could easily cause permanent damage and a fire, so homeowners who believe this is the issue should shut down the furnace and call a professional for help.
When filters become clogged with dirt and dust after being left in the furnace too long, they can develop clicking noises of their own as air resistance increases and problems with air flow develop. If the heater is clicking and heating more slowly than usual, the problem may be an air filter that has not been replaced in the last several months to a year. This problem is easy to deal with, however, as you only need to replace the filter and see if the clicking issue is resolved after doing so.
In many gas furnaces, the gas is lit through an electrical starter system that uses two electrodes to create a spark. If these electrodes fail or become misaligned, the clicking may be a sign of the unit trying to power up and work but for some reason is consistently failing. If the gas furnace does not work at all, it may be a sign of electrical problems.