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Gas-fired furnaces


July 10, 2009
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I have a gas-fired furnace. What sounds does the furnace normally make?

Thereare many different gas-fired furnaces, and the normal sounds will varyfrom brand to brand. To determine what is normal for your system, visityour furnace every so often and give a listen when things are runningwell. That way, if there is something that qualifies as a JDSR (JustDoesn’t Sound Right) you will be a step ahead of the game. Here aresome sounds that many normal gas-fired furnaces make:
  • At thebeginning of the heating cycle, you may hear the sound of a smallblower operating. This will be within the first minute of thethermostat's call for heat. The component creating this noise is a ventblower motor. Some furnaces will operate a small blower at thebeginning of the cycle to remove any gas from the combustion area andflue (vent) pipe before allowing the furnace to fire. At the beginningof the cycle, this may be the only sound you hear as the operation ofthis blower initiates the start-up process. A pressure switch sensesthe difference in pressure that the blower creates and this allows theburner to start.

  • After about a minute or so, you willlikely hear a rapid sparking or clicking sound. This is the sound of anelectric spark that will light the pilot on some furnaces. Once thepilot light is lit, the spark will stop.

  • After thesparking, you will hear the whispering of gas flowing into theappliance. You should be able to hear this because the main blower isnot going to be operating yet.

  • After a minute or two, themain blower will turn on. In the heating mode, the temperature withinthe heat exchanger controls the blower. At the beginning of the cycle,the gas rack will be lit, but there will be no fan operation. At theend of the cycle, the gas rack will turn off, but the blower willremain on to dissipate the heat inside the appliance. This helps theheat exchanger cool slowly and keeps it from cracking.

I have a gas-fired furnace. What might indicate that there is a problem with my system?

  • Yousmell gas. If you do, get out of the building immediately and call theFire Department. Don't hesitate. Let the experts locate and fix theproblem. Do not call the Fire Department from inside your home. Go to aneighbor’s house or call from your cell phone from outside.

  • There'sa noticeable reduction in airflow coming from the supply grills.Check,clean, or replace your air filters first. If the filter is clean,there may be a problem with the blower motor or the blower itself. Ifthere is an electrical problem with the motor, it may not be turning atthe correct speed, and that will slow the flow of air. This willeventually lead to the overheating of the appliance and the opening ofthe internal safety switches, which are there to protect the system.Other causes for this situation include a dirty blower wheel, internalduct blockages, and closed or blocked supply grills.

  • No airmoving and there's no heat. Listen for any noises, such as blowersrunning or things clicking. If there are no noises, there is probablyan electrical problem within the system. Check the main fuses andcircuit breakers to see if there is power going to the system. If themain fuse is blown, or the circuit breaker is tripped, determine thecause before replacing the fuse or resetting the breaker. There may bea problem with the combustion blower. Remember that the combustionblower starts the entire process.

  • Cold air is blowing fromthe supply grilles. This may or may not be normal. If the fan is in theon position, the blower will be moving air through theduct systemcontinuously, not just when the heaters are energized. If the fan isnot in the on position, and the thermostat is calling for heat, theremay be a problem with the fan switch. The fan switch determines thetemperature at which the blower turns on and off when in the heatingmode. If this switch is not set properly, blower operation may seemerratic. To avoid an unnecessary charge, check to see if the thermostatis calling for heat before you call for service.

There is a smell of gas coming from the furnace. What should I do?

Turnoff the appliance (if you are already next to it), and leave thestructure immediately. Call the Fire Department and let the expertslocate and fix the problem. Do not call the Fire Department from insideyour home. Go to a neighbor’s house or call from your cell phone fromoutside.