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The fan normally runs for a few minutes or so after the gas shuts off.
This removes the heat from the furnace and transfers it into the living space instead of just letting it possibly cause overheating of the furnace until it dissipates. If it is a fan that blows outside, that fan also needs to run for a few minutes to clear exhaust gasses from the furnace - to keep them from dissipating into your home.
I am not familiar with this unit but It sounds like the thermal switch that turns on the blower (bonnet control) has failed. There is also a safety that will shut it down if it does not detect the appropriate air flow in a short time. You can check here for more information on the blower: http://www.tommark.com/FascoDraftInducerCrossReferenceGuide.pdf
The ventpipe heat limit, fan limit, and chamber limits are only a few of the safeties that turn a furnace off when something is getting too hot. I recommend a qualified technician have a look at it.
Hello, sounds like the limit switch is opening on the furnace.Usually when a furnace goes off on its high limit is becuase the airflow is being restricted making the furnace overheat. a limit switch is there to protect the furnaace from overheating, most cuases are either dirt or the blower motor not running up to speed, anything that cuts down on the airflow will cause the high limt switch to open.
Could be a defective thermostat in the plenum.
When a gas furnace shuts down the fan(blower) will run until the plenum cools to the safe range. Usually 1 and 1/2 minutes.
Intermittent operation is a defective thermostat or relay.
I am assuming you are talking about a Natural Gas or LP furnace. The blower motor is powered by a separate 120 volt circuit, so flipping the 240 volt breaker for the furnace will not shut it off. There should be a regular light switch at the furnace that will turn off the blower for servicing the unit. When you take the cover off the unit, there should be instructions on how to re-set the igniter. If it won't re-set, try shutting off the gas for a few minutes, then turn it back on and then reset. If none of this works, and you can't find the owners manual, call a service technician.
After troubleshooting like mad, I found that the flame sensor (rod that sits within the flame) was fouled with carbon on the surface. I believe this shields the rod from proper heat detection and the sensor reports that there is no flame and to shut off the gas valve to avoid explosion. Once I pulled out the rod and cleaned it with a wire brush, I have no problem whatsoever--fixed.
Hello Genna,
I am not a furnace guy but I may be able to give you a few ideas of things to diagnose the problem and get it resolved. First off, I assume you have a thermostat that you use to to turn your heat on. If it is a generic ( meaning : non programmable model) then you should be able to try this simple test. Remove the cover ( typically they snap on) around the thermostat so you can see the control mechanism inside. You should see few skinny wires ( similar to those used on a telephone wire) solid copper in various colors.. When you rotate the dial on your thermostat to call for heat, there is a small glass vial that has a drop of mercury ( which is electrically conductive) in it that makes contact and shorts two control wires together ad that is what calls for heat on your furnace. When the temp inside the room where the thermostat is reaches the tempertaure you requested, the bimetal mechanism either contracts or expands to reposition that glass vial to shift the mercury off the contacts and your furnace shuts off... That is the basics behind how your thermostat and furnace work ( generally speaking) If you can identify the two wires inside the thermosat that are shorted together when that vial of mercury shorts them inside it.. you can temproarily unhook them ( they are low voltage.. normally 24 Volts or less) so no worries about getting ashock or anything.. and short them together for a minute or two.. by doing that... your furnace should turn on and heat should flow.... Once you start your furnace this way.. unhook these two wires and your furnace should shut down .. It may take a minute or two ( depending on the control for it) If it doesn't..then your problem is on the furnace side and you may need to get the furnace control system serviced or replaced.. if it does shut down, then your problem is your thermostat and thats a simple replacement.. Also.. make sure the thermostat was properly leveled on its base.. The position of the thermostat ( meaning level) will dictate when that mercury makes contact and your furnace switches on.. OK..I tried to explain the works of this to you but here is a link to a Honeywell site that explains it in simpler terms.. The part about shorting the two wires together and then opening them will aid you in identifying where the problem actually is.. here is the link: http://homerepair.about.com/od/heatingcoolingrepair/ss/thermostat.htm
Hope this helps you more than confuses you..
Regards
you say you hear a click. this indicates the relay is opening and closing, BUT the contacts may have welded itself shut. try replacing the relay - contactor.
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