No heat, fan working

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  #1  
Old 11-09-05, 06:23 PM
Shaenon
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No heat, fan working

Hi, we have a gas furnace that automatically ignites the pilot light.
There is no heat being produced however, but the fan works - so cold air is blowing.
I have played around with the thermostat and set the heat higher than usual. I can hear the furnance click and an igniting sound...but then it quickly shuts down. I can see a green light on the furnance, I am not sure what this means.
This has been happening for a couple of weeks, however, every 4 days or so the heat will come on for a whole day.
Does anyone know what the problem could be?

Thanks! Any help is much appreciated - we are freezing!
 
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  #2  
Old 11-09-05, 06:58 PM
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No heat

The fan comes on because it is on a timer & will come on whether or not the furnace lights.
While you are looking at the burner area of the furnace, have someone turn up the thermostat. Does the pilot light? Does the main burner(s) light? Is that green light on the gas valve or in the blower compartment?
 
  #3  
Old 11-09-05, 07:38 PM
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What brand is it?
 
  #4  
Old 11-10-05, 04:31 PM
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Originally Posted by Grady
The fan comes on because it is on a timer & will come on whether or not the furnace lights.
While you are looking at the burner area of the furnace, have someone turn up the thermostat. Does the pilot light? Does the main burner(s) light? Is that green light on the gas valve or in the blower compartment?
There is no such thing as a timer with a furnace fan. It strictly runs off sensing temperature. It may 'appear' to be timed, because often, it just so happens that the cycle is quite repitious.

The burner flame needs to be watched before you can come to any conclusions here. Tell us EXACTLY what happens throughout the sequence. All modern high efficiency furnaces act sort of this way:

The house calls for heat.

A relay triggers the ventor exhaust motor to come on.

After a number of seconds, the flame tries to ignite by either a hot surface ignitor that glows real bright yellow, or an automatic sparker.

Then the burner fires up.

The burner has to burn for like 2 minutes before anything else happens so that the fan switch will determine that the temperature outside the heat exchanger is hot enough to allow the fan blower to come on and circulate nice warm air throughout your house. The reason for the delay is, that if the fan came on right after the burner came on, you would feel a windchill effect in your house. So, you have to let the furnace insides warm up to over 100 degrees first, before the fan will start.

Then, either the thermostat getting shut off will kill the flame...OR...the high limit on the fan switch will kill the flame.

....................................

Now, knowing THIS information..tell us what YOUR furnace does, or doesn't do right, according to what I posted.
 
  #5  
Old 11-10-05, 06:26 PM
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Bonehead
You may want to learn about "fan timer boards" before you make such a harsh statement.

Once again what brand & how old is it?
 
  #6  
Old 11-11-05, 04:13 PM
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I recommend anyone into furnace repairs to go here:

http:***********.htm

mbk3---I am looking into these. Briefly tell me the purpose to the fan timer, if you will. Maybe *I* will learn something for a change.
 

Last edited by mattison; 11-13-05 at 08:13 AM. Reason: Lets keep the DIY help here.
  #7  
Old 11-11-05, 06:10 PM
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Hi Bonehead
The "Fan timer board" or" board" as is the case in most furnaces, allows the fan to come on without using thermal activity directly on a fan switch. They board goes thru a sequence and once the flame is reconized it turns fan on in predetermined amount of time. Also some furnaces used to use 'heat asist fan controls. Once t'stat was energized it would send 24 volts to resistance wiring in fan switch & fan would start even if burner never came on. I believe once you investigate, you will see how prevelant they are.
Best wishes in your quest for knowledge
 
  #8  
Old 11-11-05, 06:34 PM
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Now I wish I never said what I did about the fan timer board because do you know what? When I went to Google and brought one of these up on the screen, I actually recognized the picture! I changed one of these out on a malfunctioning furnace about 3 years ago. (And the diagnosis was correct. The new one fixed it.)

So, I apologize for making it out that posters who mentioned fan timer switches or boards did not know what they were talking about. It was more of a case like my uncle who had the guests at his house check their shoes for the dog doo-doo he smelled, only it was HE who had the dog doo-doo on his shoes!!
 

Last edited by mattison; 11-13-05 at 08:14 AM.
  #9  
Old 11-11-05, 06:54 PM
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Bonehead

This may actually belong in a Private Message but your admission of error and your appology on a public forum deserves, in my book anyway, an ATTABOY. It takes a big person to make such a statement. Apology Accepted.
Please hang around & help us out. We can use all the help we can get. I certainly don't know it all & hopefully never will & I think the other regulars will feel the same way. There will be some point in time where you will come up with a solution everyone else has missed. Welcome Aboard.
 
  #10  
Old 11-11-05, 07:34 PM
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Bonehead......We all learn everytime we listen. Thank you for your sincere reply
 
  #11  
Old 11-13-05, 08:18 AM
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Shaenon,

If you remove the front cover of your furnace where the burners are look to the far left side and there should be a small rod about 3" inches long extending into the burner area. Using a dollar bill or scotch bright pad lightly clean this rod and see if this solves your problem.

A common problem of a furnace firing then shutting down like yours is doing is a dirty flame sensor.
 
  #12  
Old 11-13-05, 10:01 AM
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Originally Posted by Grady
This may actually belong in a Private Message but your admission of error and your appology on a public forum deserves, in my book anyway, an ATTABOY. It takes a big person to make such a statement. Apology Accepted.
Please hang around & help us out. We can use all the help we can get. I certainly don't know it all & hopefully never will & I think the other regulars will feel the same way. There will be some point in time where you will come up with a solution everyone else has missed. Welcome Aboard.
Thank you. And I was most intrigued by your statement that you hope you never know it all. Just like...do we all REALLY want to know what really went on at Roswell in 1947, or if some crop circles are for real?

I think in a forum like this, we will all learn something from each other. All it could take is learning one little thing, instantly, from some poster, that could save a person a lot of timin future encounters.
 
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