Central AC/Heat Blower Fan won't shut off

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  #1  
Old 12-28-00, 10:18 PM
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Older central ac/heat unit Heat=GAS
Burner seems to cycle as per thermostat
Blower fan run continuously-never shuts down
1 yr. old cheap white-rodgers thermostat replaced today
No change.
What do you think could be causing this? Will running the fan continually cause any damage? It's cold and if I kill the power, I have nothing.
I appreciate your opinion. I plan on doing the repair myself.

 
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  #2  
Old 12-29-00, 07:47 AM
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Talking blower running

since it is an older furnace ,most likely the fan/high limit switch has gone bad. this controls the fan and/or burner if the temp goes too high.if the burner shuts down with the t-stat then this fan limit is bad.it depends on how handy you are....the limit is normally above the burner..a small cover box...you would have to kill the power.remove wires..mark the wires..remove limit..take it to a heating supply house and get the proper length for your furnace...when re-installing make sure you don,t turn the dial alot...also you have to know how the old was hooked up as some high limits use a combination of high and low/control voltage...there may be a tab you have to remove to seperate the two.....
 
  #3  
Old 12-29-00, 09:00 AM
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Thanks, SeaBee
Thanks so much for your reply and helpfulness. I have located the limit switch but have another question. You stated the following: >>>>"also you have to know how the old was hooked up as some high limits use a combination of high and low/control voltage...there may be a tab you have to remove to seperate the two....."
I'm not sure of what you mean. If I mark my wires and compare the switches, wouldn't i be able to tell if it uses a combination of high and low control voltage? What will happen if I hook it up without removing the tab and I really should have? Will I burn up the switch? Or will i get a second chance to remove the tab? I'm guessing this unit was originally installed around 1979. Did the older units use the hi/low combo more than not?
I know I'm capable of doing this. Thanks again. I'm getting tired of going in and out, flipping the breaker just to kill the fan. Having it run constantly can't be good for it.
 
  #4  
Old 12-29-00, 03:41 PM
fjrachel
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IF you have dual voltage at the fan/limit and do not remove the tab you will burn up your gas valve and almost everything else in the low voltage circuit. There are instructions with new fan/limits that show location of tab. The only sure fire way to know is through the schematic or test with volt meter. Also you may have something as simple as a shorted circuit in the t'stat control wiring to the furnace. To check, turn off power, go to controls that t'stat wire goes to in the furnace, remove the wire going to the 'G' connection(it may be a wire or screw point), tape it off, put covers back on, turn of electric. If the fan stops running it's the wiring. If not, back to the fan/limit or fan/ac relay may be stuck.

[Edited by fjrachel on 12-29-00 at 05:48]
 
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