Furnace Blower Problem


  #1  
Old 12-28-02, 07:03 AM
plmask
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Unhappy Comfortmaker Furnace Blower Problem

I have an 8 year old 92% Comfortmaker furnace problem. On several occations over the last few months, I have found the blower continuously running with no heat coming from the vents. The thermostat is calling for heat, but the flame is not on. If I switch the unit off then back on using the thermostat switch, the furnace begins heating again. The length of time it will keep working varies, but it will eventually do the same thing again. It seem to do it more frequently when the outside temperature dips below 30 degrees F. I have cleaned, then replaced the flame sensor thinking that to be the most likely cause, but the intermittent problem persists. The control unit is an Inter-City 50A50-131. I have also tried disconneting the water sensor below the combustion chamber to see if that was the problem with no change. Is there something else I can try, or is there a service technician in my future? Thank you.
 
  #2  
Old 12-28-02, 07:24 AM
DalJones
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...I think you and I have the same furnace and maybe the same problem....
 
  #3  
Old 12-28-02, 08:52 AM
plmask
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I read your post regarding you issue with a Comfortmaker furnace. Your issue looks different than mine. The ignition sequence on mine works fine, the furnace actually works most of the time. I think what mine is doing is that somewhere during a normal heat sequence or at the end of one, the flame goes out and the blower keeps running. The blower will not shut off until I do it manually at the thermostat (or power switch on the furnace) and then back on. The unit then works fine again till the next cold blower episode.

Pete
 
  #4  
Old 12-28-02, 09:29 AM
bigjohn
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On todays furnaces, when one of the limit switches opens, the burners are shut down and the blower runs full time on the heating speed. Things to look for are dirty firlters, dirty blower wheel, vents closed off, return grill blocked, maybe the flue pipe is partially obstructed, dirty cooling coil, if you have cooling, - anything that would trap heat in the furnace or cause poor airflow. It's also possible that you could have a malfunctioning limit switch but you would have to catch it right when it opens. Going off intermittently makes me think- limit switch going on the blink little by slowly. If it was the wrong temperature range, it would have started causing problems when the furnace was still new.
 
  #5  
Old 12-28-02, 10:43 AM
plmask
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Hello John, I just caught the unit blowing cold air and noticed the controller status LED blinking 3 times with a pause, then blinking 3 times again. Do you know what kind of fault that indicates? It is an Inter-City 50A50-131. Thanks for the reply.

UPDATE: I found the code table on the furnace. 3 flashes means the pressure switch is stuck open. I checked the vaccuum lines and they seem OK, the diaphram also seems to work OK when the combusion blower starts to spin. I plan to check the switch status the next time it happens with an ohm meter. Any other thoughts or suggestions?


Pete
 

Last edited by plmask; 12-28-02 at 11:32 AM.
  #6  
Old 12-28-02, 01:08 PM
bigjohn
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Make sure the hoses are clear of any water buildup. Look closely at the arrangement, the switch is probably on the negative pressure side of the combustion air fan meaning that if the hoses are deteriorated and leaking air in, the switch will have difficulty closing. It's also possible that the switch diaphragm has a pinhole leak. The switches are often 2 position, on startup it has to be open which is the ready to start position, then it closes when the combustion air blower comes up tp speed. The combustion air blower is also an item to check. See if it is slowing down during operation. This usually would mean worn bearings in the motor.
 
  #7  
Old 12-28-02, 03:24 PM
plmask
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I noticed some moisture in one of the vacuum lines to the switch after reading your post. There are two vacuum lines going to the switch, one from the combustion blower and the other from the combustion chamber. The hose from the blower was the one with some condensation in it. I replace both lengths of hose.

I also ran a snake down the 2 inch fresh air inlet pipe as well as the 2 inch exhaust vent to make sure they were free from any obstructions. Both were clear. I'll let you know if this fixes it. Thanks again.

Pete
 
  #8  
Old 12-28-02, 07:12 PM
H
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Location: Massachusetts
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vacuum/air proving switch

the only way to know if the switch is working properly is to check the vacuum line with a magnehelic gauge, and compare it with the manufacturers requirements. One way to see if the flue is causing the blockage, is to momentarily remove the stack during a failed firing and if it fires off, you know it is with the stack. You should also feel a lot of air blowing out of the exhaust port.
 
  #9  
Old 12-29-02, 07:49 PM
plmask
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So far, so good. No problems since my last post. The real test will be when the temperature falls below 30 degrees F.
 
 

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