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I have about a 30 year old Lennox furnace in a home I just bought a month ago or so. Furnace was working great until about a week ago. I noticed the AC wasn't getting to the set temp. Found that the blower motor / fan was making a clunking noise. Seems as though it would try to start for a split second then stop. (That was the clunking noise) Sometimes the blower will get up to speed for a few seconds or sometimes it would run just fine? Any suggestions? I should mention that I plan on replacing the unit sometime in the fall so I'm not going to put tons of money in to having a HVAC company come out to repair.
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Sounds like maybe the fan relay is not pulling in good. Could be bad points on the relay, could be the 24volt is low so relay does not pull in good.Originally posted by jimmyd75 View Post
Start by checking 120 volt at furnace. Check the 24 volt and see if that,s low
Check the system for bad and lose wires. You might have to take some pic. so i can see the controls on your furnace. If you take pic. you can send them to
paulm989@hotmail.com. Paul -
Thanks, I will be able to check the voltages when I go over there at lunch. I do not have a camera with me but can take them tomorrow and send them to you. Is ther any way I can jumper out the relay to see if thats the issue?Comment
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I don't think a old G12 was air prep t so they would have had to add a relayOriginally posted by jimmyd75 View Post
to start the blower. There are closed points and open relays so i can't tell you how to jump it out with out without seeing it. also if you jump it, the blower will run all the time and not turn off.
on your thermostat the wire that is on the G post is the one that goes to the relay to start the blower. If they used color code, it would a green wire.
The reason i asked you to check the voltage, is if the line volt is low then your 24 volt goes down. The blower relay needs 24 volt going to it. If that 24 volts drops down to 22 or less , the relay can start to shatter or not make at all.
also if the points are getting bad in the relay, it might not start the blower every time. PaulComment
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I checked the incoming voltage and it was about 118VAC. Not quite sure where to check for 24VDC? I will send a bunch of pictures tomorrow.Comment
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Yes you can jump out the fan switch. If you put a jumper wire with a switch in that jumper you can turn the switch on and off a lot and see if the fan works.Originally posted by jimmyd75 View Post
Let the motor come to a stop each time. Some motors have end switches in them to start the motor If the switch does not come back to the start position,the motor will not start. So if you know you have 120 volt going to the motor and you still have a problem, it would be in the motor. Does this motor have any wires going to a capasator? How many wires are coming out of the motor and what color are they? PaulComment
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Paul, I sent you furnace pictures to the E-mail you provided.If you didn't get them let me know. There is also a picture of the wiring diagram. I work in the machine tool industry as a controls designer but these types of wiring drawings just confuse me to know end?Comment
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In your pic. you took the cover off the electric box. The Brown control in the center of the box is your fan relay. On the top left is a yellow post and on the bottom left there is a white post. Check 24 volt ac power across those two post when the stat is calling for cooling, You should have 24 volts and the fan should be running. If you get 24 volts and blower does not run then the relay is bad. To jump across the relay on the 120 volt. You would jump from the top right corner to the bottom right corner, that would put 120 volts to the blower.Originally posted by jimmyd75 View Post
If you jump out the relay on the 120 volt side and the motor hums or tries to run real slow, then your start capasator could be getting bad, It is just under the relay on the blower. PaulComment
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