furnace; fire lights - no fan; #2 WITH PICS


http://groups.google.com/group/alt.home.repair/browse_thread/thread/21b6c2fcb8dc6a09 #
Here are two pics of the furnace controls
http://cakebekensail.angelfire.com/index.html
Pic 1 shows something that looks like at temperature sensor.
Pic 3 is the works. Along with something that looks like a pressure sensor, there is ther relay box. When I switch the fan off and on with the stat upstairs, you hear a realy click in the unit.
Everything works fine except the delay fan operation. It never engages. The fan is now set to run continously to procect the firebox.
I suspect the device shown in pic 1. Ideas? Hell I could replace all the parts for what service would charge me.
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http://www.high-performance-hvac.com/hvac-photos/HVACPhotos/fan-limit-control.html I going for it. I'll let you know.
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As I said in my post in the other thread:
It sounds like the fan limit switch, which is what you have pictured in pic #1.
If you remove the 2 screws you will find a "probe" that extends into the plenum. It probably looks something like a big spring.
As it heats up (or cools down) it is supposed to turn the disk seen in the picture and turn the blower on and off.
The dial should have three settings: On, Off and an over-temp setting that closes the gas valve if the plenum air temp gets too high - assuming it is working enough to do it's job.
You might be able to "fake" the device by moving the On setting tab to a position where the blower comes on. If it does, then the fan limit switch or the probe is either stuck or bad. As I'm sure you can see, it's fairly easy to replace.
Do you have a humidifier on the furnace?
I did and once found that the probe was so caked with mineral deposits that it couldn't do it's job anymore.
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That is the fan/limit switch. Jumper the red and blue wires and if the blower comes on,the switch is your problem. The switches do not give a whole lot of trouble, but they do fail, and replacement is easy. As I said in the earlier thread, be sure and cut the little brass jumper at the bottom of the switch, or you can/will fry most or all of the 24 volt components in the system. BTDT. Larry
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On Jan 28, 8:20 pm, snipped-for-privacy@webtv.net (Lp1331 1p1331) wrote:

As I look at this fan switch, I see that the wires attach at 4 positions. nw, ne, se and southwest (north being straight up). What points of the rose do I short to turn the fan on. I got a little confused about the wire color references. Thanks, you've all been a lot of help. Long live home repair.
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Have you tried to adjust the On limit to make the blower turn on at a much lower setting - like whatever temp the plenum is at right now?
If your limit switch is anything like the ones I've dealt with, it's a mechanical device that simply closes contacts when the metal in the plenum probe expands and contracts, turning the dial so the the adjustment tabs physically close the contacts. There's no real "electronics" involved.
If you adjust the On setting so that it physically closes the contacts you'll have the short you are looking for and you'll know that the limit switch is the culprit.
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It says push (or pull) for on. The "wheel" can be pushed in. I did so-- no fan. I'll move the on setting to a very low value and check it out. Thanks
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The left side of the switch is marked "fan" and the right side "limit", The switch justs connects the two wires on the left internally and completes the circuit to the fan. The two wires on the right side are connected internally until the temperature reaches the high limit safety setting and the switch opens the circuit, cutting power to the gas valve. If you look closely at the very bottom center of the switch you should see where the brass jumper was that I mentioned that needs to be cut on the new switch. Larry
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I think I may have said Red and Blue , I meant Black and Blue wires -- leave the red alone! Sorry Larry
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