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Old 03-27-2012, 10:10 AM   #1
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Carrier HVAC


I have the same problem with my Carrier HVAC (# 394JAW 048065, yr 1985). I just replaced the pilot system and gas valve for heating problem and it worked for 3 days. Now it does not light (no heat) and blower runs continuously. I have to unplug the system to shut it off. Thermostat is disconnected so I know it is not the stat. Disconnected the limit switch, still the same. Disconnected the black wire to the blower, still the same. Any suggestions/recommendations to diagnose and solve the problem will be greatly appreciated. Thank you.

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Old 03-27-2012, 05:46 PM   #2
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If its a open limit that's powering the fan, then for test purposes only, the limit should be bypassed, not disconnected.
Shut off the power to the furnace, bypass the limit and then turn the power back on to see if the fan has stopped running.
If its stopped then you need to determine why your limit control tripped.
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Old 03-27-2012, 08:24 PM   #3
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When you say bypass, does it mean to short those two wires that connect to the limit switch? I just want to make sure. Thanks.
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Old 03-27-2012, 08:48 PM   #4
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The wires that connect to the limit are part of a complete safety circuit that says the furnace is OK. When the furnace overheats, the limit breaks that circuit to shut down the gas to prevent further furnace damage and house fires. By taking the limit out of that circuit and just connecting those two wires together, you can test whether the tripping limit is the cause of your furnace troubles but leaving it bypassed is way too dangerous beyond just doing this one test.

If the fan has stopped and the gas fires up with a call for heat, when you've bypassed the limit, then you either have a faulty limit or an overheating problem.
Did this problem show up after you changed the gas valve?

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Old 03-27-2012, 09:10 PM   #5
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HELLO WHERE YOU GO.......
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Old 03-28-2012, 10:01 AM   #6
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Hello Ben's Plumbing, HOW,

Had to run yesterday and wasn't able to respond. However, I jumped short the limit switch wires and still the same: the blower runs as soon as I turn on the unit. Before replacing the pilot and gas valve, the heater would ignite/burn for about 5 minutes then the burner would be off before satisfying the thermostat but the blower would continue to run blowing cold air. After replacing the valve every thing was fine for about 3 days, but this time the blower just go on without igniting the burner and no pilot light either. Thank you for your responses.
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Old 03-28-2012, 11:02 AM   #7
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Have you checked ALL your limits and roll out switches? Follow the limit or roll out switch wires from one to the next.


And also try disconnecting the 24V stat wire "G" from the furnace board. If this line receives power through a short, crossed stat wires, poor stat wiring conections etc. your fan will run forever.
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Old 03-28-2012, 11:59 AM   #8
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I did jumped short the limit switch at the circuit board, disconnected the wire form G: the blower still runs. Also, I have the thermostat disconnected and all the wires are free (not connected). Should I install the stat? I don't see any other limit switch that I can reset. I cannot figure out where the relay is to see if it is stuck. Could it be the circuit board? Thanks!
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Old 03-28-2012, 05:43 PM   #9
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If you disconnected the G stat wire from the furnace board and the fan is still running then the stat circuit is not the problem.

With a 1985 you might have a fan/limit control that's separate from the board. It would be about the size of a pack of cigs, have three or four wires attached to it and might also have a white summer fan button sticking through it's cover like a cig filter. It ususally located on the inside wall of the furnace 1 - 2 ft above the burners.

Do you see such a thing? Picture help!
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Old 04-03-2012, 09:10 PM   #10
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I did not notice your reply so I decided to change the control board (ICM 271). However, after replacing it the whole system will not turn on: no fan, no pilot spark, no response from the therstat. I followed wiring from the old board which is exactly the same as the new board. I had the stat to on and then to auto but no success. The outlet (to plug the system has power so it is not the main power circuit board. What other ideas or test can you recommend? Any suggestions will be greatly appreciated. And oh, I can not find the limit control above the burner but there is this switch at the bottom by the blower. I was unable to load the picture. Thank you.
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Old 04-03-2012, 09:28 PM   #11
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Duplicate message below. My "puter's dying.

Last edited by how; 04-03-2012 at 09:34 PM.
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Old 04-03-2012, 09:31 PM   #12
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Do you have 24 volts from the transformer or a blown furnace fuse?

Last edited by how; 04-03-2012 at 09:42 PM.
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Old 04-03-2012, 11:18 PM   #13
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I don't have a voltmeter, but I checked with my pocket continuity tester and there is power from the transformer (black and white wires). The 3 A fuse on the control board looks okay.
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Old 04-04-2012, 12:56 AM   #14
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elvin,

Most likely, you transformer is blown from touching the low voltage wires, probably from bypassing the limit or by replacing the gas vavle. Did you check to make sure you ahve 24v coming out of the transformer? it should be blue and red. with the old carrier, the fan will run cotinously if your transformer is blown. You will still get 120v even if it is blown, make sure you are getting 24v. If you don't have a voltmeter, just replace the transformer. From what I am reading here, sorry to say this, but you actually created more problems. The new circuit board that you are getting is an upgraded version from the old one, it has a built in 3 amp fuse. The reason you are not getting anything on is because your transformer is not producing 24v to the new circuit board, which then nothing will turn on. Replace the transformer and let us know.

Last edited by Mr.HVAC; 04-04-2012 at 01:04 AM.
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Old 04-04-2012, 01:30 AM   #15
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I will try replacing the transformer. It will take a while since I have to order one online. I'll let you know. Thank you.

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