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Discussion Starter · #1 ·
Hello,
I have an old (really old, I think) Carrier furnace, Model# 5GS0___ (can't even read the rest of the model number). The blower fan stays on even when the thermostat temp is achieved, when the thermostat is set to "off" and "auto", even when the thermostat is unplugged. This has only started happening recently.

After some research, it seemed like the limit switch was the likely culprit so I bought one online and replaced it.

Old limit switch: L250-40, model# HH12ZA251
New limit switch: L250-40, model# HH12ZB250

The new one is supposed to replace HH12ZA250--not exactly my model number, but close. Purchased from this site.

However, even after replacing the limit switch and the rusty-looking connectors on the old wires, my blower fan still stays on. There is an old diode in series with one of the wires that connects to the limit switch. I'm not sure why that's there, but maybe that's the problem? Otherwise, is there something wrong with my control board?

Does anyone have any ideas on how to fix this? This is very frustrating and any help would be appreciated. Thanks in advance.
 

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limit

is this the limit you installed?
If so then this the limit then it is the wrong part to make the fan run continuously unless your unit has a control board that senses an open limit in the circuit. I'm not sure of the breakdown on your model but does it have a fan/limit switch with a white button on the cover near the end of the furnace? Can you give me a manufacturer?
I'll see if I can find a pic of your furnace so I know the type and can help you
 

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Discussion Starter · #3 ·
The old limit switch model number is my original post. So is the new limit switch model number and a link to it. It is the kind that is 3 inches long with a metal disk at the end. Here it is again:

New limit switch (link)



I don't have a fancy limit switch with a white button for overrides. It's just this ordinary one with pre-set limits. I don't know the manufacturer of the limit switch, but the furnace is a Carrier (model info in the original post).

Here is all the info I have on my furnace:
Model 5GS0_(9,5)0-_(2) [I couldn't read the full # so values in () are guesses]
Series 201
Serial D9C25_(c)39 [I couldn't read the full # so values in () are guesses]
In 50,000 BTU
Cap 40,000 BTU
58ES900410

Here is all the info I have on the (old) limit switch:
L250-40
HH12ZA251
36T01B3
42125

Thanks for your help!
 

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Troubleshooting fan problem

If I read your post correctly the furnace will run to temperature but the blower stays on continuously, correct?
If the gas valve will work then you don't have a limit problem. A fault in the saftey circuit will not let the gas valve energize. Here are a few things I would check.
1. Which speed is the blower running on continuously (Hi speed is for cooling, low or medium for heat.
2. If you remove the green thermostat wire from the terminals on the control board does the blower shut down after a time delay. The heating circuit usually doe not use the thermostat to control the fan on a gas furnace and the green wire is your fan wire.(assuming color standards were kept)
3.If the blower is running in low/medium speed and removing the green wire doesn't shut down the fan then I would remove the wire from the terminal marked "w" just to make sure the thermostat isn't calling for heat to run before changing the control board.
Hope some of this helps, If not I'm sorry.
 

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It's the fusible link(diode) that you mentioned in your first post. Seen it a thousand times.

Replace it and you are back in business. Might be tough to find so why not just call a pro out to replace, he can do a full maintenance while he is out there and give your furnace a clean bill of health.
 
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