Updating Fan/Limit switch in older gas furnace


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Old 10-12-09, 03:37 PM
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Updating Fan/Limit switch in older gas furnace

I have an older natural gas forced warm air furnace and the fan limit switch is acting up, its stuck with the fan on all the time. The furnace is 50's vintage but works pretty well. The combo fan/limit switch is an old White Rodgers unit that is not available any more, it uses long temperature sensor tubes that go up from the switch to some holes and apparently are stuck into the air flow.

As this kind of switch with its long sensors appears to be no longer available, I'm thinking I should just upgrade it to a more modern combo switch, something like a Honeywell L4064B2236 like this one:
Amazon.com: Honeywell L4064B2236 combination fan and limit furnace control: Home Improvement

The only rub is there isn't room to install it right where the current temperature sensors are currently placed, which are about 3" down for the very top of the heater. I would have to put a hole in the 14" x16" duct at the top of the heater for the sensor, so it would end up around 5" above the current location. The re-wiring won't be an issue, I'm pretty experienced with that.

Also are you supposed to buy these kinds of sensors with a length that sticks them right in the middle of the air flow?

Thanks,

CalBoy101
 
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Old 10-12-09, 04:55 PM
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The fan limit switch is a critical piece of safety equipment for a furnace. I've seen the side of furnaces so hot the outer sheet metal was glowing a dull red color because they were not wired properly and shut the fan off and sllowed the burners to continue to run.

So I wouldn't recommend you change the location of the fan/limit switch. From where it is, it can "see" the heat of the burners and react properly. Change the location and it may not act properly.


I'd continue to shop around for a replacement fan/limit switch that will fit in the space available.
 
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Old 10-12-09, 04:57 PM
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Honeywell

The problem with using the Honeywell is you will be farther away from the heat exchanger & this could cause overheating of the heat exchanger before the fan comes on. What is the number of the old White Rodgers? I might be able to find a cross reference.
 
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Old 10-12-09, 05:08 PM
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Grady and SeattlePioneer-

The White Rodgers unit is pretty old, its their Type 534 Style B 21 B .

The concern I have about replacing that one is I can't even tell where the temperature sensors go, they just feed in through holes about 3" down from the very top of the unit below where the main duct is attached. To see how they are located I think I'd have to take the main duct off, which looks pretty difficult now without some major work, unfortunately before I bought the house some building was done around this heater boxing it in pretty tight. Also it looks like its pretty hard to find these old kind of controls now.

One thing I noticed since the last post, there is a manual temperature gauge on the side of the duct, about 6" up from where the duct starts, I'm assuming it was put there to help set the temperature limit settings on the control, which are right now set at typical settings, 110 F on the fan on and a reasonable number on the limit (can't remember it exactly, I'm at work now).

Its right about the height where I had been planning to put the replacement control, so I'm figuring if it was a good place for the temp gauge it must also be an ok location for the replacement control. I could use this gauge to make sure the replacement control is working properly.

Thanks for the help-

CalBoy101
 
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Old 10-13-09, 02:10 PM
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Originally Posted by Grady
The problem with using the Honeywell is you will be farther away from the heat exchanger & this could cause overheating of the heat exchanger before the fan comes on. What is the number of the old White Rodgers? I might be able to find a cross reference.
Grady, I see your point on this, but I'm hoping I may be ok on this issue. The furnace is pretty tall, about 5 feet, with the burners mounted about 2 feet from the ground. So since the original sensors appear to be at the very top, I'm hoping I'd be ok to put the new sensor about 6" above that.

What I'm figuring to do is to go ahead and hook the sensor up, set the fan on temperature pretty low and then see if the fan switches on in a reasonable amount of time, I've had the house for a while so I know about how long it should take for the fan to switch on.

I figure to do the same test with the limit switch, disconnect the fan motor, start the heater and make sure the limit switch kicks in before the heater itself gets too hot.

I guess how hot is too hot is the question, I do have an infrared gun, do you have any feeling on what temps for the heater sheetmetal are expected when the limit is about to shut off?

Money is tight here, so is my time as I'm working 7 days a week to make ends meet, so I appreciate the help you guys are giving me.

Thanks,

CalBoy101
 
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Old 10-13-09, 06:25 PM
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IR Gun

A far better tool than the IR is a dial type thermometer with a stem long enough to go into the plenum at least a few inches. How hot is too hot is a tough question but I wouldn't set the limit over about 180º.
 
 

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