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Old 12-03-2009, 10:59 AM   #1
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Dayton 3E837 Limit Switch Question


We have an older unit heater that we think has a problem with the 3-prong fan limit switch. I believe that this is also referred to as the fan control thermo-disc switch, however if this is incorrect please let me know.

The problem with our unit heater is that the fan shuts off before the room temperature reaches the set temp on the thermostat. The fan runs for some time and then stops. It sounds like the unit re-enters a warming up cycle and then the fan will run again, however it takes a very long time on cold days to reach the set temp.

Although it seems unlikely, another possible issue might be the distance from the gas meter to the unit. It is about 125 feet and we upsized the line and ran the calculations which seemed to be fine, however could it be possible that the unit is choking on too little gas?

Thanks for any feedback.

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Old 12-03-2009, 12:48 PM   #2
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Dayton 3E837 Limit Switch Question


Could be the gas line is still too small. What size line did you run. And what size is the unit heater.

Does the burner shut off then the fan. Or is the burner still on when the fan shuts off.

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Old 12-03-2009, 01:22 PM   #3
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Dayton 3E837 Limit Switch Question


Do you have a copy of the Instalation and Operating Instructions? If not, PM me your e-mail address, I'll see what I can do for you.
In addition, are there any letters after the P/N? (3E837A, B or C?)
It appears that that unit may be upwards of 32 years old.


TJ
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Old 12-03-2009, 02:19 PM   #4
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Could be the gas line is still too small. What size line did you run. And what size is the unit heater.

Does the burner shut off then the fan. Or is the burner still on when the fan shuts off.
We ran a 1" line for most of the length. It is 1.5" coming off the meter. The burner sounds like it is still on then the fan shuts off.
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Old 12-03-2009, 02:28 PM   #5
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Dayton 3E837 Limit Switch Question


1" is good for about 130,000 BTU input at 125" and .5" or less service pressure.
Thats if its schedule 40 steel pipe. If its CSST, then its much less.

What temp are you trying to reach from what temp.

If its 50 in the building. Then you can expect it to cycle several times like that.
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Old 12-03-2009, 02:43 PM   #6
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Dayton 3E837 Limit Switch Question


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If its 50 in the building. Then you can expect it to cycle several times like that.
The temperature in the shop can start as low as 50 degrees, and when we want to warm it up to 65 degrees is when we have the constant on/off. Is this how these older units are supposed to function or is the issue back to the thermo-disc switch?
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Old 12-03-2009, 02:47 PM   #7
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Dayton 3E837 Limit Switch Question


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Originally Posted by edanajo View Post
the fan shuts off before the room temperature reaches the set temp on the thermostat. The fan runs for some time and then stops. It sounds like the unit re-enters a warming up cycle and then the fan will run again, however it takes a very long time on cold days to reach the set temp.
Post a schematic. This should be easy to troubleshoot with a multimeter or just a test lamp.
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Old 12-03-2009, 03:45 PM   #8
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Dayton 3E837 Limit Switch Question


This is what I could find.
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Dayton 3E837 Limit Switch Question-wd.jpg  
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Old 12-03-2009, 04:21 PM   #9
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Dayton 3E837 Limit Switch Question


What your describing, is typical operation for both older and newer units.
When the thermodisc cools X amount. it shuts off the blower to both protect the heat exchanger, and prevent cold air from being blown on you.

If it continued to blow. You would get condensation inside the heat exchanger. And it would rust from the inside out.
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Old 12-03-2009, 06:04 PM   #10
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Dayton 3E837 Limit Switch Question


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Originally Posted by beenthere View Post
What your describing, is typical operation for both older and newer units.
When the thermodisc cools X amount. it shuts off the blower to both protect the heat exchanger, and prevent cold air from being blown on you.

If it continued to blow. You would get condensation inside the heat exchanger. And it would rust from the inside out.
Thanks for the info and responses. Lets say that I was not concerned about the rust issue (ie. very arid climate in Utah and since the unit is older I would plan on replacing it soon anyways). Is there a way to keep the fan blowing until the heat reached the set temp on the t-stat? Thanks
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Old 12-03-2009, 09:57 PM   #11
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Dayton 3E837 Limit Switch Question


When the motor stops running, put a voltmeter across the fan switch to confirm that the switch opens and is causing the motor to stop. Open switch = full supply voltage.
Also confirm if the limit switch is opening.
If the 'stat is opening there should be 34(?) volts across it.

This is a very interesting circuit.
Apparently the 'stat calls for heat, and if the limit switch is closed the 34v across the gas valve heats the resistor which eventually closes the fan switch.
When the 'stat is satisfied the fan switch eventually cools and stops the fan.

I guess if the resistor heats up, and then opens due to thermal expansion because of a crack, you could get your symptom.
To check this you'd need to measure the current through the resistor.
If the resistor current stays, the fan switch has become oversensitive to the resistor's heat.

Last edited by Yoyizit; 12-03-2009 at 10:04 PM.
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Old 12-04-2009, 03:52 AM   #12
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Dayton 3E837 Limit Switch Question


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Thanks for the info and responses. Lets say that I was not concerned about the rust issue (ie. very arid climate in Utah and since the unit is older I would plan on replacing it soon anyways). Is there a way to keep the fan blowing until the heat reached the set temp on the t-stat? Thanks
Utah may be dry. But your gas contains moisture. So no. You can't just make it run no matter what. Without causing a hazzard by running it like that.

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