Changing Tstat in an old school portable

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Old 11-05-20, 01:27 PM
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Lightbulb Changing Tstat in an old school portable

Hey all,

We have a retired school portable on our property and it has a built in system that uses the tstat below:


old tstat wiring




I've also attached the wiring diagram that it was using here:


wiring diagram for old tstat. only bottom left table includes the actual wires being used. table shows all codes for wires, but white is "occupied signal" which I'm not sure is relevant on new diagram.


all of the wires in the whole system labelled.

We're wanting to replace this one because we can't program it and it doesn't actually have the ability to stop heating once it reads a set point. we've just been manually shutting off the breaker once it gets too hot. So we installed a Nest tstat in our house, and wanted to use the old Honeywell RTH6360D. So I went ahead and rewired it according to the original letter codes (Y to Y, W to W...etc), but when I rig it up, the heat doesn't come on when the set temp is higher than the current temp. So it's not working.

Did I not wire something in right?

I put all the wires to their codes (I think anyway), but couldn't figure out what to do with the last white wire since it's labelled as "occupied". No idea what that means and can't find any HVAC convention that uses that. Here's the current wiring set up:

new honeywell tstat wiring

Any help is appreciated.
 
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Old 11-05-20, 02:24 PM
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Welcome to the forums.

On the surface it looks like the wiring is correct.
I have a feeling you may need to ground or supply 24v to that white wire to tell unit the area is occupied.

I would try connecting it to the C/black wire first and see if that does it.
If not.... try it to the 24v line.
 
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Old 11-05-20, 02:25 PM
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Yeah I see where you're going with this. Will try and update!
 
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Old 11-05-20, 02:44 PM
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Okay update: that worked! Did the white wire to common (black), which turned the system on again but it was out of phase. Then I tried white to 24V AC (red) and that worked.

But our original problem persists... kind of. The fan won't turn off. The heating element only comes on when it needs to, which is good. But when the current temp is above the set temp, the fan stays on. Even if we turn the system off on the tstat, the fan stays on.

Not sure how to attack this problem...basically we just want the fan on when the heating element is on.
 
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Old 11-05-20, 02:47 PM
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In thermostat setup.... 205 needs to be set to 4 - electric forced air.
 
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Old 11-05-20, 03:01 PM
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Thanks for the suggestion- we tried that and it didn't work. Fan still blowing.

This system has a design for a motion sensor, so that it should only turn on when the motion sensor is active, like when a school class is in session. I wonder it's rigged so that it's always on....
 
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Old 11-05-20, 03:23 PM
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So basically this white wire needs a 24V signal to run the fan, and it will always run if that condition is met. I tried wiring the white to the green wire (fan) but that didn't do anything.

This is getting tricky....might try wiring in a transistor to the white, green and red wires. But we're going to be out of town for a couple weeks after today so I might have to shelf this one for a bit.
 
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Old 11-05-20, 03:50 PM
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I tried wiring the white to the green wire (fan) but that didn't do anything.
I would think that would do it.
 
 

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