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Old 01-14-2013, 03:10 PM   #1
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Odd thermostat wiring....

I have an old house built in 1947. When I bought the house a couple years ago, the inspector told me that the big box in the attic was a doorbell transformer that was no longer in use, but now I've found that it obviously serves a purpose....switching power on and off for my 240v living room wall heater.
After removing the thermostat just to take a look and make sure everything was ok (I've found some other things that made me cringe), I see that its a single pole line voltage thermostat, CONNECTED TO PHONE CORD. After figuring out there must be more to this than meets the eye since my house hasn't burned down, I go up into the attic and find the phone cord is going to what I thought was the old doorbell transformer. So now it appears that the battery operated thermostat is sending a signal to this box, which is apparently what is actually turning the heater on and off. I've never heard of anything like this, it seems pretty odd. Could it actually be a repurposed doorbell transformer? Or something else? Figure its safe?


Last edited by tebentheuer; 01-14-2013 at 03:13 PM. Reason: typing error
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Old 01-14-2013, 03:36 PM   #2
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Got pix?

If it's a 24 volt system, all may be legit ...



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Old 01-14-2013, 03:41 PM   #3
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A line voltage thermostat will work on 12 volts or 24 volts etc. to activate the controls and relays of a furnace.

The telephone wire like cable is safe for 24 volts. Of course that does not carry enough power (watts) to feed electric baseboard heaters or heating elements and generate heat directly.

Some older furnaces required a doorbell transformer, sold separately, to power the thermostat circuit.
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Old 01-14-2013, 04:05 PM   #4
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Actualy it is safe up to 90 volts, which is ring voltage, but low amps.
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Old 01-14-2013, 04:59 PM   #5
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There must be a relay somewhere... have someone turn the thermostat up and down while you listen near the big box in the attic... you should hear a "click" sound. If not, the relay may be in the wall heater itself. Might have to take the front cover off to find it.

Once you find the relay, you can compare it's rating to the amperage of the breaker that runs the wall heater. If the relay rating meets or exceeds the breaker, you should be pretty safe.
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Old 01-14-2013, 05:17 PM   #6
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Thanks for the replies all. Looks like the relay has to be inside the box in the attic. I know there isn't one at the heater since I've already pulled it out to clean the whole thing out. There's 10 gauge wire running from the panel to the box, then from the box to the heater. So, it looks like the box is the relay. I'll check the voltage on the line to the thermostat, but now I'm pretty certain that everything is fine. The thermostat wires are connected to two little screw terminals on the top of the box, my guess is if they were for 120v AC they would be connected inside the box itself or a junction box nearby. I've just never heard of something like this used for a wall heater, but since the house has been changed so much over the years, the relay was probably originally for some other sort of setup entirely.


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