thermostat wiring


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Old 02-15-04, 06:08 AM
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thermostat wiring

hi,
i have an oil burning forced hot air furnace. there is very old, rope insulated, 3-wire thermostat wire connected to my programmable thermostat. i want to replace it with newer wire, but I noticed something a little confusing.

the thermostat wire is joined together with wire coming from the furnace cut off switch. there are only 2 wires from the switch box. so, I have one thermostat wire that isn't connected to anything.

the thermostat seems to be operating properly (and has for over a year), but I'm not sure if the wiring is correct or not. I'm not sure if this is something that can be addressed in this forum, or if I just call an electrician, but if there is an easy answer, I thought I'd just take care of it.

Thanks!
 
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Old 02-15-04, 08:15 AM
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For just a heating system you should only need two wires, usually red and white.

Explain what wires you have, what colors and where they are connected.
 
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Old 02-15-04, 12:05 PM
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Your description of the "old" thermostat cable suggests that the original connection was the M-H "Series- 10" heating-control which, at the time, was "standard" for automatic-control of residential heating- plants. Red, Black, and White wires connected to "R", "B" and "W" terminals.

The Series-10 thermostat had a bi-metallic element responsive to temperature changes, and on a "call-for-heat" the element would close in sucession, a "run" circuit, then a "start" circuit, which expains the 3-wire thermosat-circuit.

In "responding" to the rise in room-temperature, the element would first open the "start"-circuit, then the "run"-circuit. The bi-metallic element was a mechanical form of "differential" control. The purpose of a differential-control is to prevent "short-cycles" of frequent "On"-"Off" operations, a situation that would exist if the bi-metallic element had only a single "On"-"Off" contact.
 
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Old 02-15-04, 06:35 PM
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"If it ain't broke, don't fix it!"
Your wire still works, and just because it is old doesn't mean it's not safe. Since it is only 24V, I say leave it alone.
 
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Old 03-14-04, 06:17 AM
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the old thermostat wires are yellow, green and brown. their thermostat connections are as follows:
yellow - RC
green - W
brown - G

the old yellow wire is connected to a white wire coming directly out of the furnace

the old green wire is connected to the red wire out of the furnace

the brown wire isn't connected to anything
 
 

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