How to add extra 24VAC wire (C wire)


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Old 07-06-15, 07:11 PM
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How to add extra 24VAC wire (C wire)

I have recently bought a Honeywell rth9580 programmable WiFi thermostat. Unfortunately I have old electric heating/cooling system probably from 70's. The system is based on two relays (I guess heating and cooling). Since transformer doesn't have 3rd wire (C?) by default I would like to make one by connecting extra wire to transformer chassis. Having terminal C I would like to connect wire G to terminal C and jump terminal G with Y. I measured the voltage between transformer chassis and Rh wire: idle 34VAC, calling for heating/cooling 24VAC. Here are my questions:
1) Is this setup will work?
2) Is it safe to connect thermostat since voltage on idle is a bit higher than it supposed to?
3) Am I right to assume that transformer won't overheat because thermostat will try to drain to many amps?
 
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Old 07-06-15, 07:39 PM
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34 volts is not correct.

A 24vac HVAC transformer only ever has two terminals. One terminal is technically connected directly to the R terminal. The other terminal can be a blind connection.... like just a crimp or wirenut... and may just connect to ground.

If you know where the two wires that go to the outside unit are...... one connects to the Y and the other is the C connection.

Can you post a pic or two of the existing wiring ? http://www.doityourself.com/forum/el...rt-images.html
 
 

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