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Can I install a new thermostat by just turning the power off at the servicemen's switch, instead of at the breaker?

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Turning off what power? –  BMitch Dec 29 '13 at 3:05
    
Can you be more specific? Do you mean the furnace emergency power switch? –  Edwin Dec 29 '13 at 3:37
    
Yes furnace emergency power switch –  Brenda Dec 29 '13 at 5:30
    
It might be a good idea to flip both off - in the event that someone inadvertently powers one on. You want to be able to reasonably guarantee that power won't come on while working with the device. –  crush Dec 30 '13 at 20:19

3 Answers 3

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Yes, but I would

  • test the voltage at all the thermostat connections before proceeding.
  • use an insulated screwdriver.

Some thermostat systems operate at safe low voltages (e.g 24 V) but in some countries, it is common for thermostats to be switching dangerous voltages (e.g. 120 or 240 V) so changing a thermostat needs to be handled with as much care as changing a light switch or lamp fitting.

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Yes. In fact, that's what it's there for. If I remember correctly, code calls for the disconnect for a furnace to be within view of the furnace in some situations.

National Electrical Code 2011

Article 422 Appliances

III. Disconnecting Means

422.31 Disconnection of Permanently Connected Appliances.

(B) Appliances Rated over 300 Volt-Amperes. For permanently connected appliances rated over 300 volt-amperes, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance or is capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed.

(C) Appliances Rated over 1⁄8 Horsepower. For permanently connected appliances rated over 1⁄8 hp, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance. The disconnecting means shall comply with Sections 430.109 and 430.110.

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Yes, shutting the service switch off will de-energize the thermostat, in most situations.

In very, very rare cases some thermostats are powered off a transformer to give it continuous 24v. Normally on old furnaces where this is the case they will wire the transformer after the furnace service switch. However i have found in some cases (of utter stupidity) technicians have wired the transformer off of a different electrical source. The other case is on certain new style communicating thermostats. These use a wireless signal to communicate with the furnace. In some cases a transformer is used from a different electrical source to power it so that a wire is not needed from the furnace to the thermostat. The only real risk with 24v would be accidentally touching 2 wires and running something, blowing the fuse on the control board of the furnace, or a mild shock.

In other very rare cases i have found furnaces with hidden service switches, or older furnaces that never had service switches. If the house has an old nob and tube style electrical panel (that takes fuses) it is normally easier to carefully remove the R wire off the thermostat first and put electrical tape over the bare part of the wire. Then put the R wire on the new thermostat last. As a technician, this is what i will usually do myself when changing thermostats. It is not a preference of mine, but it saves me the time of locating the furnace room, finding the switch, and maneuvering through any debris between me and the switch.

The R wire is the only wire that carries continuous power (24v)

Remember that if a thermostat is controlling baseboard heat, it is almost always 120v and should never be attempted live. As stated above, a multimeter is always a useful tool to have.

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