I am taking out a Lennox non-programmable thermostat for a programmable Honeywell Model 7500 programmable thermostat. When I remove the old thermostat there are only 2 wires attached to the device. When I asked honeywell about the wiring they asked if the old thermostat was 24 volts. There is no indication anywhere on the old thermostat as to the voltage. It is about 25 years old.
Thanks for the help

Only two wires will probably need a battery operated thermostat or you will have to pull a constant hot from the 24 volt transformer.

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Originally Posted by cwroenisch

I am taking out a Lennox non-programmable thermostat for a programmable Honeywell Model 7500 programmable thermostat. When I remove the old thermostat there are only 2 wires attached to the device. When I asked honeywell about the wiring they asked if the old thermostat was 24 volts. There is no indication anywhere on the old thermostat as to the voltage. It is about 25 years old.
Thanks for the help

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They're probably trying to see if it's a "millivolt" system. Does your system have a transformer on it? If so, it's 24 volts. If not, it's a millivolt system and the two thermostats are for the most part incompatible.

To run it off batteries-put batteries in it, connect one wire to R and the other to W, and you're done. To run it off a 24v transformer system (and lose the batteries) simply run one wire from R on furnace to R on stat, w on furnace to w on stat, and C on furnace to C on stat. Turn power back on, and you're done-except for programming.