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Moved into a property with two old thermostats (one upstairs and one downstairs - one furnace and two zones). They are ancient but the furnace company said they can be replaced and that it’s a 5 wire thermostat. I’m not totally sure what this means…

could I replace it myself with a wifi version like google nest or is this not possible with these wires?
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I believe you need 6 wires, the missing one is a constant hot

Be real careful, zones are not something to mess with if you do not understand what you have installed BEFORE you purchase something to make it better.
 

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You haven't given us any of the needed information to make a recommendation. Seeing the face of the fifties doesn't really tell us the wiring or even the number of wires in the thermostat cable. I gather it to be 5-wire cable?

It really depends on the type of furnace, air handler and air conditioning pack being controlled. I don't see any controls to switch to A/C mode or to give manual control of fan (fan control is a side-effect of how "bolt-on air conditioning packs" attach to furnaces). So it might just be a dumb 2-wire "furnace only" rig.

However if you have the usual American gas furnace + bolt-on optional A/C that uses the furnace's air handler, then that setup only needs 4 wires in the thermostat cable. If you have 5-wire thermostat cable in the walls, it should be possible to hook up a smart 'stat.
 
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