I have a three zone heating system with old round Honeywell mercury switch thermostats

[simul8guy]

Two of these zones are heat-only while one of the old Honeywell T-stats controls a whole house air conditioning system.  The heating system has three zone valves and one circulator pump.  Each of the three thermostats controls one of these zone valves in the heating system.  One of the thermostats is a different model that has two selector switches (Cool/Off/Heat  and  Fan Only/Auto) to control the A/C system.

From reading a few other posts on this topic I assume my Honeywell heat-only thermostats only have two wires running to their individual zone valves in the basement.  The Heat/AC thermostat obviously has additional wiring to control the AC system in the cooling mode.

My understanding is the Nest smart T-stats (model GA01334-US) need three wires (including a common) or possibly a separate 24V power supply to function.  Is this true?  If so, it's not worth it to me to upgrade if it's not something I can't do myself.

[Houptee]

You will need the Learning thermostat for the one that has AC and Heat because it has Rc and Rh terminals to separate the two systems power wires. 

The zone valve controlled by the t stat with the AC system will not need a power adapter because it should be getting a C wire from the AC system, hopefully it has a C wire already or a spare wire not hooked up that you can connect to C on the Nest and also at the C screw in the AC unit air handler.

The other 2 for heat only, you could use the Nest Learning with just 2 wires R W and it might be ok without a C wire but no way to know until you try it. If it gives power errors you would need to add the power adapters on the 2 zone valves. 

Or use the Nest non learning models but those will definitely need the Nest power adapter device on the 2 zone valves for those to work so it gets a simulated C wire connection.

[simul8guy]

Thanks for the detailed info on the Nest T-stats specific to my system.  

[zoeuvre]

Hi simul8guy,

I'm glad to hear that your inquiry has been resolved. 

In the meantime, I'll go ahead and lock this thread within 24 hours. If you run into any more questions in the future, please feel free to create a new thread.

Thanks for your help, @Houptee. 

Cheers,

Zoe