Upgrading old 2 wire Honeywell thermostats to Honeywell T9 smart thermostat

Massachusetts311

I currently have 2 Honeywell 2 wire thermostats. I’m looking to upgrade 1 of them to a Honeywell T9 smart thermostat. My system currently is only 2 wire. I have 2 taco 571-2 zone valves, To create a c wire for the smart thermostats, do I connect a wire to where the common goes from the transformer to the zone valves to the c on the smart thermostat? I included some pictures of my current setup. Heat only oil boiler.

EdTheHeaterMan

Actually there are some particular interesting things that happen with that zone valve that make it less than a simple wire addition.

This post addresses the Taco Valve when adding a Nest. you may have a similar problem with the T9

https://forum.heatinghelp.com/discussion/comment/1783485#Comment_1783485

There may be a resistor needed. @SteveSan will have the best answer for you. just know that your zone valves may not be wired according the the factory recommended diagram. That is because the factory diagram does no include the R and the W and the C terminal designations of the thermostat and the transformer, therefore they can be reversed and still work if both zones follow the same reversed wiring design.

EdTheHeaterMan

This comment tells the story of how the Taco 571 can be wired several different ways.  As long as you stay consistent there will be no problems. 

https://forum.heatinghelp.com/discussion/comment/1785090#Comment_1785090

When you need to add the C wire for a smart thermostat like the T9 you need to be sure that the R and the W from the 2 wire thermostat are connected this way.

I have modified the manufacturer diagram on the Left to show that R from the 24 VAC transformer is connected to R on the thermostat. W from the thermostat is connected to 1 on the Taco zone valve. And the common terminal 2 on the Taco zone valve is connected to C on the transformer

With that wiring confirmed, you can easily add the C from the thermostat to C on the transformer as shown on the Right diagram. You may find that is all you need as long as you double check the original wiring before you start.

EdTheHeaterMan

It is unclear from your pictures if your thermostats are correct or reversed.   On a two-wire thermostat, it will not matter. However, on a Common C wire it will make a difference.   On one of the zone valves a White wire is connected to #1 and on the other zone valve a Red wire is connected to #1.   The wire color does not matter because electricity is colorblind.  What does matter is that W on the thermostat is connected to #1 on the zone valve.   If that is backwards, then you will have a problem connecting the C terminal on the new T9 thermostat

If you make a mistake, you may let all the factory installed smoke out of the transformer or the thermostat or the boiler control. Once that happens you can never get that smoke back in. I like to use these Smoke Keepers so that will not happen.

SteveSan

@EdTheHeaterMan

Massachusetts311

Thanks for the responses @edtheheaterman and @stevesan. I wired it as you guys both posted and it’s working great.

SteveSan

Glad we could help. Happy New Year!

If you ever have any questions, please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000 and just ask for Technical Services