The dreaded Wi-Fi T-stat "C"- wire question

[Schu338]

Looking to install (4) Wi-Fi T-stats in a vacation home (including a garage). Almost all require a common wire from the "cooling" transformer, or leave the RC-RH jumper in place on a heating only app.

I do have electrical experience, I'm just an old fart to new technology.

In my situation, I have "C" from the heating transformer, but not the cool and would be difficult to do so. Since I have 2 separate power systems, I cannot leave the jumper in place. Does anyone know of a Wi-Fi stat that can use the heating common?

Not sure about using the C-wire "bridge" or cheater" some stats offer; just something else to go bad.

Bosch BCC-100 claims I can, I'm just not sure.

Thank you for your anticipated help.

 

[LS6 Tommy]

You can't "cross" the transformers on a two transformer system. You can install an isolation relay to avoid "bucking" the transformers, but IDK if that will accomplish what you want.

Tommy

 

[Schu338]

You can't "cross" the transformers on a two transformer system. You can install an isolation relay to avoid "bucking" the transformers, but IDK if that will accomplish what you want.

Tommy

Yeah, I understand that. Logistically difficult. In 2 of the locations the heat and a/c control wiring go in different directions. Again, if C can come from heat transformer, problem solved. Haven't found that solution yet. Thank you for your reply.

 

[Milton Shaw]

Get a big enough transformer to run both and rewire and eliminate both of the old ones. You only need one transformer to power all systems that I have seen. The VA rating and voltage should be posted on them and all I have ever seen the transformer is in the furnace.

 

[brewchief]

Get a big enough transformer to run both and rewire and eliminate both of the old ones. You only need one transformer to power all systems that I have seen. The VA rating and voltage should be posted on them and all I have ever seen the transformer is in the furnace.

That doesn't work if your dealing with hot water heat and an air handler in the attic.

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[Stuff]

Option 1: Re-purpose the G wire for a C. Put a jumper between Y and G at the air handler.
Option 2: Venstar add-a-wire - "multiplexes" Y and G into one wire to free a wire to be C.
Option 3: Install a 24vac transformer at the thermostat connecting to Rc and C.
Option 4: Jumper Rc and Rh and use the C from the heating system. This works as long as both systems use 24vac and the voltage between Rc and Rh is zero. If not 0 then swap the leads feeding one of the transformers.

 

[Schu338]

Option 1: Re-purpose the G wire for a C. Put a jumper between Y and G at the air handler.
Option 2: Venstar add-a-wire - "multiplexes" Y and G into one wire to free a wire to be C.
Option 3: Install a 24vac transformer at the thermostat connecting to Rc and C.
Option 4: Jumper Rc and Rh and use the C from the heating system. This works as long as both systems use 24vac and the voltage between Rc and Rh is zero. If not 0 then swap the leads feeding one of the transformers.

Stuff- The air handlers have fan coils for hot water heat. I don't think options 1 and 2 would work because when the coil aquastat calls for fan, the fan feed (G) will back-feed in to cool (Y). No room for option 3. Option 4 is a possibility I will investigate.

I really don't understand why all wi-fi stats use the cooling transformer C. They could use separate commons (CR/CC) with a jumper like RC/RH. How difficult is that? I am pretty sure I'm not the only one with this situation.

Thank you for your time and reply, I appreciate it.

 

[Stuff]

For Option 1 - depends on air handler as aquastats are commonly line voltage. If an issue you could add a simple peanut relay to isolate. Assumes system doesn't need thermostat to call for fan on heat cycle.
For Option 2 - the Venstar would provide separate Y and G at the air handler to prevent back-feed.

 

[99LeCouch]

Emerson Sensi is reputed to be a 4 wire capable smart thermostat.

How old is the equipment? If it is capable of communicating, the OEM communicating thermostat likely has Wifi and a remote app. Also can run on 4 wires, at least most of them.

 

[brewchief]

Emerson Sensi is reputed to be a 4 wire capable smart thermostat.

How old is the equipment? If it is capable of communicating, the OEM communicating thermostat likely has Wifi and a remote app. Also can run on 4 wires, at least most of them.

I've done plenty of sensi stats with 4 wires, the touch screen sensi does need a common wire but the original one does not.

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[nsula_country]

Here is some Sensi wiring info.

https://sensi.emerson.com/en-us/support/how-do-i-wire-my-thermostat

https://webapps.emerson.com/resources/wire-picker.aspx

The second link is wiring compatibility program.

CT