I need help wiring a thermostat without a c wire

[Ameren]

I have an Emerson st75u thermostat I’d like to wire to the heating/cooling. Heat is an old 2 wire boiler system. Cooling is a new Unico 3 wire. Two wires for cooling and one for fan. They are in separate locations and I could pull c wire from the air conditioning which in in the attic space directly above the thermostat. Emerson has a c wire emulator part no. SA-11. I could also Power the thermostat with a wall transformer. My problem is controlling bot the boiler and the Unico air unit with the same thermostat. Can anyone help please?

 

[mikeford]

Any chance the thermostat came with instructions? How was it wired before?

 

[Ameren]

It did. The manufacturer says what I want to do can’t be done but from what I’ve read online and seeing products like the Emerson sa11 thermostat c wire kit it should be doable. The two units were wired into an older Honeywell battery powered non WiFi Thermostat that had a cool/off/heat slider. I could use two separate WiFi stats with external power supplies to do operate each one independently but sort of wanted to use just the one stat. As we have a distinct transition period between seasons here in NE Ohio I could also wire the stat seasonally to run the heat or air or work out a cool/off/heat Slider switch wiring of my own using a triple pole triple throw switch. Your thoughts?

 

[mikeford]

If I get this right, C is the common wire, its the power, and this new thermostat won't work without power. Thing is, each of the units, boiler and AC should have power coming from its controller. Maybe Emerson tech support could walk you through it, but I can guess you will need a meter to test voltages etc.

 

[NutBucket]

C is actually ground. I think you could conceivably run a common ground between the two units and it should work.

How is your existing thermostat wired?

EDIT: Read your other post. So you had R and Rc wired separately? In that case I think a single C wire from either unit will do the trick. It should be doable.

 

[MadScientist]

Post your question in this forum: https://www.thermostatforums.com/

 

[ch33zw1z]

My own acendote (long long story), my tankless system has an aquastat controller with no C connection. We (hvac guy and me) ran the required C wire to the neutral on the 24v transformer and it's working ok for a few years.

 

[bbhaag]

Probably won't see the OP again but I'll add my two cents just incase someone else stumbles across this thread in the future.
I think your best bet is to drop a new line from the attic AC unit to the Sensi thermostat you want to install. You said this is doable with your attic access and then you won't have to worry about hooking up and extra external kit to get the required C wire.
Then run a new line from your boiler system over to the Sensi thermostat. This might be a little more tricky but I think it is the only way to get your HVAC system on one thermostat instead of two.

 

[NutBucket]

I was thinking about this. I think the OP's situation is exact reason why thermostats have Rc and Rh terminals...and why you jumper them together if you have an integrated system (ie forced hot air + AC). Since a thermostat is just a heat-controlled relay, it makes sense. Therefore a C wire from either the heater or the AC would suffice since it is only needed to power the thermostat.

 

[bbhaag]

I was thinking about this. I think the OP's situation is exact reason why thermostats have Rc and Rh terminals...and why you jumper them together if you have an integrated system (ie forced hot air + AC). Since a thermostat is just a heat-controlled relay, it makes sense. Therefore a C wire from either the heater or the AC would suffice since it is only needed to power the thermostat.

Yep that was my logic as well. He only needs one C wire to power the thermostat not two coming from each system. Since he mentioned the AC unit in the attic was easier to access that is why I recommended running a new wire from it to power the thermostat.
He also mentioned wanting to only have one thermostat to control both heating and cooling instead of one for each so that is why I said he would then have to run a new line from the boiler to the Sensi.

 

[NutBucket]

I don't think so. He stated he has two wires for the boiler so those would connect to Rh and W terminals.

 

[bbhaag]

Yeah I understand that but wouldn't he need to run a line from the boiler to the Sensi if he wanted to just use one thermostat for both heating and cooing?

 

[NutBucket]

You are right. I missed the bit where he stated separate locations

I swear I skim through these posts way too fast sometimes.

 

[bbhaag]

You are right. I missed the bit where he stated separate locations

I swear I skim through these posts way too fast sometimes.

Yeah I think I read the OPs second post incorrectly to. haha
After I re-read it, it sounds like both the boiler system and the AC go to one thermostat now that is a slider style that runs off of batteries. If that is the case then you were correct that the wires for the boiler system should already be there. Then all the OP needs to do it run an additional wire from the AC unit for the C wire and he should be good to go.....maybe??
haha IDK it's kind of hard to understand the OPs posts.

 

[mikeford]

What is odd is that HVAC wiring hasn't changed much in close to a century, yet most lay people don't have a clue about.

 

[bbhaag]

What is odd is that HVAC wiring hasn't changed much in close to a century, yet most lay people don't have a clue about.

Yeah I think for the longest time people just never upgraded there thermostats. If it was in the house when you bought it you just left it there unless you bought a new system then the installer wired everything up for you. If you did replace your thermostat and not your entire system chances are it was a four wire programable thermo with a couple of AA batteries to keep it powered.

Smart thermostats changed EVERYTHING and now that more and more people are wanting them it's made them take the thermostat off the wall for the first time and seeing the wiring behind it for the first time makes them have questions. I mean lets face it before the smart thermostat no one in the HVAC industry used the C wire it was a niche thing that nobody ever used. Now if you want a smart thermy you better have that C wire or it ain't gonna work.

 

[NutBucket]

Yeah, no kidding. We just bought a house built in '04 and the C wire isn't connected. Forget smart thermostats, why bother with batteries in the first place? At least there are plenty of conductors so I can easily add one.

 

[ch33zw1z]

Yeah I think for the longest time people just never upgraded there thermostats. If it was in the house when you bought it you just left it there unless you bought a new system then the installer wired everything up for you. If you did replace your thermostat and not your entire system chances are it was a four wire programable thermo with a couple of AA batteries to keep it powered.

Smart thermostats changed EVERYTHING and now that more and more people are wanting them it's made them take the thermostat off the wall for the first time and seeing the wiring behind it for the first time makes them have questions. I mean lets face it before the smart thermostat no one in the HVAC industry used the C wire it was a niche thing that nobody ever used. Now if you want a smart thermy you better have that C wire or it ain't gonna work.

Your thermostat may work without the c wire, but maybe not as designed. I'll see if I can find my old thread, pretty sure I posted here

Edit:

Found it: https://forums.anandtech.com/thread...hile-calling-for-heat-hvac-questions.2488619/

So yea, do the c wire, or could have weird issues. Like buzzi g when calling for heat, or calling for heat in July to charge it's battery.

And the HVAC guys really had no idea what to do with it, and or how smart thermostats work in general.

 

[mikeford]

And the HVAC guys really had no idea what to do with it, and or how smart thermostats work in general.

The programming is new, but the operation is exactly the same except the relay/switch closures are controlled by a processor instead of mechanically.

Until recently I would rate most of the fancy thermostats as sucks alot. Our first was $200 and did not include battery backup, nice pretty display though.

 

[ch33zw1z]

The programming is new, but the operation is exactly the same except the relay/switch closures are controlled by a processor instead of mechanically.

Until recently I would rate most of the fancy thermostats as sucks alot. Our first was $200 and did not include battery backup, nice pretty display though.

Yea, they generally work the same, but the interfaces seem to throw the guys of (imxp), so they'll get used the them.

 

[bbhaag]

^^ Yep I had the same issue with the HVAC tech not really knowing how our Nest was supposed to be wired. Our main floor system is a little bit more complicated. It's a two stage Bryant heat pump and then a gas powered Bryant forced air furnace in the basement. When we moved in two years ago I removed the proprietary Bryant thermostat and tossed it in favor of the Nest.

Little did I know at the time that the Nest needed 8 wires and I only had 5. After the tech couldn't figure it out I spent hours online researching it myself and then ended up removing the existing 5 wires and running a new 8 wire line from the Nest down to the furnace and then out to the heat pump.

Here's a pic of how my Nest is wired. It was a pain running the new wiring but in the end worth it.