Combining Heat + AC thermostat into a new Wifi-enabled thermostat


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Old 11-26-15, 12:10 PM
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Combining separate Heat + AC thermostat into a new Wifi-enabled thermostat

Hi,
I'm looking for some help in combining two existing thermostats on the 2nd floor into a new Wifi thermostat such as this Honeywell model.

Here is the relevant information:
- House is a two floor colonial, plus a basement, built in 1994. 2nd floor has 4 bedrooms and 2 full baths
- Heating: 4-zone hot water baseboard heat (furnace in basement). 2nd floor all on one zone, with existing thermostat in master bedroom
- AC: 1-zone central air (air handler in attic right above 2nd floor) with existing thermostat in hallway of 2nd floor
- Heat thermostat: Old-school mercury one with red and white wire attached. Additional blue and green wires are there but unused (wrapped around sheathing). Wiring goes in-wall down to furnace closet.
- AC thermostat: Also an old-school mercury one, with a red, white and green wire (going to the "R", "Y" and "G" connections). These wires go straight up into the attic and to air handler above the hallway.
- Boiler: Crown Boilers Model ABF140SPD. Pictures show wiring.


Pictures here: https://picasaweb.google.com/1068100...NCw74qUirPT1wE


Goal:
- Move the wiring of the Heat thermostat to the hallway where the existing AC themostat is (I can route this myself through the attic)
- Use one Wifi-enabled thermostat in the hallway to control both heat (on the 2nd floor) and AC (whole house).

- Do I need to do any additional wiring (keep reading about newer thermostats requiring "C" wires)?

thanks
 

Last edited by petesamprs; 11-26-15 at 01:19 PM.
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Old 11-26-15, 01:18 PM
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That can certainly be done,but the jumper between R & RC must be removed and you will need to get the "C" wire connected ,use the blue wire connected to C on the stat and the other end to the C terminal on the furnace, connect the AH conductors as they are now at the old stat, R to RC, Green to G and white to Y. Do you have a voltmeter? It hard to tell from your pics which terminal on the control is the common,the goal is to have 24VAC across R&C at the stat,Red from the boiler to R on the stat,White from the boiler to W on the stat.
Geo
 
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Old 11-26-15, 08:24 PM
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Thanks, Geo. Let me just make sure I follow you.

After I bring my heat wires over to the hallway, I'll have the following wires at the hallway location:
- Original AC wiring: "R", "G" and "Y"
- Original heat wiring: "R" and "W"
- Unused blue heat wire: "C"

So you are saying when I wire up the new stat, do the following:
- the "R" for the AC will go to "Rc", while the "R" for the heat will go to "R"
- Wire up the "G", "Y" and "W" as indicated
- Wire up unused blue as "C" and connect other end to furnace
- remove R/Rc jumper on new stat

So main question is how i figure out the common on my furnace. I have a voltmeter if you need me to test something.

thanks
 
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Old 11-26-15, 08:56 PM
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Almost.... but not quite.

The C wire needs to come from the A/C unit. You will need R,Y,G and C to the A/C air handler.
You will only need two wires from the heating system.

On the stat end from the A/C air handler
red-------> Rc
green----> G
yellow---> Y
????? ---> C

from the furnace...
red---- > Rh
white--> W

Remove R/Rc/Rh jumper
 
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Old 11-27-15, 04:51 AM
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If you can post a clear pic of the control where the stat wires connect to the boiler that would be a help.
 
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Old 11-27-15, 05:00 AM
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PJ, "C" will come from the boiler,from the AH he has a 3 wire R,G,W, that white will go to "Y", unless I am missing something.
Geo
 
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Old 11-27-15, 05:52 AM
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Thanks. I'll head up to the attic to check out the air handler situation and take some pics of the wiring/transformer. The wire from the air handler to the existing stat is only 18/3, so I'll need to run an additional wire alongside the existing one for the common. Should be easy to do.

Out of curiosity why does the common have to come from the air handler's transformer instead of the furnace transformer?
 
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Old 11-27-15, 06:39 AM
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I am not sure either,I would take it from the boiler.
Geo
 
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Old 11-27-15, 08:19 AM
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I added some more pictures to my picasa link showing the wiring for the air handler: https://picasaweb.google.com/1068100...NCw74qUirPT1wE
 
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Old 11-27-15, 08:42 AM
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Many boilers do not have a common terminal available. They only have two terminals labeled T. These terminals are actually R and W.

When designing a thermostat if you take this knowledge into consideration it would be a better idea to make RC the terminal that powers the thermostat when separating the R and Rc terminals.

You need to grab 24 volt common from the same unit that powers Rc.


I can't read this transformer tag but I would expect this blue wire to be 24 volt common.

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Old 11-27-15, 09:32 AM
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Very clear, thanks.

One of my other pictures shows a closeup of the transformer label. You are correct that the light blue wire is attached to the "com" label on the transformer. However, the yellow wire on the top right also attaches to another "com" label. Is there supposed to be two common wires for the transformer?

Assuming the blue one is the right one, what is the best way to splice into it to run my wire to the tstat?
 
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Old 11-27-15, 10:31 AM
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Thanks, that is a clear picture. Yellow is high voltage 240 volt common and blue is 24 volt common.

I would crimp a spade connector to a wire and slip in on to what appears to be an available piggy back connection at the relay on the right with a line pointing to it labeled C.
 
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Old 11-28-15, 04:30 AM
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Makes sense. Didn't even notice the piggy back here.

Will do this and report back.

thanks
 
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Old 11-28-15, 09:12 PM
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Accidentally touching R to C will damage the transformer. I recommend removing power before installing the thermostat.
 
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Old 12-01-15, 12:57 PM
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Thanks, yes I was going to turn off AC handler power at the breaker before wiring.
 
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Old 12-03-15, 08:37 AM
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Sorry, stupid question. Where is best place to buy that female spade/disconnect connection? I couldn't find it at home depot and i'm not sure the ones online are the right ones (and they sell them in large quantities).
 
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Old 12-03-15, 09:44 AM
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http://www.amazon.com/Gardner-Bender.../dp/B000BVXVJK

I always use yellow so that I can fold the stripped piece of wire over, the crimp each piece of the connector down on the wire.

I think blue us actually the one that has 18 Gage stamped on it.

Yellow is better for my technique.

Radio shack, Ace hardware, maybe Frys or MicroCenter.


I get them at AC suplly houses like Johnson or Johnstone supply.

Kinda like thumb then fingers...
 
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Old 12-04-15, 02:15 PM
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I had an epiphany when reviewing the wiring for my air handler. I've uploaded some new pictures on the picasa link: https://picasaweb.google.com/1068100...Cw74qUirPT1wE#

There are two sets of wires (each is 18/4) leaving the air handler:
1) Wire bundle #1 uses 2 out of 4 available wires and goes to my condenser outside. The colors used are red and white. The red is actually connected to the blue common wire ("B" below) from the AH via wire nut "C" below, and the white connects to the white on the other wire bundle (thermostat)
2) Wire bundle #2 uses 3 out of 4 wires and goes down to my thermostat. The colors used are red, green and white. Red connects to the transformer, Green to the relay and white to the white on the other wire bundle (condenser). THE BLUE WIRE IS UNUSED AND I SEE IT UNUSED AT MY THERMOSTAT AS WELL (missed this before).

Therefore, rather than running a new common wire from the available piggyback on the relay down to my thermostat, couldn't I just simply take the unused blue wire from my thermostat bundle (labeled "A") and wire it into wire nut "C" to immediately create a common wire for my thermostat?



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Last edited by petesamprs; 12-04-15 at 02:31 PM.
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Old 12-04-15, 03:13 PM
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Yes, you can connect to that blue wire that runs to transformer C.
 
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Old 12-04-15, 04:18 PM
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Love the "high voltage" gloves.
 
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Old 12-04-15, 08:16 PM
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Ha, actually they are my mechanics gloves. All i had for the work in the attic.

Alright she's up and running. Did what I said above - fished the heating 2-wire out of the master through the attic, and brought to existing hallway thermostat. Tapped into common at air handler.

The only slight confusion during initial setup is it asks you what type of HVAC system you have (forced air, heat pump, hot water/steam). Confusing for me since I have hot water heat and forced air AC. I chose hot water assuming it was asking about heat - looks to be working properly. Thanks again for everyone's help.

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Old 12-07-15, 06:54 PM
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Ok, next question. I really like the Honeywell RTH9580 so I bought a second one. If you go back to my first post, you'll see that I have 4 heating zones in my house. The 2nd floor heating zone is now combined with my AC on my first Honeywell.

I want the new Honeywell thermostat to control one of my heating zones on the 1st floor. Obviously since this is for heat only I'll need to get common from the furnace. The existing thermostat on the 1st floor is also an old-school mercury one using only a red and white wire, but the existing wiring does have an unused blue and green that I can use for common.

My furnace is gas fired and uses a Honeywell Aquastat relay Type L8148E. From what I can see, the wiring works as follows:
- Each zone valve has 4 wires: 2 yellow and 2 red.
- The 2 yellow ones form a closed loop with the thermostat and transformer
- The 2 red ones form a closed loop with the Tv and T terminals on the Aquastat

The wire from my thermostat down to furnace closet, where it meets the zone valve yellow wire and transformer wiring, has an unused blue wire. Likewise, the wire from the red side of the zone valve to the aquastat below has an unused blue wire.

Therefore, two questions:
- Can I simply connect these two blue wires (my thumb and ring fingers below) to achieve a continuous common between thermostat and aquastat
- from previous threads for this aquastat, it seems the right place to tap into for the common is the jumpered W/Z (picture below). I assume I keep the jumper on and tap either one? Note that something is already tapping the Z terminal (vent damper?)

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Last edited by petesamprs; 12-07-15 at 07:46 PM.
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Old 12-07-15, 08:39 PM
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You're opening a can of worms here.

The Common will not be coming from the aquastat...... it will be coming from the transformer that powers the zone valves. That should be an external transformer.

If you had one stat directly to the aquastat.... then the C would come from there.
 
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Old 12-08-15, 05:54 AM
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Do you have a volt meter? You must read 24 VAC at the stat between the Red and common,so connect the blue wire to one side of the transformer and check for 24 VAC at the stat,if you have it ,then connect Blue to C and Red to R ,if not, connect the Blue to the other terminal on the transformer and remeasure the VAC.
You may want to consider the RTH6580 its much cheaper and will do the job,especially for only one zone, I have 2 and the work great.
Just a thought.
 
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Old 12-08-15, 05:43 PM
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Thanks guys.

PJ- Below is the picture of the transformers for each of the zones. For some reason I only see 3 transformers connected to my house line voltage, even though I have 4 zones. So one of my stats may go directly to the aquastat, but need to figure out which one my tracing the wires. Why would the system be designed to have 1 go directly and the rest have their own transformers?

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Geo - yes, I have a voltmeter and can test for 24VAC across terminals at the stat

I need some more explanation on how the wiring will work. Right now, the stat, transformer and zone valve effectively form a small loop (triangles are wire nuts):
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I understand the purpose of the C wire is to provide continuous 24VAC, creating a "return" for the hot 24VAC from the red wire. Isn't that what my white wire was already doing?

So when everything is said and done, you are saying it should look like the below? That would mean that one of the transformer terminals will have two wires (red + blue or white + blue, depending on which creates 24VAC across the red and blue at the stat).

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Old 12-08-15, 06:05 PM
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You have four zone valves and one circulator.... correct ?
You may one transformer controlling two zone valves.
Since the two red wires from each zone valve are isolated from the yellow stat wires.... they are all in parallel.

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Old 12-08-15, 08:08 PM
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Correct, 4 zones and 1 circulator.

This is getting interesting...I mapped all my wiring to figure out what was going on. Turns out that 3 of the 4 zones are connected to the transformer facing you just below the red switch. The basement zone is by itself connected to the one facing left. The one facing right on the red switch is for something else (phone system?) since the wiring doesn't end up at the furnace.

So here's the exact current wiring for the 3 zones on the single transformer:

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So here is what i'm thinking for the revised wiring. I think this will work for the zone I care about, but will it affect the other two zones in any way?

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Appreciate all your help. This is all new to me but I'm fascinated by how it all works.
 
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Old 12-08-15, 08:20 PM
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Looks good. With only a two wire stat..... reversing the R and W is not an issue. When you introduce the common connection to the stat then the R and W need to be connected correctly.

You won't have any interaction.
 
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Old 12-09-15, 05:55 AM
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That's it, check between the R & C at the stat to be sure you have 24 VAC if not reverse the Blue at the transformer, the White wire in the 2 wire configuration is a switch leg.
 
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Old 12-11-15, 06:03 PM
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Worked perfectly. Thanks again.
 
 

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