Question about A/C & heating thermostat wiring


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Old 11-20-18, 04:54 AM
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Question about A/C & heating thermostat wiring

We just put up an addition to our house that will have a separate hot water baseboard zone. We plan on installing a nest E thermostat. We will also in the future have an AC unit in the attic and are laying the ducts for that.

I need to run the thermostat wires and am a little confused about what type of wire. Going back to my boiler for heat...what gauge should it be?

Then I also want to run a separate wire that will go into my attic for AC while my walls are open and not closed yet, but don't know what type of wire would control an AC zone. Is it the same as a thermostat wire in terms of size? I see 20/2, 20/3, 20/4, etc,

I've been buying a lot of my heating supplies from supplyhouse.com and am looking at these wires
https://www.supplyhouse.com/Thermostat-Wire-14700000

https://www.supplyhouse.com/PVC-Ther...ables-14010000

Not sure about the gauge and don't want to install an incorrect sized wire. Any help is appreciated.
 
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Old 11-20-18, 08:30 AM
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First - a Nest E isn't designed to handle separate heating and cooling systems. It has a single R connection. Other thermostats like the Nest 3rd gen, Ecobee, and Honeywell have separate Rc and Rh terminals to handle the cooling and heating connections.

18 gauge is standard for thermostat wire. When contractors are trying to save pennies they use 20 or even 22.

For the boiler you will need 18/2 but is good to run a few extra for spares and future issues. *assumes 24vac valves. rare but some home systems are line voltage 120vac .

For the air handler 18/5 to include a C wire.
 
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Old 11-20-18, 09:56 AM
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Thank you very much for the info! I wasn’t aware the E couldn’t handle separate systems. That stinks because we like the white thermostats and they are less priced. I thought they could somehow.
 
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Old 11-20-18, 11:57 AM
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You can get the Nest E to work using a simple relay for the heating. Not too difficult. Others have cheated and combined the R's from the heating and cooling. This sometimes works fine but other times blows fuses or has other issues.
 
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Old 11-20-18, 03:18 PM
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Hi, I would use 18/5 for both, why buy 2 different types of cable.
Geo
 
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Old 11-20-18, 03:25 PM
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My boiler is in the basement and ac will be in the attic which is why I need to run 2 separate lines. Was originally thinking 18/2 for each.
 
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Old 11-20-18, 04:04 PM
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You will probably need a cable to the condenser unit outside also, you will need more than 2 conductors from the attic to the Tstat if you want fan control, I would check the wiring Dia. from the AC unit to see what the min # should be.
Geo
 
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Old 11-20-18, 09:04 PM
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Originally Posted by Astuff
You can get the Nest E to work using a simple relay for the heating. Not too difficult. Others have cheated and combined the R's from the heating and cooling. This sometimes works fine but other times blows fuses or has other issues.
@astuff could you explain using a relay a bit to me? I researched a little and do see where the E only has an R connection and the 3rd gen has an Rc and Rh. How would I hookup the E using a relay and where would that relay go? What type of relay is it. I'm just thinking I'll have wires going downstairs to my boiler for heat and up to my attic to a future AC system, how do I combine those R wires to the E stat the best way. Is it where the Rc and Rh go into the relay and out comes a wire for R? How does it work exactly?
 
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Old 11-20-18, 09:42 PM
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Four wires required to the A/C. Two wires required to the boiler. The relay goes at the boiler.

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Old 11-21-18, 06:10 AM
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Originally Posted by PJmax
Four wires required to the A/C. Two wires required to the boiler. The relay goes at the boiler.

Attachment 98838
Thanks @pjmax, that helps a lot with understanding how to wire this up.
 
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Old 11-21-18, 09:06 AM
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I posted the minimum wires needed. Always good to have spares tagging along. You can use any 24vAC single pole relay. It is not critical. The relay will get its power from the A/C system. The nest E thermostat is run in the standard A/C - gas furnace setup.
 
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Old 11-25-18, 07:33 AM
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Thank you for all the information. I was able to get two nest e thermostats on black Friday and am thinking I'll run 18/2 to the boiler for each/each heating zone and then into the attic maybe 18/6 for a future AC system which I'll zone out the same with only two for the house, upstairs and downstairs.
 
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Old 11-25-18, 08:36 AM
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If only controlling the boiler then recommend at least 18/3. Nest sometimes has problems controlling heat if only 2 wires and no A/C connection. Needs a C/common wire to work.
 
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Old 12-03-18, 08:44 PM
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This is exactly what I had happen. I have 18/2 currently for my old thermostat hooked up to my boiler and hooked up a new nest e thermostat to R and W1 to test it out. It keeps cycling the boiler to charge the thermostat and heats up the house. I'm not ready for AC yet and probably will install that in the spring so for right now to use my nest, I think I need 18/3 for that C wire so it takes power elsewhere.

Where would I pull power from off my boiler? I'll have to run a new 18/3 thermostat wire and later run an AC thermostat wire. I have 18/5 for my future AC which will go into my attic and only have 18/2 into my basement to my boiler. I can switch it to 18/3 into the boiler for now before I do my AC, just not sure where to hook up the C wire to.
 
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Old 12-03-18, 09:22 PM
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There is no readily available C connection on a Hydrostat 3150 controller.
 
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Old 12-03-18, 09:37 PM
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This is low voltage, correct? Should I run a lead from my electric panel? What's the best solution so I can use this nest on the boiler only for now?
 
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Old 12-03-18, 09:40 PM
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Not good. That one only has TT terminals. One is likely the R and the other a W for the thermostat but the C/Common doesn't have a terminal. From picture appears to be a HydroStat 3150 - it's got digital circuitry so even the TT terminals might not be pure 24vac. If it is you could solder a wire to where the transformer connects to the circuit board to get a C. Need to explore with a multi-meter.

Safest to do would be to add a switching relay like https://www.amazon.com/SR501-4-Switc.../dp/B008G8S7M6. More kludgy would be a fan control center such as https://www.amazon.com/Emerson-90-11.../dp/B000PY2856

With Nest some have been helped by swapping the R and W wires. Worth a shot.
 
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Old 12-03-18, 10:07 PM
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Yeah I tried swapping the wires but the boiler keeps cycling every so often so the heat was rising. Hooked up my old thermostat on batteries for now. I thought maybe I could get low voltage C from my electrical panel since it’s not too far from my boiler but wasn’t sure. It is a hydrostat 3150.
 
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Old 12-03-18, 10:19 PM
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The transformer in that unit is not connected to ground so there is no snagging a C from ground.
You would have to pull that 3150 and connect your C to the back of the board.

For a quick easy way to connect the nest...... get a 24vAC plug-in wall wart.
 
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Old 12-04-18, 05:11 AM
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Something like this https://www.amazon.com/Replacement-H.../dp/B07B4K1LKZ

I don’t have any outlets near the thermostat but do next to the boiler. The C Wire would go to this?
 
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Old 12-04-18, 06:59 AM
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I forgot about the Nest transformer trick.

With a plug-in transformer normally 4 wires to Nest. Boiler TT to Rh and W1. Transformer to Rc and C.
For three wire you would attach one of the transformer leads to the boiler's R wire and the other lead would be C. You then use a multi-meter to measure between C and W1. Should be close to zero indicating on phase. If 48vac then need to swap wires on transformer.
 
 

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