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I have a Nest thermostat that I am trying to wire in a simple but unique configuration. My apartment uses fan coil units for heating and cooling. The building changes over from hot to cold water for the summer months and back to hot water for the winter months. The old thermostat jumped the Y1 and W1 terminals. When the building changes over for the winter months, the mode on the thermostat is switched manually so the display shows the heating set points, but the same fan that blows air over the cold coils in the summer will now blow air over the warm coils for heating.

Unlike the old thermostat, the Nest does not allow the Y1 and W1 terminals to be jumped. It is programmed to see separate circuits on those terminals that turn on cooling and heating systems, respectively. I want to create a circuit that, when the Nest closes the circuit between the R and Y1 terminal, the fan turns on but the W1 terminal does not have a current. When the Nest closes the circuit between the R terminal and W1, the same fan turns on but Y1 does not have a current. After reading about diodes, I thought that I might be able to use two diodes to make the circuit perform as I described above, but then I became (more) confused when reading about using diodes on AC circuits. Can the circuit be built similar to my picture with the diodes below with a resistor to stop the diode from burning up or am I totally on the wrong track.

Nest CircuitNest Circuit w/Diodes

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    Diodes won't work with an AC circuit. The simplest solution might be to put a small manual winter/summer switch in there... not necessarily the most convenient, but the simplest... – Chris Stratton Nov 18 '17 at 22:02
  • @Trevor - what happened to your responses? – Fredster Nov 19 '17 at 5:28
  • @Fredster This thing wont let me edit the schematics in this forum, so I deleted it. Did you check with the Nest folks if their stat is smart enough to handle your situation anyway? They seem to have quite the support system set up. If not the summer/winter switch mentioned above is probably a better bet for you anyway. – Trevor_G Nov 19 '17 at 5:58
  • If you are able to use the TS with Y1 W1 jumpered, look into Heat+Cool mode which doesn't require a summer/winter switchover: nest.com/support/article/What-is-Heat-Cool-mode – Stavr00 Nov 20 '17 at 19:26
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Original Answer Deleted: Can not support over on this forum.

You really have a couple of conflicting requirements here.

1. The NEST Wire Detection

Since the NEST is "smart" enough to figure out what wires are connected to it via some method and algorithm in the controller, which no doubt is proprietary, and since said testing happens only when it powers up, you can't just use a simple summer/winter switch.

Without knowing exactly how that works it is foolhardy to try to design a solid state circuit to augment the thermostat to monitor both lines and turn on your fan when either comes on.

Though I am fairly sure there will be a way to fool the thermostat into thinking there are two contactors attached.

2. Space

The "add two AC relays" approach will fix the issue, but suffers from the fact that 24V AC relays are rather large and you are adding a contact life issue into the equation. As such, unless your house or apartment wiring has an extra line that is not being used so you can hook up the relays inside, or close to, the heater/cooler unit, this is an "ugly" approach.

Recommendations

You really need to do some experimenting.

First: Despite what they are telling you, I'd be trying a jumper between the W and Y terminals. They obviously don't want to recommend that connection, but I don't see why it should not work. The isolation between two relays is not that great in the first place.

With your fan wire connected to one terminal, jumper the other with a small resistor say 10R. If it works and the resistor does not get really hot with no demand from the thermostat, you are golden. If the resistor gets hot, remove it immediately! If the NEST pops up an error code, or simply does not switch the line, then you know for sure it's a no-go.

Second: Failing the above, then you need to figure out how to fool the thing into thinking there is a wire connected. I would start out hooking up a fairly large resistor, perhaps 100K, between the Y pin and the C pin. If it does not fool it, try smaller resistors down to about 1K - 2 Watt. If you still can't fool it you will need a more "relay like" dummy load with inductance.

If you can`t jumper it and you can't fool it, you are stuck with the two relay solution or finding a different TSTAT.

If you CAN fool it, then you have two new choices.

  1. Hook the fool-it resistor or circuit to both the W and Y pins so it powers up thinking both are connected, then use a summer/winter switch to connect the appropriate one to your fan as need be. Since the resistors will be in there always, the bigger the resistor you can use to fool it, the better.

  2. Have someone design you a solid state circuit that incorporates the fooling device, monitors the W & Y outputs, and turns on the fan when appropriate.

  • Trevor, thanks for providing a solution to my problem. i'm going to build the circuit. If the Nest provides power to both the Y1 and W1 terminals simultaneously, I assume the fan would just turn on? Can I find a solid-state 24V AC relay in a small size? I ultimately want to be able to put the circuit in a box used for a typical light switch. – Fredster Nov 18 '17 at 19:55
  • @Fredster that's te issue.. I could not find small ones, but I did not look long. – Trevor_G Nov 18 '17 at 19:57
  • looks like your second solution can be built in a small footprint! If I understand this (at least partially), if Y is closed, the current flows through R2, into the diode bridge and starts the fan. The higher ohm R1 ensures current flows through R2. If W is closed, the transistor M1 closes and connects the circuit to the diode bridge which again starts the fan. Is that close to a proper explanation? Why is C2 needed? – Fredster Nov 18 '17 at 21:21
  • @Fredster the bridge and M1 form an AC switch since the current always goes the same way through the MOSFET. D5 and D6 form a half wave rectifier along with C2 that sets up a simple DC power supply that turns on the gate of M1 through R1. D7 keeps the gate voltage at a max of 12V. R2 is there to discharge C2 when the Y and W switches open, which in turn, turns off M1. – Trevor_G Nov 19 '17 at 1:56
  • @Trevor - In my diagram, the thermostat closes R with Y1 or R with W1 depending on mode (Air Conditioning or Heating). In your circuit, I don't see the analogue between my diagram and the connection at the bottom of the 'diamond' (the bridge). I'm missing something. – Fredster Nov 19 '17 at 5:07
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When dealing with the Nest and other smart 'stats, the trick is that you need to put things on their outputs which will vaguely emulate a relay. That's because they electronically "sense" whether something is attached to Y and W.

If it senses nothing attached to a wire at configuration time, it will assume that feature is not there.

A switch that selects the line that is active is incompatible with that. Same is true of any scheme coming to a diode. Now, you could emulate a relay coil with a big resistor between W and C, and Y and C... at the expense of wasting some power and making heat in a potentially inopportune place.

The Y and W lines are meant to go to the coil of a contactor. The contactor interrupts mains power to the fan. The fan does not run on 24V.

Rectifiers method

In this scenario, you have two bridge rectifiers, then onto the fan relay coil. This gives the relay bouncing ball DC from either W or Y, without them interfering with each other. The relay coil doesn't care. It is laminated for AC, so eddy currents shouldn't be a problem.

The idea is the rectifier will be transparent to the Nest. It will see the coil load it expects to see (without any inductive kick). It doesn't know it is seeing the same coil on both wires.

  • rectifier 1‘s AC side goes to W and C.
  • rectifier 2's AC side goes to Y and C.
  • Both rectifiers' DC sides are paralleled and go to the fan contactor coil.

The contactor operates the fan, of course. Super important: the two wires from the rectifier output to the fan must be separate, they cannot be made common with R or C, or you will dead short the rectifiers.

Two contactors method

Simpler would be what Trevor describes in his since deleted answer. I saved this schematic out of his answer. Except where it says 'fan', that is actually the two sides of the fan contactor. Paralleling AC won't hurt here since only one will be on at a time, still they should be right next to each other.

schematic

  • my diagram did not have the fan relay in the picture. I oversimplified. This was my first post on stack exchange and I'm confused about what happened to Trevor's diagrams (removed?). Trevor's second picture was a little different than the way the nest works. I envisioned a circuit that closed the circuit with Y1 if in air cond. mode or closed the circuit to W1 if heating mode was operational. I can't find relays small enough to fit where I need them, so his second schematic with the bridge seemed like a better answer. I think I may still not be articulating where I'm confused. – Fredster Nov 19 '17 at 5:58
  • I;m not sure the polarity is right on those relay coils though. I think I got that backwards. common on the relays should go to the R side if Y and W are sinks.. But I'm not convinced of that either. – Trevor_G Nov 19 '17 at 6:02
  • @Trevor, in the solid state schematic you posted, the Y or W is a switch, but the Nest closes the circuit by connecting the R (from my drawing) with Y or W. A wire comes from Y to diode and W to diode. Then runs through circuit, but where is the single output to the fan relay to turn on the fan? – Fredster Nov 19 '17 at 6:06
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    @Trevor, I will not be able to test the simple jumper because I'm not at my apt and won't be for a while. It will be the first thing I do after I open the door. Nest can fix this in software, just offer the option to the homeowner/installer. With their new home security product just coming out, I doubt they will put as much effort into this as they once did. – Fredster Nov 20 '17 at 5:31
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    @Trevor, it took me a while to get to my apt, but I was finally able to jump Y1 & W1 as you suggested. After installing the jumper, the Nest-E identified the wiring change and sees both a cooling and heating system. It looks like it works. The only thing I ws unable to do was run the Nest test for the A/C because its too cold here and the Nest won't cycle the compressor at current outdoor temperatures. Thanks again for all of your help and insight! – Fredster Feb 25 '18 at 3:35
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I'm in a similar situation- a second apartment with a "two-pipe" heating/cooling system where I want to control the system's 120V fan motor using a Nest E thermostat. I also want to be able to remotely monitor the apartment's temperature using my smart phone, which the Nest will give me.

I'll be using a simple solution that I've bench tested and it worked as planned. The bench test included all the components in my schematic below with the exception of the fan motor. I was easily able to switch between the "heating only mode" to the "cooling only mode" and the Nest E thermostat responded correctly to operate the simulated fan motor on and off.

My next step will be to install these components into the live system (later in December). I'll confirm back afterwards on how it all worked out.

So here's the setup:

  1. Relay/Transformer- Since the Nest requires low voltage (24VAC) to operate, I'm using a relay/transformer to bridge the Nest thermostat over to the 120V fan motor. I selected an Aube RC840T-120 relay/transformer. (Thanks Anthony Ngu.) The RC840T-120's transformer feeds the Nest with low voltage power (using the C and R wires). The Nest returns a control signal back to the RC840T-120's relay coil to turn the fan motor on and off (using either the W1 or the Y1 wires, depending on the position of the SPDT switch.)

  2. SPDT Switch- Rather than trying to "trick" the Nest into believing there was both a heating system and a cooling system simultaneously available, I used a SPDT throw switch to select when the system is operating in the "heating only mode" or in the "cooling only mode". The Nest did not have trouble sensing when the system's heating or cooling features were available for use.

Wiring schematic-Nest to control two-pipe heating-cooling system.

  • When the Nest E is originally configured if it doesn't 'see' a connection on 'Y1' or 'W1' (assuming other wires are set up properly), it will not configure for heating and cooling - whichever wire is not attached. What I don't understand about your solution is how the Nest E sees the Y1 and W1 connections when its in setup mode so that heating and cooling functions are available during normal use. I have not yet tested @Trevor's idea of just jumping Y1 and W1 (since I haven't been to the apartment in several months) – Fredster Feb 11 '18 at 22:03

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