Circuit to make Nest thermostat think there are separate heating and cooling systems

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.