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How to wire a replacement low-voltage thermostat to replace a 120V or 240V control

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Guide to using a Nest or other Low Voltage Thermostat with a switching relay to replace a line voltage 120V or 240V room thermostat for electric heat, fan heaters, radiant floor heat, convector heaters.

This article series describes types of line voltage wall or floor thermostats used to control heating or heating & cooling equipment where switching of 120V or 240V devices is required.



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How to Replace a Line Voltage Thermostat with a Nest® or other Programmable Low-voltage Thermostat

AUBE RC840T -SwitchingRelay at InspectApedia.comReader Question: how do I wire a Nest thermostat to a 120V or 240V electric heater?

2017/01/29 reader (anonymous by private email) wrote:

My current thermostat has 2 wires connected to it coming from the wall.

I researched this and see that this is single pole. I want to replace this thermostat with the Nest.

However, since this voltage currently is 110V/220V, I bought the Aube RC840T-240 to convert it down to 24V so the Nest will work.

How do I connect the 2 wires from the wall to the Aube RC840T-240 that has 3 wires?

Reply:

There should be wiring instructions in with the relay. Typically when a thermostat is controlling only heat the conventional thermostat wires are the red and white wires or R & W wires and the third wire would not be used.

But I don't know you're a specific situation that's why I suggest we need to see the wiring instructions for the product and switching relay you intend to use, in this case the Aube RC840T-240. I will give these below if you don't already have them.

Reader follow-up:

Lux Productgs ELV4 Programmable Line Voltage Thermosat at InspectApedia.com

This is the current thermostat [shown above], a Lux Products ELV4 Programmable Line Voltage Thermostat (available at Amazon and from other sources - Ed.)

When I disconnect it, there are only 2 wires connected to it. I know it is single pole, but I don't know if it is 110V or 220V as the thermostat says it is compatible with both. It is not 24V as the wires are thicker.

I've attached the diagram for the AUBE RC840T-240 as well as the actual transformer, but since I only have 2 wires from the wall, I do not understand the diagram and how to hook it up.

Example wiring diagram for the Aube RC840T switching relay for use with the Nest Thermostat to control 240 or 120V electric heat - at InspectApedia.comDoes this make sense? Thanks!

Reply:

Typically R & W are the heating connections in a 24VAC control, and the "C" wire is for cooling.

Electric heating baseboards are usually 240V. A switch is logically simply connecting or not connecting the 240V circuit.

Your original line voltage thermostat (TT) was making or breaking (closing or opening) a single pole single throw on-off switch on a single 240V or 120V wire circuit that powered the electric heater.

If you don't already have the instructions for your control, here are the AUBE RC840T THERMOSTAT INSTALLATION INSTRUCTIONS [PDF] - retrieved 2017/01/29, original source: http://www.aubetech.com/manuel/2/RC840T.pdf

Therein, look at Figure #1 - connection to a 2-wire low-voltage thermostat.

They show that you connect the thermostat's R and W terminals (and wires) to the R and W terminals in the AUBE RC840T-240 controller.

That illustration shows how the company expects the heater's wires to be connected as well.

In their example (Figure 1) one 120V line voltage wire L2 goes from the electrical panel to the 2-wire electric electric heater, and the heater's second wire at its "other end" (logically as it's a resistive heater) connects to the RED wire in the AUBE RC840T-240 controller.

In that same figure, the AUBE RC840T-240 controller's BLUE wire connects back to the electrical circuit's L2 (line voltage) wire.

Basically the AUBE RC840T-240 controller is switching that wire circuit on and off.

The AUBE RC840T-240 controller's BLACK wire would connect to the OTHER line voltage wire L1 if that wire is present and available to you. If it's not present I suspect it's not going to be used and you'd cap off that wire with a twist-on connector.

If you're not certain you know how to do this correctly and safely (Watch out: there are risks of death by electrocution) I would contact Honeywell (owner of Aubetech) or aubetch.com directly to ask their tech support what they advise

Here are the instructions for a related product, the Aubec 850T http://www.aubetech.com/manuel/2/RT850T.pdf

Here are separate instructions illustrating how to wire a NEST THERMOSTAT WIRING for 240V HEAT CONTROL when using a switching relay / transformer to control an electric heater through a switching relay, using an example from from WarmlyYours.com a radiant heating company.

I attach both sets of instructions. You'll see in the WarmlyYours pages when the second line voltage wire comes into use.

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