Convert a Line Voltage Thermostat to Low Voltage Nest
How to wire a replacement low-voltage thermostat to replace a 120V or 240V control
CONVERT LINE to LOW VOLTAGE THERMOSTAT - CONTENTS: 120V or 240V room thermostats for control of electric heat, electric baseboard heat, convector heaters, fan heaters, radiant floor electric heat - LINE VOLTAGE 120V Wall Thermostat Wiring
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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.
How to Replace a Line Voltage Thermostat with a Nest® or other Programmable Low-voltage Thermostat
Reader 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?
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.
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.
Does this make sense? Thanks!
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.
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
Aube Products, 705 Montrichard Avenue Saint-Jean-sur-Richelieu, Quebec J2X 5K8 Canada Tel.: (450) 358-4600 Toll-free: 1-800-831-AUBE Fax: (450) 358-4650 Email: email@example.com The product is described at"RC840T Series Switching Relay", at http://www.aubetech.com/products/produitsDetails.php?noLangue=2&noProduit=42
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.
Contact WarmlyYours at 800-875-5285 or at WarmlyYours USA, 590 Telser Rd, Suite B, Lake Zurich, IL, 60047 USA or in Canada at 300 Granton Drive, Unit 4A, Richmond Hill, L4B1H7 Canada, Tel: 800-875-5285.
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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 "The Nest Learning Thermostat", Nest Thermostat, 900 Hansen Way
Palo Alto, CA 94304, Tel: 855-4MY-NEST, Email: firstname.lastname@example.org, website http://www.nest.com/, retrieved 1/24/2013.
 Honeywell Controls, the company wants you to use their contact form at this web page: http://www51.honeywell.com/honeywell/contact-support/contact-us.html
Honeywell Consumer Products,
39 Old Ridgebury Road Danbury, CT 06810-5110 - (203) 830-7800
World Headquarters, Honeywell International Inc.,
101 Columbia Road,
Morristown, NJ 07962,
Phone: (973) 455-2000,
Fax: (973) 455-4807 1-800-328-5111
 Honeywell Corporation, Honeywell T451A, T651A, T694A,B,D,F Light Duty Line Voltage Thermostats, [installation, troubleshooting manual], 1992, Honeywell Consumer Products,
39 Old Ridgebury Road Danbury, CT 06810-5110 - (203) 830-7800 copy on file as Honeywell_Line_Voltage_T651A.pdf
World Headquarters, Honeywell International Inc.,
101 Columbia Road,
Morristown, NJ 07962,
Phone: (973) 455-2000,
Fax: (973) 455-4807 1-800-328-5111 copy on file as Honeywell_Line_Voltage_T651A.pdf
"T451, T651, T694 Thermostats provide on-off control of valves, fans, motors, contactors, electric heat elements, duct furnaces and fan coil units in heating/cooling systems. Switching sub-bases provide manual control of heating, cooling and fan."
 White Rodgers Thermostats and HVAC controls,
Homeowner information: http://www.emersonclimate.com/en-US/brands/white_rodgers/Pages/wr-homeowner-info.aspx
Contractor information: http://www.emersonclimate.com/en-US/brands/white_rodgers/wr_contractor_info/Pages/white-rodgers-contractor-info.aspx
White Rodgers Product Catalog (don't misspell the company's name as White Rogers Thermostats) -
http://www.emersonclimate.com/Documents/thermostats.pdf - Thermostat Catalog
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