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Guide to line voltage thermostats 120V or 240V room thermostats for electric heat, fan heaters, radiant floor heat, convector heaters. This article 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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I have a two wire honey well thermostat T451A - I want to replace this thermostat with a programmable. Presently
The Honeywell T451A room thermostat (illustration at left) is a line-voltage (120V) wall thermostat, a member of the product family including the T651 and T694 light duty line voltage wall thermostats.
These controls are widely used to directly control 120-V equipment such as valves, fans, motors, contactors, and electric heat systems as well as fan coil units in heating/cooling systems. Illustration at left of the Honeywell T415A and T651A from product literature. 
The T451 is a heat-only controller - I'm guessing yours may be controlling electric heat. I understand why you asked for help. Honeywell's own website includes a "thermostat finder" feature but is unable to give even one suggestion for a programmable line-voltage room wall thermostat. Happily the news is better than that.
Honeywell gives a succinct description of this class of wall thermostats:
Honeywell's newer replacement for the T451 includes the Honeywell TH114 which is also non-programmable and NOT what you want.
Programmable Line Voltage Heat-Only Hemostats
Examples of modern line-voltage programmable room thermostats include:
Honeywell TL8130A1005 - for electric baseboards and fan-forced heaters. (This is a single-function or "heat only" single-pole-single-throw (SPST) thermostat. Approximate Price: $50. U.S. designed to control 240V heating equipment.
Honeywell YTL9160AR1000 - line voltage, heat only, for electric baseboard or fan convector or fan-forced heaters, is an interesting development in wireless thermostats.
The kit includes a Wireless EConnect programmable wall thermostat and one Electric Heating Equipment Interface Module (EHEIM). Each EHEIM can support a maximum load of 3000W at 240V - figure $150. U.S.
The Honeywell EConnect™ TL9160AR is shown at above-left.
Honeywell's EConnect™ wireless thermostats can accept up to 8 EIMs on a single thermostat - sketch below right. [click any InspectApedia image to see an enlarged detailed version]  Honeywell EConnect™ wireless thermostats can control multiple electric heating baseboard units but only up to a total amperage load specified by the manufacturer, e.g. 12.5A - sketch, below left.
Lux PSPLV512 - 120V or 240V programmable wall thermostat intended for electric baseboard or radiant type electric heat, about $50. U.S.
Lux Luxpro Programmable Line Voltage Thermostat -( $70. U.S.) 120/240V SPST 1,900W at 120VAC (noninductive). 3,800W at 240VAC (noninductive).
[illustration at left]
Radimo Radistat-Pro, line voltage 120/240V thermostat intended for electric radiant floor heating ($120) available through Sears
This thermostat, or its double, appears under a range of name brands, so we suspect they're all the same product sold with re-branding as Radimo. Laticrete, etc
Dayton's line voltage room thermostat shown at left is the Dayton 1UHG4 heating-only digital, programmable room thermostat suitable for electric heat control. About $57. U.S.
Specifications from granger.com include:
Line Voltage Thermostat, 5+1+1 Programmable, Switch Type SPST, Switch Action Open / Close on Rise, Number Of Switches 1, Control Range 41-95 F, Differential +/- 1 Deg F, Height 4 1/4 In, Width 3 5/8 In, Depth 1 1/4 In, Temp Sensitivity +/- 1 Deg F, Sensor Type NTC Thermistor, For Use With Heat Only, Color White, Application Programming 5+1+1, LCD Display, Enclosure Type PVC, Includes Screws and Mounting Plate
Trane also produces a range of line voltage thermostats (Trane Line Voltage Thermostat M TC126 004 NIB, Trane TX 126 004, Trane T4039M1095, Trane THT0567) and programmable thermostats (ComfortLink™II, Trane XL900-XL200 compatible with 24-V systems when a 24-V relay accessory panel is added), with more information from from Trane.
Trane's wide array of electric unit heaters (wall mounted, ceiling suspended & others) include some models that can be operated from a room thermostat (24V WD1 or WD2 controlling 240V heaters or WD3/WD4 120V control operating 240V heaters with additional line voltage and 600-volt controllers available). 
Line Voltage Thermostats for both heating & cooling
Honeywell T6052A1015 - heating or cooling, but not programmable, SPDT thermostat. About $140. U.S. Supports 2-stage heat and 2-stage cooling.
The typical wiring connections for older, non-programmable line voltage thermostats is described ain Honeywell's T451A, T651A, T694A,B,D,F Light Duty Line Voltage Thermostats from which we excerpt and adapt some advice from Honeywell:
Watch out: disconnect all electrical power before making any electrical connections, tests, or even touching wiring on line voltage thermostat systems.
While some old timers figure out wiring by shorting thermostat wires together, that is imaginable only for low-voltage thermostat systems.
Touching live line voltage wires risks death by electrocution as well as fire, shock, and equipment damage.
See this 1949 Honeywell Control Handbook for an explanation of older line voltage (120-Volt) and 24-Volt thermostats and other controls such as the T40 series 4-wire 120V T44 Thermostat
The installation manual for your particular line voltage room thermostat will include connections for additional wires that may be necessary when the thermostat is being use in more complex installations such as connections to a winter-summer changeover switch, connections to control a separate blower fan motor, etc.
Hook up a wall plug on an electric heat circuit?
Reply: no but you can install a wall electrical receptacle to control a portable electric heater
Interestingly when first asked about this I said I didn't think so, out of concern that there may be no neutral wire or ground wire suitable for wiring an electrical receptacle or "wall plug" up to an electric baseboard heat circuit.
Watch out: and indeed, normally you would not be able to safely wire a 120V wall receptacle into the 240V circuit that controls electric baseboard heat since that circuit may be missing a suitable neutral and ground wire. See ELECTRICAL OUTLET, HOW TO ADD & WIRE.
But in researching wall thermostats we came across the combination line voltage wall thermostat for electric heat and wall receptacle combination shown in our photo (left).
This device is sold by Dayton, Model 1UHG6 and is intended to control electric heat as well as to provide an electrical outlet plug. Sold by grainger.com who includes these specifications:
So let's get clear just what this is: this is not an electrical outlet that you add to one leg of the 120V or 240V circuit that controls built-in electric baseboard heat. This is a device that is installed in the wall where a conventional electrical receptacle would have been installed. Dayton's 1UHG6 is, however a neat device. It is a wall thermostat that is intended to control a portable electric heater that is plugged into the wall where this device is installed.
You might note that the wall thermostat may be a bit low on the wall and you won't want it too close to the heater itself or it'll be dominated by heat local to the heater. Nevertheless it's a nice way to make your portable electric heater thermostatically controlled.
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