DIY Wired or Wireless Thermostat Replacement

Wired or wireless thermostats must be installed with correct wire connections.

Most thermostats are powered by and switch low voltage control power. Usually 24VAC. The HVAC system is equipped with one or more transformers that reduce the line voltage to safer control voltage. Some thermostats switch 115VAC or line voltage. If you don’t understand the difference and if you don’t have a multimeter to test for voltage, you should not attempt a DIY replacement.

Electricity is here to stay and if you plan DIY projects you should get some safety and technical training in its use.

Before you begin your replacement project, gather some information. What type system are you replacing the thermostat for? Following are the most common:

  • Single stage heat only. (Could be line voltage!!!)
  • Single stage cooling only
  • Single stage cooling, single stage gas heat
  • Single stage cooling, two stage heat
  • Heat pump with emergency heat

Select a universal replacement with all the features you want or require.

Remove your existing thermostat from the wall without disconnecting the wires. Use your multimeter to test for voltage. Write down the wire colors connected (and not connected) to your thermostat. Also write down the terminal designation on the thermostat or its sub base that the wire is connected to. As an example the red wire is connected to the R terminal. Note any jumper wires or jumper connectors connecting two or more terminals. An example might be Rc to Rh.

Most systems have one source of 24VAC control power or one control transformer. Some systems have a control transformer for cooling Rc, and another transformer for heating Rh. A single transformer system might require that RC and Rh be jumpered.

  • R terminal is the 24VAC power supply from the control transformer.
  • C terminal is the common terminal that completes the 24VAC control circuit. The black wire is recommended. Digital thermostats need a common for operation. This wire may not exist at the thermostat if you are replacing a non-digital thermostat.
  • Y terminal is usually connected to the first stage cooling contactor. Your thermostat will switch power from R to Y to turn on the compressor. There is probably a time delay relay built into the circuit that will delay this action. The wire color for the Y terminal is usually yellow.
  • Y2 terminal is the second stage of cooling if your system has 2 compressors. The wire color is usually blue.
  • W terminal is used for first stage of heat. This terminal uses the white wire. This wire will be run to the furnace air handler.
  • W2 terminal is second stage heat for multistage furnaces and boilers or back-up heat for heat pumps. The brown wire is normally used for stage 2 heat.
  • G terminal is for the indoor blower. The green wire will energize the indoor blower relay and will switch line power on/off to the indoor blower motor.
  • O or B terminal. Heat pumps must switch the reversing valve from cooling to heating. This terminal performs that function. The wire may be orange.
  • E terminal is for emergency heat. Wire color varies.
  • S1 and S2 terminal are for outdoor air temperature sensors.

Disconnect power to the outdoor unit. Remove the access panel or panels to the electrical section. This section contains high voltage and can cause death. Check for the presence of voltage with your multimeter before proceeding. Write down the terminal designation and the wire color attached. Do the same thing at the indoor unit and verify terminal designation and wire color.

If you are satisfied with the wiring for your system, simply disconnect you old thermostat and reconnect the correct wire to the correct terminal on your new thermostat.

Contact us if you have any questions.

Video, Replacing a Thermostat

YouTube video showing replacing a thermostat.

Click here  Replacing a Thermostat

Line Voltage Freeze-Stat, Thermostat

If you have anything that needs heat for protection from freezing, this thermostat might fit the bill. We use this stat in our food storage pantry to keep the temperature above freezing.  This thermostat can be used for de-icers, boats, RV’s, pump houses, etc.

The thermostat operates a heat lamp or 120V plug load in the well insulated room, located in an outside storage building.

Another application is in a small greenhouse to keep the temperature above 40 degrees.

Honeywell CW200A1032 Winter Watchman $20 to $30. Online.

An alternate product is the Thermo Cube
The Thermo Cube is a thermostatically controlled duplex outlet with a fixed range.  The device turns the load on at 35F and turns off at 45F.
Thermo Cube is sold by Farm Innovators, about $10.00

Thermostat Selection

If you have decided to replace your thermostat yourself you should have skills in low voltage electricity and wiring.  You will need a voltage meter to verify voltage sine there are a few line voltage thermostats installed that are 120V and can be fatal.

You will also need sufficient knowledge of the type system you have so you can select a replacement thermostat with the correct functions.
Many modern thermostats are manufactured to fit multiple system configurations.  This feature can complicate the installation.

Find an exact replacement or a thermostat that lists the thermostat you are replacing if possible.  If you are not sure, contact a professional or contact us.
• AC only split system.  Most thermostats manufactured for cooling will work.
• Gas furnace only.  Most heating thermostats will work.
• Oil furnace only.  Most heating thermostats will work.
• Electric furnace only.  Most heating thermostats will work.
• Heat Pump.  Make sure the t-stat lists heat pump use.
• Heat Pump wit auxiliary heat.  Most heat pumps have auxiliary heat.  Make sure you get a heat pump thermostat.
• AC + Gas furnace.  Get a heat/cool thermostat for gas heat.
• AC + Oil furnace.  Get a heat/cool thermostat for oil heat.
• AC + Electric Furnace.  Get a heat/cool thermostat for electric furnace.
• Multistage AC.  Make sure you get a thermostat that lists multiple stages of cooling.  This is typically two compressor units.
• Multistage heating.  Multistage heating is fairly common.  Get a thermostat that can be used fro multiple stages of heating.
• Multistage heat/cool.  Make sure the thermostat is listed for use with both multistage heat and cool.
• Electric baseboard heat.  Typically high voltage.  Exercise caution.  Make sure the thermostat is rated to switch the voltage and amperage installed.

Some systems have separate control transformers for heat and cool.  If this is your situation you will have and will need a thermostat with RC and RH terminals.  Check.

My favorite line of thermostats are the Honeywell Focus Pro 5000 series, or the programmable RTH7600

Ecobee Wi-Fi Thermostat, Wireless Thermostat

Ecobee was awarded the Innovation award from the International Air-Conditioning, Heating, Refrigerating Exposition, AHR Expo.  The “Energy Management System” won in the commercial application and the “Smart Thermostat” was awarded an honorable mention in the residential application.

The Smart Thermostat is Wi-Fi equipped and can be adjusted from any internet access as well as a free iPhone app.  A private web page for access is also free.

The Smart Thermostat list price is $469.00.

Wi-Fi Thermostat is a Wireless Thermostat

Wi-Fi Wireless Thermostat

Radio Thermostat

Affordable wireless thermostats and Wi-Fi thermostats are now available at some of the big box stores for under $100.

A major improvement over the programmable thermostat a Wi-Fi connected t-stat allows you to make adjustments from just about anywhere.

You can turn it down from the office or make adjustments from another device on from your car on your way home.  If you remember you forgot to make an adjustment while out of town, you can hop online and take care of it.

The pricey Ecobee at $500 will have a problem competing with the Radio Thermostat Company of America’s new offering at under $100.  Other offerings are free but with limited available thru some electric utilities.  The thermostat is accessed through thru Radio Thermostat website or an iPhone app.

Look for upgrades and further automation in the future.

Designed for the DIY market, installation and connectivity are not difficult.

UtilityPro Programable Thermostat

Honeywell reported that UtilityPRO™ is the top selling thermostat for demand response applications. Demand response control allows utilities to turn off air conditioning momentarily during high demand periods.

Honeywell has delivered over 350,000 touchscreen programmable thermostats. The thermostat gives utilities control over peak usage while assisting households to reduce energy expenses.

Thirty-three utilities selected the thermostat for demand response. UtilityPRO enables utilities to cycle air conditioners for short intervals whenever energy use and prices surge. The effect on comfort is reported to be minimal, but the collective reduction in demand allows utilities to stabilize costs, decrease the need for more power plants, as well as eliminate disruptive brownouts and blackouts.

Most participating utilities supply the thermostat and installation at no cost to the consumer. The thermostat has the same features and appearance of its non demand response commercial models.

The thermostat can communicate with utility company control systems allowing the utility to study consumption. The utility can also send text messages to the thermostat user.

Thermostat Troubleshooting

The primary connection interface between you and your air conditioning/heating equipment is your thermostat.
The thermostat, is a temperature switch that turns the air system on and off. Like with any switch it takes electrical power to operate. Most Residential and commercial thermostats are generally powered by 24 volts A.C. The source of this 24 volts A.C. is from the system control transformer.

When you have problems, check the following.
• Check out the circuit breaker for your air handler to ensure it’s not tripped.
• Check the power switch which will be placed close to the air handling unit or furnace. It isn’t unusual to mistake these types of switches for a light switch and turn them off.
• Some thermostats employ batteries for electrical power. Look at the batteries.
• Some older thermostats with no batteries drop the program each time you suffer a loss of power.

Thermostat location is important for proper operation.
• Can air get to the thermostat through the wall cavity? Remove the thermostat and seal the wire access hole.
• Is the thermostat too close to a door or window? Relocate the thermostat to a location where it can sense average mixed air entering the return air for the system.
• Does direct sunlight hit the thermostat? Is it close to a heat source such as a fireplace? Relocate.

Other issues
• Has your heat anticipator been correctly set? Your heat anticipator used on older mechanical non-digital thermostats must be set to the amp draw for the heating control circuit. The heat anticipator provides a small quantity of energy cost savings and prevents thermostat overshooting as it powers off the primary burners. Electronic and programmable thermostats contain built in heating and cooling anticipators that automatically set themselves requiring no manual adjustments. A mechanical thermostatis adjusted with an amp meter to establish the correct setting.
• Mechanical thermostats with mercury bulb switching mechanism, should be leveled for proper operation.

Wireless Thermostat, Programable Thermostat

A Wireless Thermostat is also available as a Programable Thermostat.

Effectively utilizing a programmable thermostat inside of your residence is one of your simplest methods to conserve energy as well as save money.

A programmable thermostat aids in making it effortless for you to save by providing pre-programmed configurations to help control your home’s temperatures in both summer season and winter season – any time your family is sleeping or away.

The pre-programmed configurations which are available with programmable thermostats are designed to provide cost savings without compromising comfort. Based upon your family’s routine, you may see substantial cost savings by adhering with these configurations or fine-tune them as best suited for your household.

The key element is to create a plan which automatically decreases heating and air conditioning in your household whenever you do not require as much cooling or heating.

Recommended pre-programmed configurations for a programmable thermostat may be:

  • Wake up 6:00 a.m.heat ( ≤ 70° F) … cooling ( ≥ 78° F )
  • Day time 8:00 a.m. Setback heat at least 8° F and Setup cooling at least 7° F
  • Evening 6:00 p.m. heat (≤ 70° F) … cooling (≥ 78° F)
  • Sleep 10:00 p.m. Setback heat at least 8° F… Setup cooling at least 4° F

A Wireless Thermostat with Programable Thermostat features is an easy way to Energy Cost Savings.

Thermostat Wiring

There is no official standard for wiring colors, or wiring terminals. To be safe it’s always best to refer to your equipment manufactures wiring diagram. Don’t trust the wire colors. The installer can use any color to complete any circuit. Verify each wire first.

In the following table is a note for each terminal and its common function. The normal color wire is listed. DO NOT ASSUME these are what you have.

Terminal          Purpose                             Normal Color          Notes

R                       24 V Power                              Red

Rh                    24 Vac Heating Power             Red

Rc                    24 V Cooling Power                   Red

C                      24 V Common                             Black

Y                   Cooling /Compressor                Yellow          Cool or 1st stage ht heat pump

Y2              Second stage Cooling               Blue or Orange

W               Heat                                                  White                           First stage heat could require jumper to Y for heat pump. Could be second stage heat

W2            Second Stage Heat                      Various                       First stage Aux heat for heat pump

G               Fan                                                    Green                      Fan switch or call for cooling or heat pump

E               Emergency Heat Relay             Various                   Disable the heat pump or make Aux heat first stage

O             Reversing Valve for Cooling         Various

X                 See notes

X2              Second stage heat, indicator light for some thermostats Various            See notes

T                 Outdoor anticipator                        Various

L                  Service Light                                      Various             Used to notify you of a problem with the system


Wires that cause greatest confusion are B and X

B terminal NEMA standard use B terminal for heating changeover valve.

Trane and  older Honeywell common side of transformer.

“X” standard for common.  some use “X” or “X2” for emergency heat relay.