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2 Wire Thermostats: What Are They? Best Picks

What is a 2-wire thermostat?

If we put it simply, a 2 wire thermostat is one that has only 2 wires coming out of its backside. As you already know, there are different types of thermostats serving different types of heating/cooling systems. Some thermostats will have 4 wires coming out of their back, while others will have 5 wires or even eight wires. So any thermostat with only 2 wires coming out of its back is generally named a 2-wire thermostat.

Click here to jump to the list of the best 2-wire thermostats

As you may already know, thermostats are available for line voltage heating systems (like a baseboard heater) and low voltage heating systems (like a gas furnace or heat pump). They can not be used interchangeably owing to myriad technical reasons. Just like this, a ‘2 wire thermostat’ can be either a line voltage model (operating at 120 V or 240 V) or a low voltage model (operating at 24 volts).

The 2 wire thermostat for line voltage systems is technically known as single pole thermostat (and the 4 wire version is known as double pole thermostats). When it comes to low voltage systems, a 2 wire thermostat is usually known as a “heat only thermostat” and is used for systems like a gas furnace (with only a heating option). A 2 wire thermostat can not be used for HVAC systems with a cooling option, or heating systems with multi stages or a heat pump.

Wiring & Color Code

In the case of a low voltage (24 volts) 2 wire thermostat, there will be a ‘red’ and ‘white’ color wire coming into its back. The ‘red’ wire has to be connected to the ‘R’ terminal of the furnace & thermostat, and similarly, the ‘white’ wire has to be connected to the ‘W’ terminal of the furnace & thermostat. Note: You’ll have to take all the safety precautions before you wire a thermostat. Remember there is the risk of electric shock if you fail to keep the safety protocols.

Keep in Mind: If you are having a single-stage heat only system (like a furnace) in your home, you can use any kind of thermostat that supports single stage systems. For example, you can use the Nest 3rd Gen smart thermostat or Honeywell RTH9585 wifi thermostat for your ‘heat only’ single-stage system. In this case, you just need to connect the ‘R’ and ‘W’ terminals of the thermostats and leave all other terminals intact without making any connection.

The Nest 3rd Gen thermostat has 10 terminals, say Rh, Rc, W1, W2(aux) Y1, Y2, O/B, G, C, etc. You just need to connect ‘red’ wire from the furnace to Rh and ‘white’ wire to W1. These 2 connections will set the Nest thermostat as a ‘smart 2 wire thermostat’ taking care of your home heating ‘smartly’.

In the case of a line voltage (120/240 volts) 2 wire thermostat, there will be  ‘red’ and ‘black’ wires coming out of its back (color can vary like ‘red’ and ‘white’ for some brands, some other use ‘black’ and ‘white’)

Differentiating – Line Volt vs Low Volt

Now let’s see how to differentiate a line volt and low voltage 2-wire thermostat. You’d be thinking it is a confusing task to differentiate, as both have only two wires. Well, the key difference here is the ‘thickness’ of the wires coming out of the thermostat. The low voltage (24 volts) model will be having ‘thin wires’ or ‘small wires’ coming out of its back, whereas the line voltage (120/240 volts) model will have ‘thick wires’ or ‘bigger wires’ coming out of its back.

Best 2 Wire Thermostat Models

We now know ‘what exactly is a 2 wire thermostat’. Let’s review some of the popular 2 wire thermostat models used in US households.

Note: Almost all of the ‘2 wire thermostat’ models available in the market are ‘non-programmable’ types. If you prefer a programmable variant or a model with WiFi or smart features, you can buy any model you like and wire it up for a single-stage heat-only system (by just connecting the two wires ‘R’ and ‘W’).

1. Honeywell TH1100DV1000 – 2 Wire Digital Thermostat

Honeywell TH1100DV

This model from Honeywell (TH1100DV) is the most popular 2-wire thermostat among all other models. This is a basic heat-only thermostat with only 3 buttons on the device. The temperature range can be set from 40 deg F to 90 deg F. If you’re using it in garage mode, the temperature range can be from 35 deg F to 90 deg F.

The buttons provided are, an up/down button for temperature range adjustments and an Off/Heat button provided on the side to turn the thermostat ON/OFF. This model provides compressor protection to the heating system.

2. Emerson 1E78-140 Non Programmable Model

Emerson 1E78 – 140

This model from Emerson, 1E78-140 is a great upgrade for your old mechanical thermostat (if you still have one in your home). Moreover, 1E78-140 is one of the best-selling model in ‘2 wire thermostat’ category after Honeywell TH1100DV.

The reason this model would be a great upgrade to old mechanical thermostats is its design. The body is designed in such a way that it covers any marks (on walls) which could occur while replacing your old mechanical thermostat. In most cases, you don’t need an additional wall plate. This model is suitable to be used for single-stage furnaces, single-stage heat pumps, or millivolt systems.

3. LuxPro PSD010B – Heat Only Model with Fan

two wire thermostat
Luxpro PSD010BF

PSD010B – is a popular heat only, 2 wire thermostat model from LuxPro. They also have a variant with ‘fan’ control, named PSD010BF – which has ‘fan’ control as an additional feature. Both models have a rectangle-shaped backlit LCD display, which shows the set temperature value. They are compatible with all heat only single-stage systems.

The base model PSD010B has 3 switches for control, an ON/OFF switch on the side and an up/down control on the front side to adjust the temperature. The variant with the fan option (PSD010BF) has 4 switches, an additional switch on the side to control the fan.

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  1. Jack says:

    I appreciate it if you could help. I have an HVAC Carrier; there are seven wires coming out the back wall but only used two wires Red and White for the thermostat. It could control both Cooling and Heating. I want to upgrade to WIFI Thermostat, so I can control while away. I used the ‘Sensi” WIFI Smart Thermostat, but it didn’t work. It would be appreciated if you could tell me what happened or point out the compatible WIFI thermostat. Thanks

  2. Jack says:

    My Carrier system have 7 wires coming out the wall but there is only used Red and while wires for the thermostat. I wanted to replace with WiFi thermostat, so I can control while away. Could you help with the Model and how to install it. It would be appreciate..
    I bought the sensi WiFi smart thermostat but it didn’t work.

  3. mike says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    We have an old apartment built in 1970s and we just moved in. It runs on circulating hot water furnace. The current honeywell thermostat is old and i am replacing soon. It only has 2 existing wires. The problem is that the heat continues even if i lower the thermostat setting to lowest possible setting. Is this how it works with circulating hot water furnace?
  4. Brad says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    I have a 2 wire line voltage thermostat – im not super clear after reading the above if its compatible with a smart thermostat without rewiring or an adapter.
  5. Miriam Rivera says:

    I need to replace the thermostat in my basement, it is an old discontinued Honeywell T651A1269 non programmable heat only with 2 wires (blk & white) & apparently a low volt of possibly 24v. What thermostat do you recommend as a replacement?

    • Mathew Kevin says:

      Are you really sure your current thermostat is a low volt model? If you are unsure, please seek help of a professional. You can also take a picture and post in online hvac forums to get advise.

  6. Vishnj says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    I recently bought google nest 3rd gen and ecobee smart thermostate . U really confused about C wire . I have 2 wire low voltage connection . Please suggest if I can use about smart thermostate or not. If yes , they r working fine but will they face issue in future without C wire( due to lack of power) ?
  7. ARSHAD A ZEEDO says:
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    I HAVE THE TWO THICK WIRE RED AND WHITE AND HEAT ONLY AND IAM STILL WORRIED ABOUT TO SWITCH TO WHICH TYPE OF SMART WI-FI THERMOSTAT THAT I CAN USE CAUSE ACCORDING TO WHAT IS MENTIONED HERE AM I ABLE TO BRING ANY WI FE SMART THERMOSTAT LIKE HONEY WELL OR NEST OR ECOBEE AND JUST AND INSTALL THEM THROGH JUST TWO WIRE CONNECTION ?
  8. Guy says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hi Kevin I was reading your blog I do have a single stage heating system. Thermostats have a red and white wire which are regular thermostats from Honeywell. You were stating that a Wi-Fi thermostat from Honeywell would work without a C wire is that correct. Thanks Guy
  9. Doug Shumilak says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Nice write up, are you saying that we can hook up a Emerson smart wifi thermostat using only two wires on a 24 volt system? Once again I only have a R wire and a W wire. Which programable thermostat would you suggest for our two wire system. It does not have to be a wifi thermostat. I was thinking of using Emerson’s UP310 model. Any thoughts
  10. Dmitriy Zadushlivy says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hello. I have heating board in my cottage with old fashion thermostat. I have only 2 wires coming to thermostat. I am looking for WIFI smart thermostat to control heat from outside the house. Mysa requiring 4 wires and according to http://www.mysa is not compatible with 2 wire heating board. Do you know, which thermostat i should buy. Thank you.
  11. Randy says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    There are applications that use a 2 wire thermostat for cooling only for example mobile home package units that have a separate heating system
  12. Marcie says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hi, I have a programmable honeywell RTH6580WF, and I only have two wires in the wall, they look like thicker line voltage…(right?) one is black and one is white. Will the thermostat work if I simply connect the white to W and black to R? I don’t have a C wire. Can I jump between the black R to the C?
  13. June Marriner says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    How to return WYZE thermostat that will not work with a two wire system
  14. jeffrey says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    when I hookup the 2 wire from my old programable thermostat to the new honeywell 9585 the screen does not light up…the wires are very skinny it seems like they dont really lock into the push connectors very well or could it be something else
  15. Todd says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    I am seeking a programmable 2 wire heat only thermostat for my oil burner. Who makes such a product?
  16. Rachel says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hi, I have a question, I am renting an apartment with a very old looking thermostat thats falling apart, but still useable. We pay for gas, so I wanted to make sure I had something that would help us cut back and be more energy efficient. Is there any reason to update it to one of these thermostats mentioned? Would they help reduce cost? It still works, but Im wondering if maybe parts wear out over the years that would maybe make it less efficient?
  17. Brittney says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    @Matthew, I was wondering if you knew if the ecobee or the Nest was compatible with only 2 wire (R and W) thermostats. We have no other wires behind our current Honeywell, and would like to transition to a smart thermostat if there were any compatible ones. Thank you
  18. Ell says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    How about LuxPro GEO?
  19. jeremy thompson says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Another question rather than comment. Is there any real advantage to adding a programmable or smart thermostat to a two -wire, heat only system? I have a boiler with radiators. I’d like to maximize efficiency, AND be able to set my temperatures on a schedule/control from my phone or via home automation. Seems my options are pretty limited with my setup and trying to determine if it’s worth the pain of installing a c-wire or other work around. Thanks in advance! Jeremy
  20. Lux DMH110 Thermostat - Best Digital Thermostat Reviews and Buying Guide says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.

    […] 24V heating and A/C systems like heat/1 cool: gas, oil, electric or single stage heat pump systems, 2-wire heat-only hydronic systems hot water baseboard and radiator, millivolt systems (wall heaters/furnaces and gas […]

  21. Ron says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Can an Emerson Sensi WiFi be hooked to a 2 wire setup?
  22. Steve says:

    Mine is more an inquiry, than a comment. I was wanting to know; if these type of Thermostats could be used with a 2 wire system having an A/C and gas heater. The switch to activate heat or cooling mode being on the condenser unit, as on the Inter-therm model we have. More so, is there an adaptable wi-fi unit for this use.