Programmable thermostat Complete Guide to Heating, Heat Pump, or Air Conditioning Thermostats
How to buy, install, wire, use, adjust, or repair room thermostats

  • THERMOSTATS, HEATING / COOLING - home - CONTENTS: Guide to Thermostats for Heating and Air Conditioning Systems - how to Find, Adjust, or Repair Thermostats - Troubleshooting & Repair Guide
  • POST a QUESTION or READ FAQs about Room Thermostats for Air Conditioners & Heating Systems or heat pumps: thermostat operation, wiring, adjustment & repair procedures

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Heating & Cooling Thermostats, how to install, set, troubleshoot & repair home page: this article series explains selection, installation, use, setting, and adjustment of heating or cooling system room thermostats or "wall thermostats".

Here in the article links found at the "More Reading" links at the bottom of this article and at the top of this article we provide a complete guide to buying, installing, using, adjusting, & repairing thermostats for heating or air conditioning or heat pump systems.

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. Our page top photograph illustrates an antique room thermostat still in use at the Franklin Delano Roosevelt FDR Estate in Hyde Park, NY.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Types of Thermostats Used to Control Air Conditioning & Heating Systems

Honeywell room thermostat

Here we illustrate a traditional Honeywell round bimetallic-spring type room thermostat that was used in thousands if not millions of homes in North America & other countries.

If your heating system is not working at all, start at HEAT WON'T TURN ON.

Article Series Contents:

Traditional & Early Clock Thermostats & How They are Set

Photograph of an oil fired heater stack relay

Heating & Air Conditioning Thermostats where are thermostats usually located, what types of thermostatic controls are used on various heating and cooling systems, how do we find all of the thermostats, how do we set, adjust, and use thermostats

The round Honeywell® wall thermostat shown in the photo above has been locked in a plastic enclosure to prevent people from changing its set temperature. Set temperature and how the thermostat works are described in excruciating detail below.

GE Telechron Thermal Control Room Thermostat (C)

Above is General Electric's Telechron thermal control (room thermostat) first produced in 1933. This was probably the first clock-controlled automatic set-back residential wall thermostat produced in the U.S. The top of the thermostat is hinged to permit acces to its interior for wiring.

GE Telechron Thermal Control (thermostat) is described by the U.S. Smithsonian Institute's National Museum of American History:

General Electric manufactured this Thermal Control thermostat around 1933. The thermostat came equipped with a Telechron synchronous motor that powered the clock attached to the thermostat.

The clock allowed the furnace to turn on and off at a certain time, and the thermostat has two wheels on the right and the left, which allowed the home owner to set a desired temperature during the day and during the evening. - retrieved 2017/12/31, original source: Smithsonian Institute, National Museum of American History, Constitution Avenue, NW
Between 12th and 14th Streets Washington, D.C., USA,

Below is a later clock-thermostat widely sold int he U.S, the Honeywell Chronotherm wall thermostat, another clock-operated automatic day-night room thermostat.

Honeywell Chronotherm thermostat from 1950 (C)

The Honeywell Chronotherm thermostat is described in this 1950's HONEYWELL THERMOSTAT GUIDE [PDF 3MB file] produced by the Minneapolis-Honeywell Regulator Company, Minneapolis 8, Minnesota, (1950) reproduced from a Dodge building products catalog. Clicking on the first page in that PDF file should return to this web article.

At HEAT ANTICIPATOR Adjustment we describe how an internal adjustment can be made to this thermostat to improve its operation in some types of heating systems.

Also see THERMOSTAT CALIBRATION where we discuss the accuracy and calibration of room thermostats.

Room thermostats for air conditioning work and are set the same as when these devices are used for controlling heating systems. We provide lots of detail about how these devices work - see THERMOSTAT SETTING INSTRUCTIONS.

Traditional Rectangular Wall Thermostats and how they are set

Traditional wall thermostat

On this wall thermostat the red pointer shows the current room temperature.

The black pointer at the center of the scale at the bottom of the little window shows the temperature that the thermostat has been "set" to.

The "Set Temperature" is the temperature that the room occupant is requesting.

Moving the thermostat adjustment lever (located at the top, side, or bottom of the thermostat depending on the model) will change the set temperature up or down, causing the heating or air conditioning system to turn on or off as needed.


Programmable, Digital Room Thermostats & How They are Set

Programmable thermostat

Digital room thermostats are illustrated in our two photographs shown here. A modern digital thermostat uses an LCD type display to show the current room temperature along with other information.

Typically the display changes in response to pushing the thermostat control buttons.

Most digital room thermostats are "programmable" - you can set the thermostat to set back the room temperature at night (in heating mode, for example) and many models allow different time and temperature settings both throughout the day and for individual days of the week as well.

Our digital thermostat shown at left indicates the time (5:43 PM) in the left side of the display and the current temperature (53 °F) in the right side of the display.

With the thermostat's plastic cover "shut" it's operation is very simple, just showing up and down arrows that temporarily call for a higher or lower room "set" or "desired" temperature.

We opened the plastic thermostat cover to show additional detailed controls and instructions.

Our Favorite Room Thermostat - The Honeywell RTH2300 / RTH221

Honeywell digital room thermostat (C) Daniel Friedman

The Honeywell RTH2300 / RTH221 series room thermostats are easy to program, permit different weekday and weekend programs, and include a "hold" button that is invaluable for keeping the thermostat at a fixed setting without losing the whole program.

Honeywell displays a "Rated Overall Best Brand" sticker on the packaging indicating that this model received a "Best" rating by an independent Frost & Sullivan study.

At THERMOSTAT SWITCHES, INTERNAL we describe how to set this thermostat's internal "Gas" or "Oil" slide switch.

At TROUBLESHOOT ACCURACY & RESPONSE of THERMOSTATS we describe the accuracy of this thermostat, typically +/- 1° F.

We describe the operation of a typical digital room thermostat for heating or air conditioning later in this article at DIGITAL ROOM THERMOSTAT OPERATION

Honeywell digital room thermostat (C) Daniel Friedman

A simpler programmable Honeywell® digital room thermostat is show in our photo at left. This is a Honeywell CT2700 Electronic Round Programmable Thermostat.

The thermostat is showing the current time (12:15 PM) and current room temperature (67 °F).

This unit does not provide additional buttons under a cover. But by pressing the center "Set" button the display changes to allow the user to set a desired time and temperature setting for day and night time temperatures.

At THERMOSTAT SWITCHES, INTERNAL we discuss setting the thermostat's internal F/E toggle switch.

Watch out: we installed and tested two of these thermostats over several years of use. While the device is simple, un intimidating, and easy to program, the lack of a "hold" feature was annoying and required more fooling around; we also would have liked an additional daytime automatic setback period that this device doesn't offer.

But the worst discovery was that our oil burner was short cycling, sometimes turning on for just a few seconds - a problem that we finally traced to a defect in one of these thermostats. (The problem ceased when we swapped in our preferred thermostat shown above). We also found the flat cable wiring of this thermostat very fragile - eventually the display cable simply detached itself from its contacts.

The Nest™ Learning Thermostat - home automation features include buiding occupancy monitoring & communicating with the Nest Protect Carbon Monoxide & Smoke Detector

Nest thermostat

"The Nest Learning Thermostat" is an electronic device that can control room temperatures and possibly other components connected to or installed as part of a home automation system. We have installed three of these in our test building and will report further on ease of installation, programming, and use as well as using and adjusting the Nest thermostat remotely from your cellphone, computer, or tablet.

Photo: the Nest® Learning Thermostat on demonstration display at a Home Depot store in 2013. [Click to enlarge any image]

Nest Learning Thermostat Installation & Use Article Series

Radiator Thermostats & Baseboard Thermostats, Automatic - TRVs & How They are Set

Radiator thermostat (C) Daniel Friedman

Several manufacturers provide thermostatically-controlled radiator valves (Armstrong, Danfoss, Hoffman Specialty, Jacobus (Maid'O'Mist) and others).

Automatic radiator/baseboard valves provide thermostatic controls that can be installed on individual hot water radiators, steam radiators, or heating baseboards. The automatic radiator or baseboard valve will automagically open or close to attempt to control room temperature to the desired level.

Shown at left is an adjustable individual thermostatic control installed on a hot water radiator in Molde, Norway.

For details see RADIATOR VALVES & HEAT CONTROLS This article describes these automatic or thermostatically controlled radiator valves (such as the Armstrong RV-4) that will allow you to set the desired room temperature.

Also see RADIATOR STEAM VENTS, 1-PIPE SYSTEMS for a description of automatic steam vents used to control the rate at which steam heat radiators will get hot. Automatic steam vents can help balance the delivery of otherwise uneven steam heat throughout a building.

Remote Control Room Thermostats for Heating and Air Conditioning & How to Use Them

Photograph of the outdoor safety switch for an air conditioning residential system, shown here with the cover open

Remote control thermostats such as the unit shown at left are usually used with air conditioning or heating split systems using an outside compressor/condenser unit and one or more indoor wall-mounted cooling or heating units..

The thermostat controls in the hand-held remote control communicates with a wall-mounted air conditioner or heater using infra red signals.

The open finned area at the bottom of this Sanyo control (at the left in our photo) permits ambient air to enter the control for purpose of sensing the air temperature.

A control such as this Sanyo™ unit can be quite sophisticated and include automatic set back temperatures, timers, etc.

See SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS for details, including troubleshooting and fixing remote control thermostats themselves
as we describe in detail

Hand held A/C remote control does not seem to be working

Have a hand held remote for TCL air-conditioner (split model TAC-30CHS/G). when we set temperature, conditioner blinks what you want it to be, then it goes to inside temperature (suppose to do this) but then it quickly jumps to 34 degrees & switches off. Guessing it is thermostat? Anne Webber 9/2/11


Anne, sounds odd to me too. Try

If those steps don't fix the problem and you don't see more help in the installation and use manual, it's time to ask for an HVACR service call as there could be a bad controller, thermostat, or control board.

Outdoor Thermostats on the Heat Pump Compressor-Condenser Unit

Heat pump outdoor thermostat control (C) D Friedman N Renn

Heat Pump Outdoor Thermostats are used to control the turning on and off of backup heat.

Our photo (left, courtesy of Neal Renn shows the thermostatic control unit in the outdoor compressor/condenser unit of a residential heat pump.


Also see Detailed Case of a Heat Pump that Put out Heat when Cooling was Required

Choices of Room Thermostat Models, Brands, Features

In the article above and at Related Links (near page top) we describe the installation and use of most types of room thermostats used for heating, cooling, even dehumidification in buildings. But there are hundreds of models and features available.

The largest selection of room thermostats we find anywhere is at Grainger who has over 250 models in their online catalog. - but beware not every thermostat description necessarily names all features.

Room Thermostat Temperature Sensor Mechanisms

Bimetallic spring thermostat (C) Daniel Friedman

A variety of methods are used to sense room temperature and to communicate that data to the thermostat's heating or cooling switching mechanism:

Bimetallic Element thermostats use a bimetallic spring consisting of strips of two different metals fused together, typically into a coil shape. As room air temperature causes the two metals to expand or contract at different rates, the effect is that the spring coils up or un-coils, moving a switch that turns heating or cooling or or off.

Bellows Element thermostats are filled with a volatile liquid that vaporizes at temperatures typically found indoors. As the liquid vaporizes pressure inside the bellows expands, translating temperature change into the movement of a contact to turn heating or cooling on or off in response to building temperature.

Thermistor-type thermostats use a tiny solid-state electronic component, a type of resistor whose electrical resistance changes in response to temperature. By combining the thermistor with appropriate additional circuits and controls the thermostat uses the change in resistance to control the switching of the thermostat's control of cooling or heating on or off in response to room temperature.

For details: at TEMPERATURE RESPONSE of ROOM THERMOSTATS we give a detailed description of how room thermostats sense & respond to room temperature, and


and THERMOSTAT SWITCHES, INTERNAL we explain methods used to fine-tune thermostat operation in response to temperature swings, thus adjusting the burner on-cycle or fan operation if the heating system uses a fan.

Also see THERMOSTAT CALIBRATION where we discuss the accuracy and calibration of room thermostats.

Steam Heat Room Thermostats, Wall-Mounted

Steam  heat thermostat, Vassar College offices (C) Daniel Friedman

Room thermostats used to control steam heat may vary from their cousins illustrated above, or not - depending on just what the thermostat is controlling. In a residential application a standard room thermostat works fine to turn on or off the steam boiler.

But in larger buildings you may find that the room thermostat for steam heat is controlling a valve or relay that admits steam to radiators in one room or in just one section of a building.

In our photo at left, this wall thermostat controls steam heat in a college office in Poughkeepsie, NY.

[Click to enlarge any image]

The upper scale indicates the current room temperature.

The lower dial-scale is the set-temperature adjustment on this thermostat. The thermostat controls a steam radiator valve that in turn admits or shuts off heat into the space managed by this device.

RADIATOR & BASEBOARD THERMOSTATS, Automatic - TRVs provide an alternative that can control individual hot water or steam radiators in buildings.

Thermostat Accuracy: Settings vs Room Temperature

Reader Question: Honeywell RTH 2300 thermostat in cool mode accuracy

13 Aug 2015 Bryan said:

Office environment with a HONEYWELL RTH2300 thermostat . . . in Cool mode Fan On and set to 75F Hold . . . cooling does not click on until 77F reached on display temp . . . and does not click off until 73F reached on display temp . . . I would think cooling should click on when above 75F . . . ie at 76F . . . and click off when below 75F . . . ie at 74F . . . is this operating within standard tolerance? . . .

is the thermostat defective? . . . is there any means to rectify to hold at 75F?

Right now there is a 4 degree variance which at 77F is slightly too warm and at 73F is slightly too cool . . . would be better clicking on at 76F and off at 74F.

Thought about changing the batteries but don’t think that should have any effect as cooling is clicking on and off OK and display reading OK just at higher and lower temps than the setting.
Any comments or suggestions to rectify would be greatly appreciated.
Thanks, Bryan


Good question, Bryan. I like the Honeywell RTH 2300 thermostat series an have installed a number of them without having the trouble you describe.

I'd expect the thermostat to respond just as you do. And I'm doubtful that batteries are the problem.

Looking at Honeywells' FAQs for the RTH2300 the company says that the thermostat accuracy is +/- 1 degree Farenheit

Your thermostat is designed to control temperature to +/- 1 F.

How often your heat turns on and off depends on may factors including the type of heating system you have, as well as how much your system needs to run to maintain your temperature setting (in other words, how cool or cold it is outside). A typical forced air system will cycle about five times in an hour (5CPH), this is normal. A typical hot water system would cycle less then that.


A Guide to Using Wall-Mounted Room Thermostats - How to Set the Thermostat

This topic has been moved to its own article now found at THERMOSTAT SET PROCEDURE

How to Set a Room Thermostat To the Desired Temperature


How a Digital Room Thermostat Works


Backup Batteries & Default Programs for Digital Room Thermostats: blank thermostat display problems

Some programmable digital room thermostats include an internal battery that will allow the device to remember the program that you have set even if there has been a power outage. This topic has been moved to its own article at THERMOSTAT BACKUP BATTERIES - for temporary or full time operation.

Heating & Cooling Control Thermostat Troubleshooting Guide

We have moved and expanded this discussion to a separate article found at THERMOSTAT DIAGNOSTIC GUIDE - link given just below.


Continue reading at THERMOSTAT DIAGNOSTIC GUIDE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




Suggested citation for this web page

THERMOSTATS, HEATING / COOLING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman