Internal parts of a thermostat and how they work How Room Thermostats Work to Respond to Changes in Room Temperature

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How room thermostats work in response to temperature:

This article explains in detail the diffrent types of temperature sensors used in heating and cooling room thermopstats and how heating or air conditioning thermostats actually work. We explain how a bimetallic spring operates to move a mercury bulb switch, how mercury bulb switches operate, how a snap action thermostat or a thermostor operated thermostat works. We define switch make and switch break modes.

This document will help repair technicians and building occupants to understand wall thermostats, their use, setting, and adjustment. Page top sketch of how a mercury bulb type bimetallic spring thermostat operates was provided courtesy of Carson Dunlop Associates.

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.

How the Honeywell Room Thermostat Senses & Responds to Temperature Changes

Internal parts of a thermostat and how they workArticle Contents



Mercury Bulb and other Bimetallic Spring Room Thermostats & How They Work

Bimetallic Springs & How They Operate as Temperature Controls

This and other older room thermostats sense room temperature by the combination of a bimetallic spring and a switch that the spring moves as its position changes in response to temperature changes in the room.

The round, coiled bi-metallic spring in the thermostat shown in this photo has been used various shapes in lots of other control devices that respond to temperature changes, such as the furnace or boiler stack relay switch.

Two metal strips, each of different properties, are sandwiched together and then coiled (center-right in our photo) to form a spring.

Because the thermal coefficient of expansion of the two metals are different, and because the two metal strips are adhered together, as temperature changes the spring will bend or flex.

As the bimetallic thermostat temperature sensing spring flexes it expands or shrinks as temperature rises or falls. This causes a movement of the spring.

How the Bimetallic Spring Moves an Electrical Switch or Mercury Bulb

A sensor or switch attached to the end of the spring is mechanically moved and in turn is used to turn a device "on" or "off".

In our photo above our little dental tool points to a mercury switch which makes or breaks electrical contact to actually turn on or off the control to which our thermostat has been attached. You can see some wires leaving the glass mercury bulb and heading off to other contacts inside the thermostat.

Just below, in our left hand photo the bimetallic coil spring has tilted the mercury-bulb so that it tips to roll its blob of mercury away from two metal contact wires that are sealed, along with the mercury, inside of the glass bulb.

In the right-hand photo, the bimetallic spring has contracted (it got cold), causing the bulb to tip so that the mercury rolls down the inside of the tube and contacts the two wires inside the bulb, completing an electrical contact to switch the air conditioning (off) or the heating system (on) depending on the mode (cooling or heating) in which the thermostat is being used.

Thermostat mercury bulb closeup in "break" mode Mercury bulb swithc in "make" mode

This swapping of the role of the switch in turning something OFF in response to a temperature drop (cooling mode) or ON in response to a temperature drop (heating mode) is why a dual-purpose thermostat will also have an extra switch to decide whether we're controlling heating or cooling.

Definition of Switch Break & Switch Make Modes

To avoid confusion about what a mercury bulb switch or any other kind of switch is doing, electricians call the left photo condition "switch break" mode and the right photo condition "switch make" mode because the switch is "breaking" or "opening" a circuit when contacts are disconnected, and a switch is "making" or "closing" a circuit when its electrical contacts are connected.

How Mercury Bulb Thermostat Switches Work & Why a Thermostat is Just an On-Off Switch, not an Accelerator

Simple wall thermostatAnyone who understands how a mercury bulb thermostat works to simply "make" or "break" an electrical circuit will see clearly and forever that a room thermostat is not an accelerator, it is an on-off switch that responds to temperature changes.

So if you're cold, and the room temperature and room thermostat are both reading 55 °F., turning the thermostat to any temperature above that will cause the heating system to turn on.

Turning the thermostat up to 95 °F will not warm the room any faster than turning the thermostat up to 65 deg F.

Our little sketch explains how the traditional mercury switch worked in the old Honeywell (R) round wall thermostats.

As room temperature changes, a bimetallic coil-spring moves to tip the glass bulb in either direction, up or down. As temperature drops the bulb tips to the left (the spring contracts) and the mercury, at point (B), connects the two contacts to turn on the heating system.

Modern thermostats no longer use mercury switches (mercury is a toxic product) but they function similarly in response to room temperature.

Mercury is a toxic substance which is no longer used in thermostat switches, but there are millions of these devices still in place in homes.

You should ask your local or state department of environmental protection/conservation if your area has special requirements when one of these old mercury bulb thermostats is to be thrown away.


How is a "Snap Action" bimetallic-spring Room Thermostat Different from a Mercury Bulb Thermostat?

Internal parts of a thermostat and how they work

As Carson Dunlop Associates' sketch illustrates, a bimetallic-spring snap-action room thermostat works almost exactly the same as the older mercury bulb thermostat illustrated above in this article.

But to eliminate the requirement to use mercury (a hazardous substance) the bulb of mercury found inside the glass capsule and used to complete the circuit or "close the switch" to turn on heating or air conditioning has been replaced by a mechanical switch.

When the bimetallic spring end approaches the fixed glass bulb, a magnet on the end of the spring pulls closed the tip of the electrical switch to operate the cooling or heating appliance.

[Click the image to see an enlarged version and to read more operating details for this type of thermostat control.]

Types of Room Thermostat Temperature Sensors & How they Work

How Bimetallic Element thermostats work

Bimetallic spring thermostat (C) Daniel Friedman

Bimetallic Element thermostats use a bimetallic spring consisting of strips of two different metals fused together, typically into a coil shape. Our photo (left) shows a coiled spring inside a mercury tube based room thermostat made by Honeywell®.

As the fused metal strip/coil is exposed to different room air temperatures, because the two metals expand or contract at different rates but are fused together, the spring "coils" or "un-coils", causing the spring end to move in response to temperature.

This movement can be used to open or close contacts to turn heating or cooling on or off in a building.

The movement of the bimetallic spring may move a mercury bulb that acts as a switch (described below) or it may operate a snap-type mechanical switch to turn heating or cooling on or off.

See TEMPERATURE RESPONSE of ROOM THERMOSTATS for a detailed description of how bimetallic springs used in room thermostats work.

How Bellows Element thermostats Work

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.

Photographs of a bellows-operated line-voltage room thermostat produced by Honeywell, Inc., are found at Honeywell or in Burkhardt.

Mercury-tube Element thermostats

Mercury bulb thermostat (C) Daniel Friedman

Mercury-tube element thermostats use a glass tube containing mercury and a pair of metal contacts at one end of the tube to turn heating or cooling on or off in a building.

The mercury tube is connected to a bimetallic spring that tips the mercury tube in response to changing room air temperature.

When the tube tips in a direction that causes the mercury to move to the end of the tube containing two metal contact wires, the mercury acts as a conductor to close an electrical connection to turn heating or cooling on.

When the tube tips in the opposite direction the mercury moves off of the contacts and the switch is opened or heating /cooling are turned off.

How Thermistor-Controlled Room Thermostats Work: photos & temperature chart

Thermistor-type thermostats use a tiny solid-state electronic component, a type of resistor whose electrical resistance changes in response to temperature.

You'll notice in our photo of a thermistor found inside a Honeywell CT2700 Electronic Round Programmable Thermostat that the device is deliberately connected using long wire leads so that it can ride in room air away from influence by heat generated on the thermostat's own internal circuit board.

Our photo (below left) shows a thermistor used in a room thermostat. We had to take the thermostat apart and use our lab microscope to take this photo - this is a detail you won't normally see when installing a room thermostat. But it's there.

Thermistor resitance curve Thermistor resitance curve

Thermostats control within a narrow range, say +/1 1 °F by using a thermistor combined with a high gain amplifier to obtain sensitivity down to 0.005oC!. What about reliability? Thermistors, properly selected for the environment, can be quite durable.

Nevertheless, some thermostats or HVAC equipment include a "safe-default" operating mode to keep the system working in the event that the thermistor should fail.

Thermistor details such as how they work, definitions of types, features, and more photographs of thermistors are at THERMISTORS.

Thermostat Heaters, heat accelerators, heat anticipators

Heat anticipator control in thermostats © D Friedman at

Some thermostats models also use a source of artificial heat (a very tiny resistor or resistance heating element inside the thermostat) to force the heating system to turn off earlier than it would have if the thermostat only responded to an increase in room temperature.

In other words, the heating system itself will be turned off before the room temperature has reached the thermostat set point.

This feature is added to prevent the heating system from "overshooting" or making the room too warm - a condition that might occur due to additional heat that will be radiated into the room from radiators or heating baseboards that will continue to be warm even after the steam boiler or the hydronic boiler circulator has shut off.

To accommodate variations in heating source and distribution design, heat accelerators or heat anticipators are usually adjustable.

Thermostat Accuracy: Settings vs Room Temperature

Honeywell digital room thermostat (C) Daniel FriedmanReader 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 76, 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.

This question appeared originally at THERMOSTATS, HEATING / COOLING

Reply: The RTH 2300 thermostat is designed to control temperatures to +/- 1° F.

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° 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.

This FAQ expands on the question of why the temperature displayed by the thermostat may not match other temperature measurements made in the home

Why does the thermostat temperature not match my temperature thermometer in my home?

Answer: Honeywell digital thermostats are designed to display the room temperature in a way similar to how people sense temperature. This means the thermostat takes into account not just the ambient air temperature but also the radiant temperature of objects in the room (i.e., wall and furniture). Room thermometers often only read ambient air temperature so they will not match the thermostat reading.

Also, Honeywell thermostats do round in the display to the nearest whole number (half number in Celsius) and to the temperature setting. For example, if you have the thermostat set to 72 degrees the system will turn on and off but the inside reading on the thermostat never changes from 72. The actual temperature did fall to 71 or up to 73 and that is what turned on the heating or cooling but the thermostat display will stay at 70 to avoid jumping up and down constantly. - retrieved 13 Aug 2015, original source:


A programmed setting can confuse us about how the thermostat should be responding.

But if you've got the thermostat in HOLD mode then it ought to respond to temperature changes around the hold temperature setting. Double check that you see the word "HOLD" in the display.

Honeywell RTH 2300 Thermostat Diagnostics

The Honeywell RTH 2300 thermostat has some self-diagnosing circuitry and other features showing up in its display

I'd give the company a call to ask about the response you're experiencing: but FIRST inspect the TT closely to be sure that no clod of dust or crud is blocking the little thermistor that senses temperature. Contact the company directly at:

Reader Follow-up:

Thanks for the prompt response and clarification.
Unit is definitely in "Hold" mode.
Not displaying "Lo Batt".
Will inspect for blockage on the temperature sensor - assuming I can figure out what that is. - Bryan 13 Aug 2015

Reply: check the thermostat's thermistor for dust or debris clogging

Thermistor in the 3M Filtrete room thermostat (C) Daniel Friedman Look closely at all sides of the thermostat. If I recall, on the right side you'll see, through a small gap in the plastic, a small thermistor sticking out in the air. That part could be blocked or bad.

Our photo at left shows the thermistor of a different model thermostat (not yours), observed from the back side of the device.

One can often spot the little orange thermistor by peering into an air gap at the side of the thermostat.

See THERMISTORS for more examples of these devices, what they look like and how to spot them.

Bottom line, the thermostat ought to respond to 1-3 degrees of temperature change. I looked for but didn't see an explicit spec on that range. If the response were too sensitive the unit would short cycle which is death for AC and some heating systems. So at 2 degrees Honeywell may say you're in-spec.

Reader follow-up:

I took the stat apart and did not see anything unusual - will have another look.

We have another H/W Stat, aslightly lesser grade model in our gym. It is sent to Cool Hold 72F and is always at 72F when I am in there.

Frustrating, sitting here, the thermostat is set to Cool /Hold / 74F and is reading 76F and the A/C is not on.

You may be right that we are within spec and I understand that excessive starting and stopping would not be good for the A/C unit, but this temperature variance is beyond a comfort level as well.

I will probably phone H/W to see what their response is.
Thanks again for your comments and assistance! - Bryan 13 AUg 2015

Reply: Check to see if the thermostat is calling for cooling or not

It sounds to me as if the thermostat is out of spec but before calling Honeywell I would check whether or not the thermostat is actually calling for cooling.

The device is ultimately a simple on-off switch. If it is calling for cooling then the cooling wires from the thermostat ought to be "making" or joined electrically - something one can test with a simple VOM at the air handler or compressor/condenser unit of your system: find where your thermostat wires go and check there.

If the TT is not calling for cooling and there is a 2 or more degree spread between what it says is room temperature and what it says you've set the temperature to (2 degrees or more below room temperature) then we'd expect the device to call for cooling.

Reader follow-up: Inquiry sent to Honeywell about thermostat's response to room temperature

13 Aug 2016 Bryan said:

1. It is longer than 5 mins between cooling starting and shutting off or shutting off and starting so not having any issues with the flashing message.

2. The response to the question about the home T/S is very curious tho . . . says the temperature never changes on the T/S despite room temp changing a degree or two . . . that is definitely not the case with our T/S . . . as stated previously if set at 75F the T/S temp reading will raise to 77F before cooling engages . . . and will drop to 73F before cooling cancels . . . it does not maintain a constant reading of 75F.

3. If the T/S is designed to control temp to +/- 1F, then I suppose that is what it is doing. If the thermostat is set at 75F and temperature raises to 76F that is +1F and within the +1 tolerance. When temperature raises one more degree to 77F then that is 1F above the 76F tolerance level and cooling begins.

Conversely when cooling and the temp falls to 74F that is within 1F of tolerance so cooling continues until the temp falls to 73F which is now beyond the 1F tolerance of 74F and cooling cancels. I suppose that within this parameter the unit is function properly although slightly beyond the comfort range - and this does not agree with the preceding statement that the temperature readout remains constant.

4. I have sent an email to H/W [Honeywell]. It will be interesting to see what their response is.

Honeywell Corporation Response to Thermostat Accuracy & Response to Room Temperature Inquiry

Just received the following response from H/W:


Thank you for contacting Honeywell.

All of Honeywell’s digital thermostats are calibrated to hold temperature within one degree of your set temperature to maintain an even room temperature. Although you need to check if the ‘COOL ON’ indicator appears on the screen just as the room temperature deviates from the set point by 1F degree. If yes, then we can be assured that thermostat is operating correctly.

If not, you may want to replace the thermostat.

Honeywell Customer Care

. . . the Cool On display is showing as the temp deviates by + or - 1F and cooling engages at +2F and disengages at -2F so the unit is functioning as designed by H/W and falling within their spec.

For our purposes it would be better if it was + or - .5F rather than a full degree . . . would reduce the temperature range . . . but apparently that is not how the unit is designed so I guess we live with it or investigate if there is another unit with a lesser tolerance.

Looks like we have solved the riddle djf . . . thanks for your assistance!


Bottom line, when the room temperature is more than one degree away from the set temperature in heating or cooling mode we should see the thermostat indicating that it is calling for heating or cooling - indeed you'll see, for this thermostat, small text indicating "COOL ON" or "HEAT ON"

At that point if your heating or cooling system is not actually turning on we know that the thermostat is asking it to to so, and that the problem is elsewhere, for which InspectApedia provides further diagnostics:




Continue reading at HEAT ANTICIPATOR Adjustment or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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