Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
CIRCULATOR PUMPS & RELAYS
CONVECTOR HEATERS - HYDRONIC COILS
COOL OFF HEAT, Thermostat Switch
DEFINITION of HEATING & COOLING TERMS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FLOODED HEATING EQUIPMENT REPAIR
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER
NO HEAT - FURNACE
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
PULSE COMBUSTION HEATERS
Reset Switch - Heater Primary Control
Reset Switch Broken - Quick Repair
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
ZONE VALVES, HEATING
How room thermostats work: This article explains in detail the 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 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
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.
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.
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.
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
Anyone 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.
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.]
How Bimetallic Element thermostats work
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-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.
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.
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
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.
Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators.
If your building uses warm air heat, see FURNACE CONTROLS & SWITCHES.
If your building uses steam heat see STEAM HEATING SYSTEMS.
Also see Heat Won't Turn Off - Stop Unwanted Heat. or select a topic from the More Reading links shown below.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs) About the operating properties of room thermostats & thermostatic controls
Question: why won't my outside A/C unit turn on?
why wont my outside unite turn on - Anon 7/30/11
Anon, start your diagnostic procedure at DIAGNOSTIC GUIDES A/C / HEAT PUMP
Question: Our A/C is running but not cooling the house - hot air comes out of the vents
question...Our air conditioning system is running, but not cooling the house. Only hot air is coming out of the vents. Can you tell me how I would know if it's the coil or the something in the unit outside - Vickie Ivie 7/13/11
Vickie if you have good airflow but it's not cool, then the indoor air handler is working but the system is not cooling the air; assuming that the outside compressor/condenser is actually running (you should check) you could be out of refrigerant (a leak to find and fix) or there could be a control problem. See the diagnostic link I gave to Anon just above. Also see REFRIGERANTS & PIPING.
Question: why does the blower fan run when the thermostat is OFF?
Why is my blower fan coming on when the dual thermostat is in the off position. - Charles 11/29/11
Charles, see Fan won't stop: What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off? for possible explanations; also look for a defrost cycle control or control board problem on your system. Charles there also could be some confusion between turning the A/C or cooling system off and turning the heating system off. Your system may be using the same blower assembly for both modes.
Question: Why does my AC system switch from cooling to emergency heat?
Why does my ac switch from cooling to emergency heat? You can't turn it off with the thermostat. Never had this problem until we had a digital thermostat installed? - Brenda 6/25/2012
Brenda, I'd ask for a diagnostic service call by an experienced HVAC tech - I'm not sure what's going on - if the system is not running through some odd defrost cycle then perhaps there is a control board problem in the unit.
Question: my A/C won't turn off - reclaimed refrigerant and reloaded it
If my a/c will not shut down and I changed out my thermostat 3 times and it still will not shut down also reclaimed all the freon from unite clean it out and reloaded it - Andy 09/26/12
Andy, check for a bad contactor switch.
Changing the same part repeatedly is probably not a reliable repair approach and suggests we're barking up the wrong tree. Unless at the changeout you also repeat a wiring area or part error.
Also, "reclaiming and reloading" refrigerant sounds odd to me. First because I'm not sure why you think this has to do with the A/C not shutting down and second because I wonder how, unless you're a trained HVAC rep, you'd have the equipment to properly reclaim refrigerant, and even if "reclaimed" I have no idea if the refrigerant is clean, doesn't have air mixed in with it, nor if you knew to install the proper charge.
Certainly if your system cannot reach the thermostat's SET temperature it will keep running. There could be a variety of reasons for not reaching the SET temp, including improper refrigerant charge.
Questions & answers or comments about heating and air conditioning thermostat operating properties.
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