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AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIRS
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
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC HEAT, DIAGNOSIS, REPAIR
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
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 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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 degF., turning the thermostat to any temperature above that will cause the heating system to turn on. Turning the thermostat up to 95 degF will not warm the room any faster than turning the thermostat up to 65 deg F.
How Bimetallic Element 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
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 degF 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
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.
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 DIAGNOSE & FIX AIR CONDITIONER / 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.
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Technical Reviewers & References
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