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HEATING SYSTEMS

AFUE DEFINITION, RATINGS
AGE of CHIMNEYS & FIREPLACES
AGE of AIR CONDITIONERS & HEAT PUMPS
AGE of FOUNDATION MATERIALS
AGE of FRAMING MATERIALS
AGE of FLOORING MATERIALS
AGE of ELECTRICAL WIRING
AGE of HEATERS, BOILERS, FURNACES
AIR CONDITIONING & HEAT PUMP SYSTEMS
AIR FILTERS for HVAC SYSTEMS

AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIRS
ANODES & DIP TUBES on WATER HEATERS
ANTI SCALD VALVES
APPLIANCE EFFICIENCY RATINGS
ASBESTOS IDENTIFICATION IN buildings

BACKDRAFTING HEATING EQUIPMENT
BACKFLOW PREVENTERS
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPE NOISES
BAROMETRIC DAMPERS
BASEBOARD HEAT

BIOGAS PRODUCTION & USE
BLEVE EXPLOSIONS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BLUE vs YELLOW COMBUSTION FLAMES
BLUERAY Recall

BOILERS, HEATING
  BOILER OPERATION DETAILS
  BACKDRAFTING HEATING EQUIPMENT
  BOILER COMPONENTS & PARTS
  BOILER CONTROLS & SWITCHES
  BOILER LEAKS CORROSION STAINS
  BOILER LEAKS, HOW TO LOCATE
  BOILER OPERATING PROBLEMS
CARBON MONOXIDE - CO
CARBON MONOXIDE WARNING

CHIMNEY INSPECTION DIAGNOSIS REPAIR
CHIMNEYS & Flues - Asbestos Transite Pipe
COMBUSTION AIR DEFECTS
COMBUSTION PRODUCTS & IAQ
COMPLETE COMBUSTION, Stoichiometric

CONDENSING BOILERS/FURNACES
CONDENSING BOILERS/FURNACES DAMAGE
CONVECTOR HEATERS - HYDRONIC COILS

DEFINITION of Heating & Cooling Terms

DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE

DRAFT HOODS - gas fired
DRAFT MEASUREMENT, CHIMNEYS & FLUES
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT INSULATION, Asbestos Paper

DUCT SYSTEM & DUCT DEFECTS
DUCT INSULATION - Asbestos Paper
DUCTS - Asbestos Transite Pipe
DUST CONTAMINATION FROM HVAC?

ELECTRIC HEAT
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT

ENERGY SAVINGS in buildings
EVAPORATIVE COOLING SYSTEMS

FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FAN NOISES

FILTERS, AIR for HVAC SYSTEMS
FILTERS, OIL on HEATING EQUIPMENT

FIRE SAFETY CONTROLS
FIREPLACE Damage & Unsafe Hearths - Settlement
FLAME COLOR, BLUE vs YELLOW COMBUSTION
FLOODED HEATING EQUIPMENT REPAIR
FLOODED WATER HEATER REPAIR
FLUE VENT CONNECTORS
FREEZE-PROOF A BUILDING
FUEL OIL TYPES & CHARACTERISTICS
FUEL UNIT, HEAING OIL PUMPS

FURNACES, HEATING
  DUCT SYSTEM DEFECTS
  ALLOY SYSTEMS FLEXDUCT
  DUCT & AIR HANDLER ODORS
  DUCT INSULATION, Asbestos Paper
  FURNACE CONTROLS & SWITCHES
  FURNACE OPERATION DETAILS
FURNACE OPERATING TEMPERATURES
  HEAT EXCHANGER LEAKS
GALVANIC SCALE & METAL CORROSION
GAS BURNER Flame & Noise Defects
GAS FIRED WATER HEATERS

GAS PIPING, VALVES, CONTROLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEATING COST FUEL & BTU Cost Table
HEATING COST SAVINGS
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES

HEATING OIL CLOUD WAX GEL POINT
HEATING OIL EXPOSURE HAZARDS, LIMITS
HEATING OIL - OLD, USEABLE?
HEATING OIL PIPING TROUBLES
HEATING OIL SHELF LIFE
HEATING OIL SLUDGE
HEATING OIL USAGE RATE
HEATING SYSTEM INSPECTION DIAGNOSIS REPAIR
HOT WATER HEATERS
HOT WATER IMPROVEMENT

INSULATION INSPECTION & IMPROVEMENT

LP & Natural Gas Safety Hazards

MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER / FURNACE DIAGNOSIS

NOISE / SOUND DIAGNOSIS & CURE
NOISE AIR CONDITIONER / HEAT PUMP
NOISE, DUCT VIBRATION DAMPENERS
NOISE CONTROL for HEATING SYSTEMS
NOISE CONTROL for PLUMBING
NOISES COMING FROM WATER HEATER

ODORS & SMELLS DIAGNOSIS & CURE
ODORS FROM HEATING SYSTEMS

OIL BURNERS
OIL BURNER INSPECTION GUIDE
OIL BURNER NOISE SMOKE ODORS
OIL BURNER NOZZLE & ELECTRODES
OIL BURNERS, RETENTION HEAD
OIL BURNER SOOT & PUFFBACKS

OIL FILTERS on HEATING EQUIPMENT
OIL FILTER MISSING
OIL FUEL TYPES & CHARACTERISTICS

OIL & GAS PIPING
  GAS PIPING, VALVES, CONTROLS
  OIL TANK PIPING & PIPING DEFECTS

OIL LINE CLOGGING FIX
OIL LINE SAFETY VALVES
OIL ODORS, LEAKY OIL TANK PIPING
OIL PUMP FUEL UNIT
OIL SPILL CLEANUP / PREVENTION

OIL TANKS
OIL TANK SAFETY
OIL TANK SLUDGE
OIL TANK TESTING
OIL TANKS, BURIED
OIL TANK WATER REMOVAL

PLASTIC HEATER VENT
PULSE COMBUSTION HEATERS
PASCAL CALCULATIONS

RADIANT BARRIERS
RADIANT HEAT
RADIANT HEAT Floor Mistakes to Avoid
RADIANT HEAT TEMPERATURES
RADIANT SLAB FLOORING CHOICES
RADIANT SLAB TUBING & FLUID CHOICES

RADIATORS

RELIEF VALVES - TP Valves on Boilers
RELIEF VALVES - STEAM TP VALVES
RELIEF VALVES - Water Heaters
RELIEF VALVES - Water Tanks

Reset Switch - Heater Primary Control
Reset Switch - Electric Motors
Reset Switch - Stack Relays

SAFETY HAZARDS & INSPECTIONS
SAFETY HAZARDS GUIDE
SAFETY, HEATING INSPECTION
SAFETY,HOME HEATING TIPS
Safety Recalls, Chimneys, Vents, Heaters
  BLUE vs YELLOW COMBUSTION FLAMES
  BLUERAY Recall
  CHIMNEYS & Flues - Asbestos Transite
  Goodman HTPV RECALL
  Heat Recovery Ventilator RECALL
  Lennox Furnace Manuals
  Lennox SAFETY WARNING
  PLASTIC Plexvent / Ultravent RECALL
  Weil McLain RECALL
SAFETY, HEATING INSPECTION
SAFETY,HOME HEATING TIPS

STEAM HEATING SYSTEMS

TANKLESS COILS
THERMAL TRACKING & HEAT LOSS

THERMOSTATS, HEATING / COOLING
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT, Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat
THERMOSTATS, WATER HEATER
  AQUASTAT CONTROL Functions
  Electric Water Heater Thermostats
THERMOSTATIC EXPANSION VALVES
Transite Pipe Chimneys & Flues

More Information

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

  • How the Building or Room Thermostat Senses & Responds to Temperature Changes
  • Honeywell traditional wall thermostat operation details
  • How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall Thermostat?
  • Questions & answers about heating and air conditioning thermostat wiring hookups
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This article explains the how heating or air conditioning thermostats actually work. This document will help repair technicians and building occupants to understand wall thermostats, their use, setting, and adjustment. Here we provide A Guide to Finding, Using, and Adjusting Thermostats for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers.

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.

Readers needing to find and fix un-wanted air leaks, heat losses, or other energy wasters should see HEAT LOSS DETECTION TOOLS. Readers should see ENERGY SAVINGS RETROFIT CASE STUDY and also see HEAT LOSS DETECTION TOOLS and INSULATION INSPECTION & IMPROVEMENT for energy saving retrofit detailed guides. Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

How the Honeywell Room Thermostat Senses & Responds to Temperature Changes

Internal parts of a thermostat and how they workThis 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.

Thermostat mercury bulb closeup in "break" modeMercury 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.

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.

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.

Simple wall thermostat

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.

 

Questions & Answers regarding this article

Questions & answers about heating and air conditioning thermostat wiring hookups

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.


THERMOSTATS, HEATING / COOLING
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT, Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

  • Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
  • Home Reference Book - Carson Dunlop The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
  • Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
  • DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
  • Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
  • Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
  • Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
  • Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
  • Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
  • Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
  • Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
  • Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
  • Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
  • Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
  • National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
  • The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
  • Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
  • The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
  • Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
  • "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
  • "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
  • "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
  • Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
  • Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
  • Installation Guide for Residential Hydronic Heating Systems
  • Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
  • The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
  • Links to our list of additional information on heating system inspection, repair, maintenance
  • ...
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