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AFUE DEFINITION, RATINGS
AGE of CHIMNEYS & FIREPLACES
AGE of AIR CONDITIONERS & HEAT PUMPS
AGE of HEATERS, BOILERS, FURNACES
AIR CONDITIONING & HEAT PUMP SYSTEMS
AIR FILTERS for HVAC SYSTEMS
APPLIANCE EFFICIENCY RATINGS
ASBESTOS IDENTIFICATION IN buildings
BACKDRAFTING HEATING EQUIPMENT
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BLUE vs YELLOW COMBUSTION FLAMES
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
BUILDING SAFETY HAZARDS GUIDE
CAPACITORS for HARD STARTING MOTORS
CARBON DIOXIDE - CO2
CARBON MONOXIDE - CO
CHEMICAL TREATMENTS for BOILERS
CHIMNEY INSPECTION DIAGNOSIS REPAIR
CHIMNEYS & Flues - Asbestos Transite Pipe
CHINESE DRYWALL HAZARDS
CIRCULATOR PUMPS & RELAYS
COOL OFF HEAT, Thermostat Switch
COMBUSTION AIR for TIGHT buildings
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
COMPLETE COMBUSTION, Stoichiometric
CREOSOTE FIRE HAZARDS
Curved Brick Chimneys - Sulphation
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
DIRECT VENTS / SIDE WALL VENTS
DIRECTORY of OIL TANK EXPERTS
DRAFT HOODS - gas fired
DRAFT MEASUREMENT, CHIMNEYS & FLUES
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
DUST, HVAC CONTAMINATION STUDY
DUST SAMPLING PROCEDURE
ELECTRIC HEAT, DIAGNOSIS, REPAIR
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
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
FILTERS, AIR for HVAC SYSTEMS
FILTERS, OIL on HEATING EQUIPMENT
FIRE SAFETY CONTROLS
FIREPLACES & HEARTHS
FLAME COLOR, BLUE vs YELLOW COMBUSTION
FLOODED HEATING EQUIPMENT REPAIR
FLOODED WATER HEATER REPAIR
FLUE SIZE SPECIFICATIONS
FLUE VENT CONNECTORS
FREEZE-PROOF A BUILDING
FUEL OIL TYPES & CHARACTERISTICS
FUEL UNIT, HEATING OIL PUMPS
FURNACE CONTROLS & SWITCHES
FURNACE EFFICIENCY, HIGH vs MID
FURNACE HEAT EXCHANGER LEAKS
FURNACE OPERATION DETAILS
FURNACE OPERATING TEMPERATURES
GALVANIC SCALE & METAL CORROSION
GAS BURNER Flame & Noise Defects
GAS FIRED WATER HEATERS
GAS PIPING, VALVES, CONTROLS
GAUGES ON HEATING EQUIPMENT
GEOTHERMAL HEATING SYSTEMS
HEAT EXCHANGER LEAK TEST
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION
HEAT PUMPS, DIAGNOSIS, REPAIR
HEAT TAPES & CABLES on Roofs for Ice Dams
HEATING COST FUEL & BTU Cost Table
HEATING COST SAVINGS METHODS
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 SMALL LOADS
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
HEATING SYSTEM NOISES
HEATING SYSTEM SERVICE & MAINTENANCE
HEATING SYSTEM TYPES
HIGH EFFICIENCY BOILERS/FURNACES
INSULATION INSPECTION & IMPROVEMENT
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
LP & Natural Gas Safety Hazards
MANUALS & PARTS GUIDES - HVAC
MOTOR OVERLOAD RESET SWITCH
Natural Gas Combustion Products
NO HEAT - BOILER
NO HEAT - FURNACE
NOISE / SOUND DIAGNOSIS & CURE
NOISE AIR CONDITIONER / HEAT PUMP
NOISE, DUCT VIBRATION DAMPENERS
NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
OIL BURNER FUEL UNIT
OIL BURNER INSPECTION & REPAIR
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 HEAT FIRE SAFETY CONTROLS
OIL LINE CLOGGING FIX
OIL LINE QUICK STOP VALVES
OIL LINE SAFETY VALVES
OIL ODORS, LEAKY OIL TANK PIPING
OIL PUMP FUEL UNIT
OIL SPILL CLEANUP / PREVENTION
PLASTIC HEATER VENT
PULSE COMBUSTION HEATERS
RELIEF VALVES - TP Valves on Boilers
Reset Switch - Heater Primary Control
Reset Switch Broken - Quick Repair
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
SAFETY HAZARDS & INSPECTIONS
SAFETY HAZARDS GUIDE
SAFETY, HEATING INSPECTION
SAFETY RECALLS, Chimneys, Vents, Heaters
SOLAR HEATING SYSTEM DESIGNS
SOOT on OIL FIRED HEATING EQUIPMENT
SPILL SWITCHES - Flue Gas Detection
SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS
STACK RELAY SWITCHES
STAIN DIAGNOSIS on BUILDING INTERIORS
STEAM HEATING SYSTEMS
Thermal Expansion Cracking of Brick
THERMAL EXPANSION of HOT WATER
THERMAL EXPANSION of MATERIALS
THERMAL IMAGING, THERMOGRAPHY
THERMAL MASS in BUILDINGS
THERMAL TRACKING & HEAT LOSS
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
Transite Pipes, Chimneys & Flues
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
Heating furnace operation: this article lists the sequence of steps in the operation of a modern warm air heating furnace as well as older heating furnaces such as the "octopus" furnace that may have little or no duct work. Understanding exactly what happens from the moment that the room thermostat calls for heat until the call for heat is satisfied can help troubleshoot a heating furnace that is not working properly. Sketch at page top courtesy of Carson Dunlop Associates.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects. Sketch at page topt courtesy of Carson Dunlop Associates.
1. The Building Thermostat senses temperature, turns on the furnace burner: As building temperatures drop a wall-mounted thermostat in the occupied space senses the temperature drop and in response, calls for heat at the furnace, causing the furnace oil or gas burner to turn "on". Details are at THERMOSTATS.
Sketch at left courtesy of Carson Dunlop Associates.
2. The Gas or oil burner: The heat source such as a gas or oil-fired burner will turn on in response to the thermostat, and it will continue to run until either the high temperature or "FAN LIMIT" temperature is reached inside the warm air plenum or until the building thermostat inside the occupied space senses that the desired temperature there has been reached.
3. Heat exchanger: Hot combustion gases produced by the oil or gas burner circulate inside of the furnace's metal heat exchanger causing it to get hot. Combustion gases leave the inside of the heat exchanger and flow through a flue vent connector to a building chimney where they are vented safely outside.
4. The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of a steel "box" called the heat exchanger, sending the now-warmed air onwards into the furnace's output side or "supply air" plenum where it is sent into the building warm air duct system for delivery to the occupied space.
5. Air ducts connect and permit movement of cool air from occupied space through furnace and deliver warm air back to occupied space: The building air duct system includes return or cool air ducts and warm air ducts.
Cool air ducts carry air from the occupied space into the furnace return air plenum, possibly through an air filter, and into the furnace blower compartment. Warm air supply ducts connect to the supply air plenum and carry warm air into the occupied space where it flows out of floor, wall, or ceiling warm air registers or diffusers. See DUCT SYSTEMS
The Octopus furnace shown at left delivered heat to the building with no ductwork whatsoever. Warm air rose into the first floor of the building through a floor grid mounted at the top of the furnace. Warm air rose to upper floors through stairwells or by ceiling-cut air registers.
6. Combination Fan & Limit Control: This control turns the furnace blower on and off at the proper times. The FAN ON setting on this control makes sure that the blower fan does not turn on (even though the building thermostat has asked for heat) so that the furnace wont' blow cool air into the occupied space.
Furnace FAN ON: When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns on the furnace blower to deliver warm air to the occupied space.
Furnace FAN OFF: The furnace combination fan and limit control FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down and the heat it contained has been sent to the occupied space.
Furnace LIMIT: The LIMIT indicator setting is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level.
See FAN LIMIT SWITCH for details.
When does the furnace blower turn OFF in normal operation?
When the thermostat has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate until the temperature at the supply plenum has reached or dropped below the "cut-in" or "fan-on" lower limit on the switch.
When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:
On most heating systems the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as soon as the thermostat is satisfied. If the furnace oil or gas burner is very high capacity, or if the furnace fan/limit controls have been set to cause this effect, the burner may on some systems cycle on and off periodically while the warm air blower continues to run.
How is heating furnace efficiency or economy measured? What does Furnace AFUE mean?
Each furnace model is assigned an AFUE number. AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better. If your furnace has an AFUE rating of 90, that means that for every dollar you spend on fuel, 90 cents worth of heat is delivered into your building. The remaining 10 cents is lost in inefficiency such as heat that escapes up the chimney along with the products of combustion.
AFUE is not the whole story of heating cost efficiency. A high-efficiency heating system that has not been cleaned and serviced may be running poorly and wasting money. In fact, an 85% AFUE heating furnace that has not been cleaned might be running at an efficiency much lower, perhaps 65%.
Furthermore, if your building is drafty or poorly insulated, you may be delivering heat at high efficiency but losing it from the building much faster than necessary. These articles can help with a more complete approach to saving money on heat: First see HEATING COST SAVINGS METHODS then for more detail check out our articles at HEAT LOSS RATE CALCULATIONS and HEAT LOSS INDICATORS and INSULATION IDENTIFICATION GUIDE
Signs of Improper Warm Air Heating Furnace Operation
Furnace Oil or Gas Burner Cycling On and Off Frequently
If a furnace oil or gas burner cycles on and off frequently while the furnace blower continues to run, the heating system probably needs service, inspecting, testing, or adjustment.
Reduced or no Air Flow at Heating Supply Registers
If you are getting some but not much air flow out of heat supply registers there is probably a problem with:
If you are getting no air flow whatsoever but the furnace burner seems to be running there is probably a problem with:
Similarly, when the thermostat is satisfied (the room has reached the thermostat set temperature), the thermostat will "turn off" the furnace. What actually happens is that the oil or gas burner will turn off immediately, but on most hot air furnace systems, the blower will continue to run, perhaps even for several minutes, as it extracts the remaining heat from the furnace heat exchanger and hot air supply plenum.
Two reasons for this continued operation of the furnace blower fan past burner shutdown include:
Make a thorough visual inspection for signs of damage or improper operation, missing parts, water damage, rust, mechanical damage, soot, smoke, improper venting, missing or damaged safety controls, etc. We detail these topics in articles listed below. Thanks to reader Cyrus for editing suggestions.
Details of Heating System Controls and Switches
For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:
Types & Combinations of Water (hydronic heating boilers) and Air (warm air furnaces) Heating Systems
A technical note is necessary about determining what kind of heat or what type of heating system is installed: some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator).
The heat exchanger in a water to air heating system is then placed inside of an air handler or blower compartment where a blower fan circulates building air from return ducts to a plenum where air is blown across the heat exchanger and then the warmed air is delivered to the occupied space through additional warm air ducts or radiators.
More about Water to Air Heat Exchanger Heating Systems
Some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator). The heat exchanger in a water to air heating system is then placed inside of an air handler or blower compartment where a blower fan circulates building air from return ducts to a plenum where air is blown across the heat exchanger and then the warmed air is delivered to the occupied space through additional warm air ducts or radiators.
Water-to-air heating systems will use both a separate water heating boiler and a blower or air handler system.
Watch out for corrosion on water to air heat exchanger heating coils, as we explain just below.
Corrosion on Heating & Air Conditioning Heat Exchanger Coils & Condenser Coils
Corrosion on heat exchanging coils in heat pumps, air conditioners, or water-to-air heating coils eventually leads to leaks and costly repairs that require replacement of the coil and re-charging of the HVAC system refrigerant. Corrosion may occur as a function of age and use and failure to keep the coil clean, but there are some special sources of corrosion in HVAC systems that you should watch for:
Some buildings are heated by a combination of separate hot water systems (circulating hot water through radiating devices like baseboards or radiators in some areas) and hot air systems (circulating warm air through ductwork into the occupied space in other areas).
These buildings will have both a hot air furnace and a completely separate hot water heating boiler installed. In this case these are completely separate heating systems and usually each serves different building areas.
Frequently Asked Questions (FAQs)
Questions & answers or comments about the operating sequence of warm air heating systems
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.