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AFUE DEFINITION, RATINGS
AGE of CHIMNEYS & FIREPLACES
AGE of HEATERS, BOILERS, FURNACES
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE
ANODES & DIP TUBES on WATER HEATERS
ANTIFREEZE for BOILERS
ANTI SCALD VALVES
APPLIANCE EFFICIENCY RATINGS
ASBESTOS IDENTIFICATION IN buildings
BACKDRAFTING HEATING EQUIPMENT
BACKFLOW PREVENTER VALVE, HEATING SYS
BACKFLOW PREVENTER, HEATER WATER FEEDER
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPES RADIATORS
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BLUE vs YELLOW COMBUSTION FLAMES
BOILER CHEMICAL TREATMENTS
BOILER COMPONENTS & PARTS
BOILER CONTROLS & SWITCHES
BOILER LEAKS CORROSION STAINS
BOILER NOISE SMOKE ODORS
BOILER OPERATING PROBLEMS
BOILER PRESSURE & TEMPERATURE SETTINGS
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
CAPACITORS for HARD STARTING MOTORS
CARBON DIOXIDE - CO2
CARBON MONOXIDE - CO
CHEMICAL TREATMENTS for BOILERS
CHIMNEY INSPECTION DIAGNOSIS REPAIR
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
CONVECTOR HEATERS - HYDRONIC COILS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DIRECT VENTS / SIDE WALL VENTS
DRAFT HOODS - gas fired
DRAFT MEASUREMENT, CHIMNEYS & FLUES
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC HEAT, DIAGNOSIS, REPAIR
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
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
FLUE SIZE SPECIFICATIONS
FLUE VENT CONNECTORS
FUEL OIL TYPES & CHARACTERISTICS
FUEL UNIT, HEATING OIL PUMPS
FURNACE CONTROLS & SWITCHES
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 INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEATING COST FUEL & BTU Cost Table
HEATING COST SAVINGS METHODS
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING OIL PIPING TROUBLES
HEATING OIL TANKS
HEATING OIL TYPES & PROPERTIES
HEATING OIL USAGE RATE
HEATING SMALL LOADS
HEATING SYSTEM NOISES
HEATING SYSTEM TYPES
HIGH EFFICIENCY BOILERS/FURNACES
HOT WATER HEATERS
HUMIDITY LEVEL TARGET
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
LP & Natural Gas Safety Hazards
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
MOTOR OVERLOAD RESET SWITCH
Natural Gas Combustion
NO HEAT - BOILER
NO HEAT - FURNACE
NOISE, DUCT VIBRATION DAMPENERS
NOISE, HEATING SYSTEMS
NOISE, WATER HEATER
ODORS & SMELLS DIAGNOSIS & CURE
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 FILL PIPE LEAKS
OIL PUMP FUEL UNIT
OIL SPILL CLEANUP / PREVENTION
PLASTIC HEATER VENT
PULSE COMBUSTION HEATERS
PRESSURE REDUCING VALVES
PRESSURE REGULATOR, WATER
RADIANT HEAT Floor Mistakes to Avoid
RADIANT HEAT TEMPERATURES
RADIANT SLAB FLOORING CHOICES
RADIANT SLAB TUBING & FLUID CHOICES
RELIEF VALVE LEAKS
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 Broken - Quick RepaiR
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
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 of HOT WATER
THERMAL EXPANSION of MATERIALS
THERMAL MASS in BUILDINGS
THERMAL TRACKING & HEAT LOSS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
TRANSITE PIPE CHIMNEYS & FLUES
VIDEO GUIDES: Heating System Videos
VIDEO GUIDES - InspectAPedia.com
WATER HEATER SAFETY
WATER HEATERS for HOME HEATING USE?
WATER HEATER NOISES
WATER HEATER SCALE - De-Liming Procedure
WATER HEATER SCALE PREVENTION
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
Combustion air defects & hazards: this article explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters. Lack of adequate combustion air causes improper heater operation, increased maintenance cost, and risks dangerous production of carbon monoxide gas. This article series explains how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.
This article series answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. Our photo at page top shows an oil fired furnace installed in a closet with an airtight door; there was no outside combustion air supply. The heating system could not work properly nor safely in this home.
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How to Recognize & Diagnose Inadequate Combustion Air, Sooting, or Burn Marks at Oil-Fired or Gas-Fired Heating Systems
The photographs above show a heating furnace with soot blow-back around the oil burner probably means there is backpressure in the combustion chamber - an improper operating condition that may be unsafe.
Watch out: inadequate combustion air supply to a gas burner (and less often to an oil burner) is very dangerous and can produce potentially fatal carbon monoxide. If you suspect unsafe heating system operation or a carbon monoxide problem be sure everyone leaves the building immediately and then call your local fire department for assistance.
Clues Indicating Possible Lack of Combustion Air & Related Safety Hazards
Also see Unsafe Air Conditioning or Heating Duct Openings which describes the risks of reduced combustion air on hot air heating systems when certain return air duct defects are present, and also see CARBON MONOXIDE - CO and CHIMNEY INSPECTION DIAGNOSIS REPAIR
Heating Equipment Combustion Air Rules of Thumb
Square Inches of Combustion Air Intake: 1 sq. in. per 1000 BTUH
For heating equipment installed in confined spaces, an old-timer's rule of thumb is to add up the total INPUT BTUH numbers from all of the data tags on all of the heating equipment installed. You want to see at least one sq.in. of open fresh air intake per 1000 btuh.
Watch out: this combustion air rule of thumb needs to be adjusted to account for the air flow restriction caused by louvers and screening over the combustion air intake opening. As a rule of thumb we
Watch out: The combustion air estimate provided by outside combustion air openings or openings into other, presumably larger building areas (see below) also needs to account for the effects of building exhaust fans, tight buildings, and similar interferences. (BACKDRAFTING HEATING EQUIPMENT)
Watch out: this rule of thumb falls apart if the fresh air is not being vented directly into the heating equipment area through an outside wall. That is, if air has to move through vent pipes or ductwork into the area where it is needed, the equivalent square inches of fresh air intake venting may need to be increased depending on the length, number of bends, angles of bends, and diameter of the fresh air or combustion air intake venting system.
Standard engineering approaches to calculating air flow through round or rectangular ductwork can solve the question of impact on combustion air of routing it through ducts.
Cubic Feet of Room Space as a Measure of Adequacy of Combustion Air: Total Input BTUH / 1000 x 50
For heating equipment installed in larger spaces, a common rule of thumb for computing the required total cubic feet of free space to assure adequate combustion air is to provide 50 cubic feet of free space per 1000 Input BTUH for the total of all of the heating appliances installed in the area. The assumption behind this old rule is that buildings leak air and that larger rooms or spaces have more air intake leaks than smaller ones.
Watch out: this formula may not adequately consider the reduction in volume of the room or open space attributed to contents, storage, etc. and it certainly does not adjust for modern tigh building construction. COMBUSTION AIR for TIGHT buildings explains how to provide outside combustion air for tight buildings.
Some writers simplify the formula to express this rule of thumb as
Example: if we have a 180,000 Input BTUH boiler and a 40,000 input BTUH water heater installed in an enclosed utility room, how many cubic feet of space in that room would make us think we had adequate combustion air?
(100,000 + 40,000) / 1000 x 50
140,000 BTUH / 1000 = 140
140 x 50 = 7,000 cubic feet.
If our room is smaller than 7,000 cubic feet we probably don't have adequate combustion air (unless an outside combustion air source is also provided).
Calculate the total cubic feet of space in a room by multiplying the room width x room length x room height
Example: if the heating equipment is installed in an open basement that is 40 ft. x 20 ft. x 8 ft. high, we have
Cubic Feet = 40 x 20 x 80
Cubic Feet = 6400 - this basement may not provide adequate combustion air for the example input BTUH total given above.
How to Convert Round Opening Diameters (say an air duct cross section) to Opening Size Equivalent
If we are using smooth-walled round ducts to bring combustion air into the space where it is needed, and before considering the restrictions on air flow caused by duct bends and length (friction losses) we start by simply calculating the cross-sectional area of the duct:
We can use any unit (cm, inches, feet) as long as we stick to the same unit through.
Example: a 6-inch diameter round air duct has a cross section (or area) of
Area = 3.1416 x (6 / 2)2 inches
Area = 3.1416 x (3)2 inches
Area = 3.1416 x (3 x 3)
Area = 28 sq.in. of space - which, if unrestricted by duct length, bends, or screens, and if we use our first rule of thumb (one inch per 1000 BTUH) would support about 28,000 Input BTUH
Watch out: square duct area is not equal to round duct area in air flow capacity. That's because air flowing through a square or rectangular duct (or chimney) does not flow uniformly - the area of the corners of the rectangle moves less air. For you who left your calculator at home and left Pi in the refrigerator, here is the square inches of cross section opening size for common round duct diameters:
Combine Combustion Air Sources to Check the Combustion Air Requirements
When room volume in cubic feet is inadequate to provide safe combustion air we can add combustion air by providing an outside air source.
Guide to a Simple Combustion Air Safety Check for Gas Fired Heating Appliances
Thanks to Tjernlund Products who recommended this procedure, we describe a simple combustion air safety check that can be performed by a homeowner or by a heating service technician.
This procedure is only intended for buildings where LP gas or natural gas heating appliances (heating boiler, warm air furnace, water heater) are installed AND where a flue gas spillage safety switch such as a Tjernlund UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment or Field Controls Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches are installed on those appliances. (Similar products are provided by other manufacturers.)
SAFETY WARNING: If AT ANY TIME your gas fired equipment has shut down in SAFETY OFF position it may be due to a resettable flue gas spill sensor switch. Check with your heating service company - you might think you can avoid a costly heating service call, BUT BEWARE: because flue gas spillage is very dangerous, including the production of potentially fatal carbon monoxide gas, don't simply reset the system without finding out what caused the problem in the first place.
MORE SAFETY WARNINGS: in addition to our safety warning above, Tjernlund explains that flue gas safety switches are intended to alert the building occupants to a potentially dangerous condition.
But flue gas spillage safety switches are not a substitute for a regular chimney safety inspection nor do they replace regular heating appliance inspection and maintenance by a trained technician. Those steps must be taken as well.
What causes flue gas spillage:
Please see SPILL SWITCHES for a discussion of the causes of flue gas spillage in buildings and for a description that contrasts inadequate combustion air with other causes of dangerous flue gas leakage into buildings.
Bachrach Corporation, a manufacturer of heating system test equipment opines that gas fired equipment is more likely to have flue gas spillage from a blocked chimney than from building depressurization due to inadequate combustion air supply. We're not sure what data supports that view.
Certainly home inspectors find many heating appliances installed in tiny closets with no outside combustion air and a door that, when shut, blocks off air to the appliance. We have also observed that gas fired heating equipment operated just fine in a building until a new owner installed a whole house ventilation fan system.
Combustion Air Defects & Safety Hazards at Mobile Home Heating System
Below our photographs illustrate several unsafe conditions at a house trailer's heating system. There are no return air flow from the conditioned (occupied) space to the furnace warm air intake, questions about adequate combustion air, and dirt blocked air flow passages.
In addition to the absence of return air to the heating furnace we notice that
The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.
Question: how do I provide combustion air for a woodstove?
I've often read about the necessity of providing combustion air to a woodstove, but there's usually not much explanation of how it's done. Is it just a pipe that ends near the stove inlet? - Chris Rich, New Castle VA
Answer: hard-ducted outside combustion air required for some woodstoves - in manufactured housing
Many wood-stove manufacturers provide a special line of stoves intended for manufactured housing. These woodstoves must have an external combustion-air inlet hard-ducted directly into the stove.
Often the combustion air supply duct comes up through the floor under the woodstove (or coal stove or airtight wood-burning fireplace), making it almost invisible from inside the room where the woodstove is located.
This makes it attractive for the homeowner, and easy for the builder to install.
See COMBUSTION GASES & PARTICLE HAZARDS for an explanation of the dangers of inadequate combustion air. See COMBUSTION PRODUCTS & IAQ for the relationship between fuel burning appliances and building indoor air quality. More about carbon monoxide - CO - is at CARBON MONOXIDE - CO and at CARBON MONOXIDE WARNING. Also see the safety warnings at BACKDRAFTING HEATING EQUIPMENT.
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