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AIR CONDITIONING & HEAT PUMP SYSTEMS
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPES RADIATORS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
CARBON MONOXIDE - CO
CIRCULATOR PUMPS & RELAYS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DIRECT VENTS / SIDE WALL VENTS
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
FAN, AIR HANDLER BLOWER UNIT
FLOODED HEATING EQUIPMENT REPAIR
FLUE SIZE SPECIFICATIONS
GAS BURNER Flame & Noise Defects
GAS PIPING, VALVES, CONTROLS
GEOTHERMAL HEATING SYSTEMS
HEAT PUMPS, DIAGNOSIS, REPAIR
HEATING COST SAVINGS METHODS
HEATING OIL PIPING TROUBLES
HEATING OIL TANKS
HEATING SYSTEM NOISES
HEATING SYSTEM TYPES
GAS LP & NATURAL GAS SAFETY HAZARDS
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
MOTOR OVERLOAD RESET SWITCH
NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
OIL FILL PIPE LEAKS
OIL SPILL CLEANUP / PREVENTION
PLASTIC Plexvent / Ultravent RECALL
PUFFBACKS, OIL BURNER
RELIEF VALVE LEAKS
Reset Switch - Heater Primary Control
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
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: Heating System Videos
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
ZONE VALVES, HEATING
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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