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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
Unsafe air ducts: this HVAC diagnosis & safety hazard article describes unsafe air conditioning or heating duct openings such as openings that may cause fatal production of carbon monoxide and move it into the occupied building space. We also discuss air or return duct openings that may pick up and distribute other gases, chemicals, mold or allergens throughout a building.
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UNSAFE OPENINGS - Air Conditioning or Heating Duct System May Draw Dangerous Combustion Gases
Also see LEAKY DUCT CONNECTIONS for other examples of duct leaks and openings. The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects.
This poorly-designed central air conditioning return duct was located in a cramped basement boiler room only five feet from a large gas-fired, natural draft heating boiler.
It is a possible safety concern. When the location of the system return air duct work and air handler is such that the system may pull dangerous flue gases back out of the gas appliance flue vents, piping and into the building heating or cooling air we cite it as a potential hazard: distribution of combustion gases may be blown into the living area.
This return air location on a heating or air conditioning system is a dangerous carbon monoxide hazard.
Such a system should be examined promptly and corrected by a qualified heating professional.
Return Air Openings Near Heating Equipment
Still more common is the presence of extra openings cut into the return ducts atop a building furnace (perhaps also serving as the air conditioning blower system in cooling season).
Often these openings are added to provide more return air to a system which is not providing sufficient cooling or heating to the building.
But making return air openings right at a heating appliance, such as shown in the photo here, risks drawing combustion gases into the building air supply as well as potentially interfering with proper appliance draft and combustion.
This is the case particularly with gas fired furnaces, boilers, or water heaters, which operate at a lower and usually natural draft, but it is also a potential safety hazard with oil-fired equipment.
Flue gases from nearby heating or water heater appliances are easily drawn into the return air plenum and air handler. This would permit circulation of flue gases into the living area and can be a safety hazard which could deliver potentially fatal carbon monoxide to building occupants.
"Hidden" Duct or Air Handler Leaks
Here is an interesting case of a surprise air leak into the return air plenum at a gas-fired hot air furnace and air conditioning system. We removed the (not working) electrostatic air cleaner (first photo) to look into the return plenum where we saw a large gap between the return plenum and the blower compartment (second photo).
When the gas burner was operating along with
the blower (in heating mode) this opening could certainly draw un-wanted gases into the duct system and might lead to CO production too.
In cooling or air conditioning mode, the blower was pulling in air from the basement (where we had a mold concern).
Carbon Monoxide Production Caused by Improperly Located Return Air Ducts/Registers
Return air openings close to natural-draft fired appliances, again particularly gas, can also interfere with proper gas (and possibly oil) burner operation by competing for combustion air, thus causing carbon monoxide production when the burner is operating.
If openings are found in the duct system near fossil-fuel fired appliances it should be reported as an indication of a system operating problem (inadequate return air) and as a safety hazard (potentially fatal carbon monoxide poisoning). Review this potential hazard with a qualified service professional. For example, should a stack pipe fail and flue gas be dumped into the furnace room it would be picked up and distributed throughout the building.
Sample Air Conditioning Or Heating System Report Language for Unsafe Duct Openings
Sample inspection report language for dangerous HVAC ductwork openings:
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