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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 DIAGNOSIS & REPAIR
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
FAN NOISES, HVAC
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
This article describes the inspection of heating and air conditioning (HVAC) duct air supply and return registers & duct zones for defects such as missing air conditioning cool air supply or return air registers, undersized air conditioning duct openings, improper cooling duct routing, cooling (or heating) air duct corrosion, leaky air duct connections, defective heating or cooling ductwork materials. We include a discussion of how to increase the supply of cool air or air conditioner output in a building.
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The photograph above shows a 1930's heating and cooling air supply register still in active use, but with leakage around the register which transmitted odors and mold from the building basement. Also see SUPPLY DUCT AIR LEAKS and see LEAKY DUCT CONNECTIONS.
How to Diagnose Stains at Ceiling Air Supply Registers
The ceiling air supply register shown in the photo at left is typical of modern residential installations except for those leak stains around the register.
Further investigation was needed to discover whether the stains were due to a roof leak above this point or an air conditioning leak into and around the supply duct or as we've seen in some northern climates, accumulation of condensation and even ice in ceiling air conditioning ducts caused by movement of moist air backwards through the duct system by natural convection when the HVAC system was turned off.
The ceiling register in the second photo at right has blown soot and debris onto the ceiling itself. Further investigation was needed to determine if this soiling was from failure to maintain filters in the duct system, mold or debris in the duct system, a failed (and dangerous) oil-fired or gas-fired furnace heat exchanger, or other causes.
Heating & Cooling source in each room:
The minimum air conditioning system inspection standard (or heating inspection standard) includes the observation of the presence of a cooling source in each habitable room in the building. Ductwork to each room (at least connected to visible supply registers) & supply registers themselves should be observed in every room, delivering conditioned air to each habitable room (we can exclude closets and utility rooms and in some locales, baths).
Beware of "dummy" supply registers that are not connected to anything.
Beware of supply registers that are connected to ductwork but have no air flow due to duct routing errors, pinched, or disconnected duct work, or similar faults. This defect can only be observed if conditions permit operating the system.
In a home inspection report, cooling system duct work defects may be reported under "Heating System" for cases where same ductwork is used for both heating and cooling. However, optimal supply and return placement for cooling is different from the optimum placement for cooling supply and return air. A heating system may deliver warm air low on walls or at floor level (warm air rises).
A cooling system may prefer to deliver cool air from a supply diffuser high on the wall or in ceilings (cool air falls) and draw cool air to the system return duct from a separate high-location in a ceiling (warm air rises to the high return duct for air conditioning). This topic is discussed in more detail at LOCATION OF DUCTS below.
Heating or Cooling Zone Control for Furnaces & Central Air Conditioning Systems
Zone control for air handling systems can be provided by:
See ZONE DAMPER CONTROLS for details about how to achieve individual zone control for warm air heating for for air conditioning systems.
Leaks in HVAC cooling or heating air supply ducts mean that cooling or heating costs are increased, since the HVAC system needs to run longer to reach the desired indoor temperature and humidity. Where more severe supply air duct leaks are occurring, some areas of the building or even the entire building may receive no heated or cooled air at all, even though the equipment is running.
Watch out: But keep in mind that even this apparently accurate calculation of the effect of piping on air pressure and airflow loss will not include the effects of leaks or obstructions in the building return air or supply air duct system such as
How can I improve cold air delivery from my air conditioner?
Older Florida home with air handler under house in crawl space. Air is ducted to floor registers. Not very efficient as cold air doesn't rise much. House has a flat roof no attic space to get up into. Any thoughts on how to improve? - D. (Anon).
Reply: Checklist of Air Conditioner Airflow Improvements
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider in improving cool air flow in your home:
We agree completely that cool air works better delivered from ceilings than from floors since cool air tends naturally to fall through the occupied space. It takes more energy to blow cool air "up" than to drop it into a room from supply registers mounted high on walls or in the ceilings. We discuss this further at LOCATION OF REGISTERS & DUCTS.
But the cost of changing ductwork is rather high and the work disruptive so that's not your first choice.
Here are some A/C cool air flow improvement steps:
Ductwork Requirements Rules of Thumb & Tables
The following tables are adapted from "How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" published by the Vermont Department for Children and Families ; similar tables are widespread through HVAC literature and resources cited at References
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