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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
Guide to heating & cooling duct work supply air flow adequacy, flow rate, temperature, and related problems.
This chapter of "How to Inspect the Central Air Conditioning or Cooling System" describes the inspection of heating and air conditioning (HVAC) duct registers & 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 such as Goodman gray flex-duct, some Owens Corning Flex-duct, and asbestos-containing air conditioning or heating duct work.
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.
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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.
Technical note: The D'Arcy-Weisbach equation for pressure and head loss can be used to calculate the actual pressure loss due to friction in a building piping or air duct system.
The Engineering Toolbox provides the D'Arcy-Weisbach formula: Δp = λ (l / dh) (ρ v2 / 2) where Δp = pressure loss (Pa, N/m2), and l = length of duct or pipe (m) and dh = hydraulic diameter (m) and finally, ρ = density (kg/m3).
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 the following:
Continue reading at UNDERSIZED RETURN DUCTS
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Frequently Asked Questions (FAQs) about leaks in HVAC supply ducts
Question: AC system suddenly quit delivering supply air to some rooms in the building
(June 29, 2011) pam said:
I am trying to troubleshoot an a/c system that overnight quit supplying air to the upstairs and two rooms downstairs. (coil is fine, air handler/blower operating fine, no observable fault of (exterior) duct system. the rooms without air are getting none at all.
I'd look for:
- a damper that someone closed in the ductowrk
Question: ok to run supply air ducts inside of cold air returns?
(May 2, 2012) firstname.lastname@example.org said:
can supply ducts be run within cold air returns? I was inspecting a new home being built in my area and noticed several areas where there were heat ducts running within cols air returns for a substantial length. Does this comply with codes?
The only place where running coaxial heating ducts makes any sense to me is within an air to air heat exchange system.
For example, if a building is bringing in fresh outdoor air and also exhausting stale indoor air, we may run one duct inside the other in order to transfer heat in the desired direction, thus reducing building energy costs.
May 2, 2012) Supply duct inside a return return said:
I noted at least two instances in the new home where supply ducts were inside a return air joist spaces between the floor joist. wondered if this was within code. These were inside for a distance of ten or more feet. Looks like in cooling season these supplies would probable become a huge area for condensation to form and drip. This also would greatly reduce the volume of air the retun may have.
Building codes often cannot and do not try to anticipate every snafu that people can imagine, instead specifying overall performance objectives.
Running a supply air duct inside of a return air passage transfers energy between the two, as we explained above. If we are only talking about recirculating building air rather then the impact on overall heating or cooling costs may be minimal, certainly of less impact than running either supply or return ducts through unconditioned space, particularly if the ducts are not well insulated.
In my OPINION there is also a risk of un-detected cross-leakage out of supply into return ducts that might reduce the effectiveness of the system.
Question: need to replace collapsed flex-duct
(Jan 9, 2013) Larry said:
If a flexible supply duct under our double wide has a 6' collapsed section, does it need replaced?
Certainly yes, Larry. Otherwise you are restricting air supply to the living area and increasing heating & cooling costs.
Questions & answers or comments about inadequate supply air in HVAC duct systems and its effect on heating or cooling air flow adequacy.
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