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Sizing Retrofit Conditioning Air Handlers InspectAPedia®  -     Sizing Retrofit Conditioning Air Handlers: How to Add Air Conditioning to Hot Air Heating Systems Air conditioning air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement

This chapter of "How to Inspect the Central Air Conditioning or Cooling System" discusses the Sizing of Retrofit Conditioning Air Handlers: How to Add Air Conditioning to Hot Air Heating Systems. Here we discuss the problems that occur if an add-on air conditioning component (to an existing hot air heating system) is not properly sized. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ADDING A/C: RETROFIT SIZING - Air Conditioning Retrofit Errors When Adding Air Conditioning to an Existing Hot Air Heating System: Improper Air Conditioning Cooling Coil Size

The typical indoor half of an air conditioning system is comprised of a blower fan which moves building air (through return ducts) across an evaporator coil (which cools and dehumidifies the building air) and then through supply ducts to various building rooms. Liquid refrigerant is metered into the evaporator coil where it evaporates or changes state from a liquid to a gas.

This state change from liquid to gas absorbs heat and thus cools the evaporator coil. As air is cooled by moving across the evaporator coil, moisture is removed from the air (cool air holds less moisture than warm air). The condensate is moved to an acceptable disposal point, perhaps by a condensate pump to a building drain.

But a larger evaporator coil does not necessarily mean we'll see an equivalent improvement in the cooling capacity of the air conditioning system. In fact if the cooling coil is too large for the air handler or "blower unit" the air conditioning system will not work properly.

The photo shows that an evaporator coil has been added atop an existing heating furnace. Using the existing blower and duct work and simply adding an evaporator coil atop the unit is a common way to add air conditioning to an existing furnace and duct system, and such air conditioning retrofits can work quite well by taking advantage of an existing duct system and air handler.

But to work properly the evaporator coil (or cooling coil) should not be too large for the air movement capacity of the existing air handling system. Otherwise the air handler won't be able to move enough air across the evaporator coil to prevent freeze-ups and the net result may be less cooling capacity rather than more. In other words, when the new evaporator coil for an add-on air conditioning system is too large, the blower will not move enough air across the coil, probably leading to the evaporator coil becoming blocked by ice.

Similarly, the air movement capability of the blower assembly, including the speed of the fan of an existing furnace needs to be matched to the evaporator coil's needs. The speed of air flow through ductwork and its delivery into a building are different for heating than for cooling a building. Some contemporary combination units which provide both heating and cooling air to a building make use of a dual speed or variable speed fan.

If when inspecting a "retrofit" air conditioning system (like the one in this photograph) you see that the evaporator coil is much larger than the furnace blower atop which it sits, the system is probably not properly designed and it may not work correctly. An expert air conditioning service technician or design specialist should evaluate the system when you see this condition.

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Technical Reviewers & References

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Daniel Friedman - principal author/editor of the InspectAPedia ^TM Website
• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Wikipedia provided background information about the definition of HEPA and airborne particle interception.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING & HEAT PUMP SYSTEMS
AGE of AIR CONDITIONERS & HEAT PUMPS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNIT
  ADDING A/C: RETROFIT SIZING
  BLOWER LEAKS, RUST & MOLD
  COOLING COIL DEFECTS
  DIRTY A/C BLOWERS
    Leaks, Rodents In Air Handlers
    Mold Growth in Air Handlers
BACKUP HEAT for HEAT PUMPS
BLOWER DOORS & AIR INFILTRATION
BOOKSTORE - Air Conditioning "How To" Books
CLEANING & Legionella BACTERIA
COMPRESSOR & CONDENSING COIL
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
  DAMAGED COOLING COIL
  DIRTY COOLING COIL
  DIRTY COIL CLEANING PROCEDURES
  FROST BUILD-UP
DATA TAGS on AIR CONDITIONERS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
ENERGY SAVINGS in BUILDINGS
EVAPORATIVE COOLING SYSTEMS
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
HUMIDITY LEVEL TARGET
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
SWAMP COOLERS
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS on A/C SYSTEMS
FURNACES WARM AIR HEATING SYSTEMS
INDOOR AIR QUALITY IMPROVEMENT GUIDE

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