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 air conditioning repair article discusses the
Sizing of Retrofit Conditioning Air Handlers: How to Add Air Conditioning to Hot Air Heating Systems.
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We describe the central air conditioner operating problems that occur if an add-on air conditioning component (to an existing hot air heating system) is not properly sized for the building or if it is not properly matched to the pre-existing blower or air handler system that was used for central heating. There may also be a need for an air handler blower fan capable of operating at different speeds for heating versus cooling. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
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.
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.
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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
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.).
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