Sizing Retrofit Conditioning Air Handlers
- RETROFIT SIZING for A/C or HEAT PUMPS - 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
- Questions & answers about how to properly size & install retrofit or add-on air conditioning to an existing warm air furnace
- References
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Retrofit or add-on air conditioning specifications: this air conditioning repair article discusses the
Sizing of Retrofit Conditioning Air Handlers: How to Add Air Conditioning to Hot Air Heating Systems. 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.
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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
We discuss the air conditioning system sizing problem at AIR CONDITIONER BTU CHART and while details are found at DEHUMIDIFICATION PROBLEMS, we also introduce the oversized air conditioner problem at LOST COOLING CAPACITY. Also see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs and see RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.
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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Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
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- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- 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
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
- Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
- Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
- Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
- "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]
- Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard- Asbestos HVAC Ducts and Flues field identification photos and guide
- Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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