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AHU blower fan troubleshooting diagnosis & repair:
This article series describes in detail the testing and diagnosis of problems with air conditioning fans and with forced warm air heating blower fans found in the air handler or AHU.
When the blower fan won't start the problem could be something as simple as a blower compartment door that is not fully closed, or a clogged condensate drain.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
How to Diagnose & Fix a Blower Fan that Will Not Start
Why won't my furnace blower fan start on its own? It will run if I give it a "spin"
My gas heater isn't working correctly. My thermostat signals the heater correctly and the burner ignites, but the fan doesn't always turn on. If I open the panel and spin the squirrel cage a little, it immediately begins to blow air and runs the heating cycle. What could this be? - Mike.
Photo: an electric motor using a pulley and belt to drive the fan in an air handler.
[Click to enlarge any image]
Reply: Check or replace the blower fan motor start/run capacitor
Often when an HVAC fan won't start on its own but will start and keep spinning when you give it a push that means that the start-capacitor for the unit has failed.
It's an inexpensive part that can be installed or replaced by your HVAC service tech or by a homeowner who has expertise in electrical wiring, using a VOM, and following simple wiring instructions.
12 Reasons that an Air Handler, A/C or Furnace Blower Van Does Not Run
Photo: a direct-drive air handler blower motor on a nicely-cleaned AHU.
Before we focus on the motor and controls of the air handler blower fan (used in both air conditioning and heating systems) let's ask if the trouble is not specifically the blower fan but more basically: the air conditioner or heater itself just won't start in response to a call for cooling or heating.
To rule out trouble outside of the fan or air handler itself, check:
A/C won't start or lost cooling capacity: If your air conditioning or heat pump system has lost its cooling capacity or won't start at all
see REPAIR GUIDE for AIR CONDITIONERS. Make sure that all electrical power switches for the system are "on" and that the thermostat is properly set.
Now we can continue by assuming you're convinced the trouble is right in the air handler. So take a look at these most-common causes of failure of the AHU blower fan to run. I've arranged this list roughly in order of probability and also in order putting the easiest and simplest things to check first.
Most homeowners can check the first six of these things by simple observation.
Watch out: turn off the electrical power before opening and inspecting the interior of the air handler or its blower compartment. There are risks of chopped off fingers or even death by electrocution if the equipment is not completely turned off.
Certainly most homeowners (except my mom) can check to see that the thermostat is set to call for heating or cooling, and you can look to see if a blower compartment door is open or the condensate tray is full of water.
But for electrical inspection and testing of components or switches, if you are not trained, comfortable and confident with using basic electrical test tools like a neon tester or VOM or DMM to check for electrical power, do not try it and don't touch anything. Instead call your HVAC service company for help.
Flooded condensate drain tray - for heat pumps and air conditioning systems that use a sensor switch to detect water in the condensate overflow pan, this condition can prevent the air handler from running in air conditioning and heat pump systems - CONDENSATE PAN SWITCH LOCKOUT
Loose or broken fan beltOn belt-driven blower assemblies - the blower fan motor runs but the fan does not turn
Loose pulley on belt driven blower assemblies - the blower fan motor runs but the fan does not turn
Damaged blower assembly shaft or bearings - a seized fan assembly (unusual). The motor will draw abnormally high amps.
Reader Question: my furnace blower fan will not engage - the burner starts but not the fan
I have recently started using my furnace (20+ years old furnace), and the blower fan will not engage. The pilot and gas burners start, but the fan does not. I have ensured that the fan is set to "auto", on the thermostat. Is there any procedure I can do to troubleshoot the issue? I have seen several topics, however none seem to be helpful. - J.B. 12/31/2013
A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. But here are some suggestions to start troubleshooting the problem of a furnace blower fan that does not start:
Step 1: I would start by confirming that the blower fan will operate. Your system probably has a manual FAN ON switch on the thermostat, or if not, there may be a FAN ON push pull switch right on the fan limit switch in the air handler.
If you can make the blower run then we know the problem is not with the blower motor or drive belt or assembly and we'd go to step two.
Step 2: I would continue diagnosis by watching the behavior of the fan limit switch as the furnace heats up; presuming nobody has messed with the switch to try to change its settings, it should act as described in the article above beginning
at FAN LIMIT SWITCH TROUBLESHOOTING
This article series answers most questions about central heating and air conditioning system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Continue reading at FAN LIMIT SWITCH TROUBLESHOOTING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
For details about the controls, components and switches commonly found on hot hot air heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a furnace or warm air heating system.
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates 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.
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.
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