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How to Diagnose & Repair No Heat for Furnaces
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This website answers most questions about hot air heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards. The limitations of visual inspection of heating systems are described. We continue to add to and update this text as new details are provided.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

How to Diagnose Loss of Heat with Hot Air Furnace Systems

This website discusses How to Diagnose Loss of Heat with Hot Air Furnace Systems. If your heating system uses water or hydronic or hot water or radiator or basboard hot water heating systems, see HEATING LOSS DIAGNOSIS - BOILERS.

How to Diagnose Loss of Heat with a Warm Air or Furnace Heating System

• Furnaces-Hot Air Heat: is any air coming out of the supply registers? Is it warm or cool?
• No warm air comes out: If no air is coming out of supply registers, is this true throughout the building (the blower is blocked or not running) or just in some areas (some registers are closed or blocked)?
• Too little warm air comes out: check for blocked air filters at the furnace air handler or at the central cool air return register(s). Check for a loose or worn belt on the air handler blower assembly.
• Check hot air heat thermostats: is the thermostat(s) turned up above ambient air temperature in the occupied space? Is the thermostat set to "heat" position (if a heating/cooling/off switch is present on the thermostat)
• Check Furnace Electrical Switches and Live Power: Is the furnace turned on? Is there electric power to all of the heating system components such as an oil burner at the furnace and power to the blower fan? Has someone left the inspection or furnace access door open on the furnace? (Modern furnaces have a switch which forces the furnace to turn off if the inspection door is opened - to avoid injury to someone working on the blower assembly.)
• Check heating furnace fuel supply: If the furnace is not running, is there fuel? Oil in the oil tank (check the gauge) or gas in the LP gas tank, or gas in the natural gas lines (valves turned on?) The furnace may go off on reset even if there is plenty of heating oil in the tank if the oil filter has become blocked - this will require a service call.
• Check furnace blower operation: Does the furnace cycle on, the oil burner runs, but the blower never comes on? If so the fan limit switch will turn off the oil burner when the heating supply plenum temperature limit is reached. Normally the blower fan comes on when the fan limit switch senses that supply plenum temperature has reached the "cut on" point, and the moving air keeps the plenum from getting much hotter. This sounds like a problem with the blower fan assembly. Check the blower compartment. If the blower motor is running and the system uses a fan belt to drive the blower fan, perhaps the belt is loose or broken. WATCH OUT: working in the blower compartment is dangerous - if power is on you can lose a finger if the fan starts!
• If the furnace oil burner is not cycling on the system may have been turned off on reset - see our "off on reset" and other advice at the links given just below.

Details of Hot Air Heat Furnace Controls and Switches

For deatails about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

• Hot Air Heating Furnace Basic Operating Steps
• What is the Function of the Hot Air Furnace Fan Limit Switch?
• Other Hot Air Heating Furnace Controls
• Furnace Oil Burner Flame Sensors
• Furnace Cad Cell Relays Explained
• Furnace Stack Relay Switches Explained

Definition of Water to Air Heat Exchanger Heating Systems

A techincal note is necessary about determining what kind of heat or what type of heating system is installed: some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator).

The heat exchanger in a water to air heating system is then placed inside of an air handler or blower compartment where a blower fan circulates building air from return ducts to a plenum where air is blown across the heat exchanger and then the warmed air is delivered to the occupied space through additional warm air ducts or radiators.

Definition of Dual Water and Air Heating Systems

Some buildings are heated by a combination of separate hot water systems (circulating hot water through radiating devices like baseboards or radiators in some areas) and hot air systems (circulating warm air through ductwork into the occupied space in other areas).

These buildings will have both a hot air furnace and a completely separate hot water heating boiler installed. In this case these are completely separate heating systems and usually each serves different building areas.

Definition of Water to Air Heat Exchanger Heating Systems

Some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator).

The heat exchanger in a water to air heating system is then placed inside of an air handler or blower compartment where a blower fan circulates building air from return ducts to a plenum where air is blown across the heat exchanger and then the warmed air is delivered to the occupied space through additional warm air ducts or radiators.

Water-to-air heating systems will use both a separate water heating boiler and a blower or air handler system.

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author/editor of the InspectAPedia ^© Website
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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.

More expert information on this topic

More Information on Inspecting and Repairing Heating Systems

• Carbon Dioxide Gas Toxicity
• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• Dust from HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• 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.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "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.)

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