How to Diagnose & Repair No Heat for Furnaces
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No heat service procedures for warm air furnaces: how to diganos loss of heat and how to get the furnace working again; when do you need to call a heating service company? What to check first.
Diagnose loss of heat in a building with warm air heating equipment
Troubleshooting heating system furnace, burner, controls, or heat distribution problems
How to inspect & repair hot air heating systems - Furnaces
Duct system inspection, defects, repairs
Questions & answers about how to fix a warm air heating furnace that is not working - how to diagnose the loss of heat in a building when the heat source is an oil or gas fired furnace.

Warm air heating furnace troubleshooting: how to diagnose & fix hot air furnace heating system problems. This article series answers most questions about hot air or warm air heating system troubleshooting, inspection, diagnosis, and repairs. This article explains the diagnosis and correction of no heat conditions for warm air or hot air furnace and air duct heating systems. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects. We explain how to recognize and fix common heating system operating or safety defects and how to save money on home heating costs.

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How to Diagnose Loss of Heat with Hot Air Furnace Systems

This article explains how to diagnose and fix warm air or hot air furnace heating systems that are not working. Carson Dunlop Associates' sketch (left) shows the main layout of a basic warm air heating system.

If you don't know whether your heat is provided by a furnace (hot air) or boiler (hot water), or whether your fuel is oil, gas, or electric, and whether your heat is hot water, steam, or warm air, see HEATING SYSTEM TYPES.

If your heating system uses water or hydronic or hot water or radiator or baseboard hot water heating systems, instead of the article found here you should see HEATING LOSS DIAGNOSIS - BOILERS.

Note: some hybrid or mixed 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) - described at HEATING SYSTEM TYPES.

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

Before turning to defects specific to a warm air furnace system, review our Oil / Gas Burner basic operating problems outlined at the top of HEATING LOSS DIAGNOSIS - BOILERS.

If the furnace oil burner or gas burner is not turning on at all check these points first:
- Electrical power to circuits powering the heater (or air conditioner or heat pump) may be turned off: at the main electrical panel and sub panels, check the circuit breaker or fuse supplying power to the unit; check the service switch at the furnace or air handler, and if the system is air conditioning or a heat pump, check the service switch outside at the compressor/condenser unit to be sure it is "on".
- Check Furnace Electrical Switches and Live Power: Is the furnace turned on? Check the electrical service switch at the furnace as well as others that may be present in the home. 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.)
- Oil Burner safety reset switch is off: the heating or air conditioning system may have been turned off on reset - see our "off on reset" and other advice at CAD CELL RELAY SWITCH (modern equipment) and also Stack Relay Switch (older oil fired furnaces and boilers).
  
  Cad cell control or stack relay buzzing or jamming trouble: if you hear or have ever heard buzzing from any heating system control that includes a relay switch, watch out for a control cover that is in contact with a relay switch in the unit. If the cover is lightly touching the relay switch the control may simply buzz when the relay is trying to move. But a too-tight control cover can also prevent a relay from operating, leading to loss of heat. Simply pulling off the cover to let the relay move freely can immediately discover and "fix" this problem. Details and an explanation are at HEATING SYSTEM NOISES.
  
  Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: AQUASTAT CONTROL Functions and CAD CELL RELAY SWITCH (hot water boilers, furnaces, and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.
- Check the heating fuel supply for the furnace: if it is a gas furnace, is the gas turned on? If your gas is from bottled or LP gas, is the tank empty? Check that no one has shut the gas supply valves.
  
  If it is an oil fired heating furnace, check the gauge on the oil tank. Are you out of oil? (see OIL TANK GAUGES for details)
  
  An oil or gas fired heating furnace may go off on safety reset even if there is a good fuel supply. And at an oil furnace, even if there is plenty of heating oil in the tank if the oil filter has become blocked the system will try to start but won't run - this will require a service call.
  
  Check for a broken or slipping coupling (or a frozen coupling) between the oil burner motor and the fuel unit. In this case the burner starts in response to a call for heat, but there will be no ignition whatsoever because no oil is being delivered to the nozzle assembly.
  
  Check these additional causes of no oil flow to the oil burner: an oil safety valve that has been "closed" (someone turned it the wrong way), low oil or no oil in the heating oil tank (the tank may not be completely empty but oil may be below the oil pickup line if the oil lines are exiting at the top of the tank), a clogged oil filter or fuel unit strainer, a clogged oil burner nozzle, a air leak in the heating oil supply piping, or a crimp or clog or restriction in the oil piping. If you recently ran out of oil and then got a delivery, and if your system uses a single oil line between tank and burner, the line or fuel unit may be air bound and needs to be bled. [8]
Once you have assured that you have heating fuel and that electrical power is turned on to the heating unit check these controls:

Heating Thermostat is in the "off" position or is set to a temperature that does not call for heating (or cooling) See FAN AUTO ON Thermostat Switch for details about what turns the blower fan on and off. Condensate tray overflow switch: If the fan blower unit combines heating and air conditioning, an air conditioning condensate pan float switch could be in the "shut off" position due to water in the condensate overflow pan or because the switch has been damaged or moved.

See DRIP TRAY DEFECTS and also Use of float switches on air conditioning condensate overflow pans.Fan limit switch may be damaged or inoperable.

See FAN LIMIT SWITCH the flue gas spillage switch: If your furnace is gas-fired, a flue gas spillage switch may have shut the system down after sensing possibly dangerous flue gases (that can contain fatal carbon monoxide).

See Spill Switches
Stack relay switch may be installed on the flue connector and may be in the "safety - off" position. To identify and reset this control see Stack Relay Switch.

No heat or not enough heat or not enough warm air volume from the Furnace: 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 or duct sections are closed or blocked)?

Heating air comes out but it is too weak or slow: check for a dirty clogged air filter (AIR FILTERS for HVAC SYSTEMS), a dirty-clogged blower fan, blocked, disconnected ductwork.
Check the warm 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). See the discussion of thermostat switches at FAN AUTO ON CONTROLS.
Check furnace blower operation: Does the furnace cycle on, the oil burner or gas 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. See FAN LIMIT SWITCH.

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. If the blower fan motor just won't start on its own but the fan will start and continue to run if given a manual "spin" , also see FAN MOTOR START CAPACITORS.

Watch out: working in the blower compartment is dangerous - if power is on you can lose a finger if the fan starts! Modern air handlers have a safety switch in the blower compartment door that should turn the blower off when you open the door.
Not enough warm air volume from the furnace and duct system: check for a dirty air filter at the return register or furnace. Check for blocked, crimped, leaky, or disconnected air ducts. See AIR FILTERS for HVAC SYSTEMS and see DUCT SYSTEM & DUCT DEFECTS

Details of Hot Air Heat Furnace Controls and Switches

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see FURNACE CONTROLS & SWITCHES and also FURNACE OPERATION DETAILS

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).

For a detailed guide to inspecting, diagnosing, maintaining & repairing mixed, hybrid, water to air heating systems see FAN CONVECTOR HEATERS - HYDRONIC COILS

Dual hot water and warm air heating systems: as we explain at HEATING SYSTEM TYPES, 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.

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DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE

HEAT LOSS in buildings
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION

HEAT PUMPS, DiAGNOSIS, REPAIR
HEAT TAPES & CABLES on Roofs for Ice Dams
HEATING COST FUEL & BTU Cost Table
HEATING COST SAVINGS METHODS

HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
[8] Suntec Model A-7400 Fuel Unit Solenoid Dumping Pump,
Suntec Industries Inc., 60 Aberdeen Drive - PO Box 5000 Glasgow, KY 42142-5000 - USA Tel : 270 651 7116 - Fax : 270 651 9276 e-Mail : info@suntecpumps.com and
Suntec Industries France, 1 Rue Lavoisier - B.P. 102 F-21603 LONGVIC Cedex - FRANCE Tel : +33 (0)3 80 70 60 70 - Fax : +33 (0)3 80 70 61 11 e-Mail : information@suntec.fr, [copy on file as /heating/Oil pumps fuel units/Sunted_Solenoid_DumpingPPump.pdf] ,
Also /Suntech_Solonoid_Safety_Valves.pdf (brochure)
web search 4/19/12, original source: http://www.suntecpumps.com/Suntecus/PDFs/Form%202740%20
-%20Solenoid%20Dumping%20Pump.pdf

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.

Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
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 TANK PIPING & PIPING DEFECTS - how to pipe oil tank fill, vent, and oil supply lines, common defects, leaks, safety concerns
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.)
"Scale formation in water heaters and methods of prevention", Krappe, Justus Maximilian, Engineering experiment station. Gas engineering bulletin; no. 6; Research series; no. 74; On cover: Engineering bulletin, Purdue university. Vol. xxiv, no. 3a. June, 1940 (Layfayette Indiana) commonly referred to in some references as "Purdue University Bulletin No. 74" - thanks to researcher Robyn Goldstein for the full citation. LCCN: 40028844 & OCLC: 1038544 - Water analysis, water softening, hot-water supply. 27pages. You can obtain this document through your local library. (full copy file at InspectAPedia 3/31/2010) Purdue B074 can be hard to locate online.
Also Bradford White Corporation (a manufacturer of water heaters) has published excerpts from that document, available at Purdue_B074_BradfordW.pdf
Links to our list of additional information on heating system inspection, repair, maintenance
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How to Diagnose Loss of Heat with Hot Air Furnace Systems

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

Details of Hot Air Heat Furnace Controls and Switches

Definition of Water to Air Heat Exchanger Heating Systems

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair