How to Diagnose & Repair No Heat for Furnaces
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Troubleshooting heating system furnace, burner, controls, or heat distribution problems
How to inspect & repair hot air heating systems - Furnaces
Duct system inspection, defects, repairs
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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.
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
Electrical power to the heater (or air conditioner) is off: 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? 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 Flame Sensors (modern equipment) and also Stack Relay Switch (older oil fired furnaces and boilers).
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 Functions and Cad Cell Relay Switch Flame Sensors (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? If it is an oil fired heating furnace, check the gauge on the oil tank. Are you out of oil? See OIL TANK GAUGES
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
Flue gas spill 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 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 are closed or blocked)?
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 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 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. 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 shoud turn the blower off when you open the door.
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 these articles:
Definition of Water to Air Heat Exchanger Heating Systems
A technical 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.
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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 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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