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Hot Air Heating Furnace Basic Operating Steps
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This website answers most questions about central 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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links 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.

How does a Warm Air Heating Furnace work?

The Building Thermostat senses temperature, turns on the furnace burner: As building temperatures drop a wall-mounted thermostat in the occupied space senses the temperature drop and in response, calls for heat at the furnace, causing the furnace oil or gas burner to turn "on". Details are at THERMOSTATS. Sketch at left courtesy of Carson Dunlop.

The Gas or oil burner: The heat source such as a gas or oil-fired burner will turn on in response to the thermostat, and it will continue to run until either the high temperature or "FAN LIMIT" temperature is reached inside the warm air plenum or until the building thermostat inside the occupied space senses that the desired temperature there has been reached. At that time the burner is turned off but the blower will usually continue to run for a bit longer for reasons we'll explain in more detail below. See OIL BURNERS or GAS PIPING, VALVES, CONTROLS

Heat exchanger: Hot combustion gases produced by the oil or gas burner circulate inside of the furnace's metal heat exchanger causing it to get hot. Combustion gases leave the inside of the heat exchanger and flow through a flue vent connector to a building chimney where they are vented safely outside.

The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of a steel "box" called the heat exchanger, sending the now-warmed air onwards into the furnace's output side or "supply air" plenum where it is sent into the building warm air duct system for delivery to the occupied space.

Air ducts connect and permit movement of cool air from occupied space through furnace and deliver warm air back to occupied space: The building air duct system includes return or cool air ducts and warm air ducts. Cool air ducts carry air from the occupied space into the furnace return air plenum, possibly through an air filter, and into the furnace blower compartment. Warm air supply ducts connect to the supply air plenum and carry warm air into the occupied space where it flows out of floor, wall, or ceiling warm air registers or diffusers. See DUCT SYSTEMS

Combination Fan & Limit Control: This control turns the furnace blower on and off at the proper times. The FAN ON setting on this control makes sure that the blower fan does not turn on (even though the building thermostat has asked for heat) so that the furnace wont' blow cool air into the occupied space.

Furnace FAN ON: When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns on the furnace blower to deliver warm air to the occupied space.

Furnace FAN OFF: The furnace combination fan and limit control FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down and the heat it contained has been sent to the occupied space.

Furnace LIMIT: The LIMIT indicator setting is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level.

See  FAN LIMIT SWITCH for details.

When does the furnace blower turn OFF in normal operation?

When the thermostat has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate until the temperature at the supply plenum has reached or dropped below the "cut-in" or "fan-on" lower limit on the switch.

When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:

On most heating systems the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as soon as the thermostat is satisfied. If the furnace oil or gas burner is very high capacity, or if the furnace fan/limit controls have been set to cause this effect, the burner may on some systems cycle on and off periodically while the warm air blower continues to run.

How is heating furnace efficiency or economy measured? What does Furnace AFUE mean?

Each furnace model is assigned an AFUE number. AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better. If your furnace has an AFUE rating of 90, that means that for every dollar you spend on fuel, 90 cents worth of heat is delivered into your building. The remaining 10 cents is lost in inefficiency such as heat that escapes up the chimney along with the products of combustion.

AFUE is not the whole story of heating cost efficiency. A high-efficiency heating system that has not been cleaned and serviced may be running poorly and wasting money. In fact, an 85% AFUE heating furnace that has not been cleaned might be running at an efficiency much lower, perhaps 65%.

Furthermore, if your building is drafty or poorly insulated, you may be delivering heat at high efficiency but losing it from the building much faster than necessary. These articles can help with a more complete approach to saving money on heat: First see HEATING COST SAVINGS METHODS then for more detail check out our articles at HEAT LOSS CALCULATIONS and HEAT LOSS INDICATORS and INSULATION IDENTIFICATION GUIDE

Signs of Improper Warm Air Heating Furnace Operation

Furnace Oil or Gas Burner Cycling On and Off Frequently

If a furnace oil or gas burner cycles on and off frequently while the furnace blower continues to run, the heating system probably needs service, inspecting, testing, or adjustment.

• An over-sized or gas burner that is putting heat energy into the furnace too fast for the blower to extract it can cause this symptom.
• A dirty or blocked air filter or a dirty or blocked blower fan can cause this symptom because it is reducing the rate of air flow across the heat exchanger. If you want to save money on heating costs be sure the warm air flow is not reduced or obstructed by dirty or improper air filters.
• Improper furnace control settings can cause furnace oil or gas burners to cycle on and off too often; check with your heating service technician.

Reduced or no Air Flow at Heating Supply Registers

If you are getting some but not much air flow out of heat supply registers there is probably a problem with:

• the air filter(s) - dirty or damaged - see AIR FILTERS for HVAC SYSTEMS
• the blower fan is dirty or has a loose fan belt - see DIRTY A/C BLOWERS
• the duct system itself, such as collapsed, blocked, or disconnected duct work - see DUCT SYSTEMS
• if the system is a combined heating and air conditioning system, the cooling coil may be blocked by dirt - see DIRTY COOLING COIL
• if the system has always delivered only poor air flow at the supply registers, the blower may be dirty or defective, or the return air supply may be inadequate - see INCREASING RETURN AIR

If you are getting no air flow whatsoever but the furnace burner seems to be running there is probably a problem with:

• the blower fan motor or fan itself has failed
• the duct system has become totally blocked or disconnected - see DUCT SYSTEMS
• the furnace is not running at all - see NO HEAT - BOILER / FURNACE DIAGNOSIS

A Guide to Hot Air Heating Furnace Inspections

• Operating sequence of a hot air heating furnace:
  
  The building or building room cools,
  as temperature drops the thermostat switch closes (calls for heat),
  oil (or gas or electric) burner is turned on, oil is pumped from tank to burner, ignited, burned,
  Hot gases pass through a fire chamber (steel can)
  Hot gases pass through a heat exchanger (steel separating building air from combustion chamber gases)
  and out an exhaust flue,
  past a barometric damper to a chimney to outdoors.

• ASHI 9.2.C. The inspector shall open readily open-able access panels provided by the manufacturer or installer for routine homeowner maintenance
• Understanding functions=understanding implications: E.g. : wet basement, steel furnace, high CO risks from rusted heat exchanger.

Heating System Operating Sequence Inspection Tips

Similarly, when the thermostat is satisfied (the room has reached the thermostat set temperature), the thermostat will "turn off" the furnace. What actually happens is that the oil or gas burner will turn off immediately, but on most hot air furnace systems, the blower will continue to run, perhaps even for several minutes, as it extracts the remaining heat from the furnace heat exchanger and hot air supply plenum.

Two reasons for this continued operation of the furnace blower fan past burner shutdown include:

1. Cooling down the heat exchanger avoids cracks or damage that could occur if the furnace heat exchanger gets too hot.
2. The cool down cycle of the blower avoids wasting heat by failing to distribute it to the living area from the furnace.

Make a thorough visual inspection for signs of damage or improper operation, missing parts, water damage, rust, mechanical damage, soot, smoke, improper venting, missing or damaged safety controls, etc. We detail these topics in articles listed below. Thanks to reader Cyrus for editing suggestions.

Details of Heating System 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:

• Hot Air Heating Furnace Basic Operating Steps
• Fan Limit Switch on hot air furnace heating systems (gas or oil fired)
• Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
• Cad Cell Relays on oil fired furnaces or boilers as flame sensors & safety devices
• Stack Relay Switches Explained on oil fired furnaces or boilers as flame sensors & safety devices
• Draft Hoods on gas fired heating equipment, function and safety
• Draft Regulators & barometric dampers on oil fired heating equipment
• Gas Piping, Valves, Controls including gas regulator assembly and other controls

Types & Combinations of Water (hydronic heating boilers) and Air (warm air furnaces) Heating Systems

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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• Thanks to our reader Cyrus for editing help and critique 2/1/2009
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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More Information on Building Diagnostic Inspections and Repairs

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• 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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