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Furnace sketch (C) Carson Dunlop AssociatesHot Air Heating Furnace Basic Operating Steps
Troubleshooting warm air heating systems

  • FURNACE OPERATION DETAILS - CONTENTS: Sequence of steps in furnace operation - how warm air heating systems work. Troubleshooting heating system boiler, furnace, burner, controls, or heat distribution problems. How to inspect & repair hot air heating systems - Furnaces. What are the basic components of warm air or furnace heating systems? Typical operating temperatures of warm air or hot air heating systems
  • POST a QUESTION or READ FAQs about the operating sequence of warm air heating systems
  • REFERENCES
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Heating furnace operation:

This article lists the sequence of steps in the operation of a modern warm air heating furnace as well as older heating furnaces such as the "octopus" furnace that may have little or no duct work.

Understanding exactly what happens from the moment that the room thermostat calls for heat until the call for heat is satisfied can help troubleshoot a heating furnace that is not working properly. Sketch at page top courtesy of Carson Dunlop Associates.



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How does a Warm Air Heating Furnace work? Sequence of Operating Steps

Furnace sketch (C) Carson Dunlop Associates

This article series answers most questions about central heating 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. Sketch at page top courtesy of Carson Dunlop Associates.

1. 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 Associates.

2. 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 BURNER FLAME & NOISE DEFECTS

3. 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.

4. 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.

LARGER VIEW of an octopus furnace5. 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

The Octopus furnace shown at left delivered heat to the building with no ductwork whatsoever. Warm air rose into the first floor of the building through a floor grid mounted at the top of the furnace. Warm air rose to upper floors through stairwells or by ceiling-cut air registers.

6. 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 RATE CALCULATIONS

and HEAT LOSS INDICATORS

and INSULATION IDENTIFICATION GUIDE

What are the Typical Air Temperatures Supplied by Warm Air Heating Furnaces?

High efficiency furances and duct size Carson Dunlop AssociatesA conventional gas fired warm air furnace will provide heated air at 140 to 170 °F at the supply plenum. Naturally the temperature of this air drops as it moves through the ductwork system and will be cooler exiting the air supply registers.

A high efficiency gas fired furnace will provide heated air at lower temperatures, typically at 110 F to 140F at the supply plenum.

Watch out: as Carson Dunlop Associates warn (sketch at left), under-sized or restricted ductwork will reduce airflow over the furnace heat exchanger and can thus overheat and even damage the furnace heat exchanger.

So if air temperatures at the supply plenum (or even at supply ducts) are abnormally high don't think that's a great heating system - instead you may need the system checked by an HVAC expert technician.

See FAN LIMIT SWITCH for details on the temperature operating limits and controls for warm air heating systems.

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.

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:

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

A Guide to Hot Air Heating Furnace Inspections - Troubleshooting

Photograph of an oil fired heater stack relay

Heating System Operating Sequence Inspection Tips

Photograph of an oil fired heater stack relay

The rust on the exterior around the gas burners of this gas fired furnace could be due to improper and unsafe operation.

Expert inspection should be ordered to assure that this system is safe and functional.

If the furnace turns on immediately when the thermostat is turned-on or up from the living area what does this mean? Normally a hot air furnace oil or gas burner will turn on immediately when the thermostat calls for heat.

However the blower or fan unit that delivers warm air to the living area will not turn on until the fan limit switch in the hot air supply plenum or furnace top senses that the air in that area is warm. This prevents blowing cold air on building occupants.

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.

List of Heating System Controls and Switches on Most Furnaces

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

How to Improve the Warm Air Supply

Reader Question: Heating Furnace Runs but Air that is Not Warm Enough

This discussion has been moved to a separate article
at WARM AIR SUPPLY IMPROVEMENT

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.

More about 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.

Watch out for corrosion on water to air heat exchanger heating coils, as we explain just below.

Corrosion on Heating & Air Conditioning Heat Exchanger Coils & Condenser Coils

Corrosion on heat exchanging coils in heat pumps, air conditioners, or water-to-air heating coils eventually leads to leaks and costly repairs that require replacement of the coil and re-charging of the HVAC system refrigerant. Corrosion may occur as a function of age and use and failure to keep the coil clean, but there are some special sources of corrosion in HVAC systems that you should watch for:

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.

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