Warm air furnace installation, troubleshooting, repair guide:
This article series answers just about any question about forced air or warm air furnace central heating system troubleshooting, inspection, diagnosis, and repairs in residential buildings and homes.
We explain how furnaces work, what controls and settings are used, what goes wrong, and how to fix it.
This page is the starting point for our series of heating furnace diagnosis and repair articles. Sketch at page top courtesy of Carson Dunlop Associates.
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If you have no heat, see HEATING LOSS DIAGNOSIS-FURNACES.
At this website we 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.
First make sure you understand what type of heating system is installed
If you don't know what kind of heat your building uses, we explain how to figure out the answer in more detail
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These photos will help you determine what kind of home heating system you have. Here we're showing heating equipment just as you'll see it in your home, with all of the access covers and panels in place. Articles at this website offer lots more detail including photos of individual heating system controls and components along with explanatory text.
Warm Air Heating Systems - Furnaces: If the heat in your building is provided by warm air that flows out of floor registers (above left), a wall air supply register (above right) or a ceiling air supply register (photo at left) and on into the occupied space, then the air which warms the living space is probably being delivered through large or small diameter ducts, registers, air filters, and a furnace blower.
The heated air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system then your heat is provided by a warm air furnace (sketch at page top, for example)
Cooler air (hopefully also from the same occupied space) flows back to the furnace through one or more air returns and ducts into the furnace return air plenum from which it enters the furnace itself to be re-heated.
Some older warm air systems (illustrated below) are less sophisticated and may have no ductwork at all, and worse, may heat cold air from the basement and send it one-way into the occupied spaces of the home.
Your heating furnace may located in a basement, in a crawl space, in an attic, or even in an outdoor utility closet or an attached garage. In all cases, some heating equipment (oil, gas, coal, wood, geothermal, electric, solar) is used to transfer heat to air that is then delivered to the occupied space of the building.
Illustration of Simple Gravity Furnaces using a Floor Register (Ductless Warm Air)
Older hot air heating systems were comprised of a furnace that heated air, sometimes just air from the basement. The warm air rose into the upper areas of the building by convection (warm air, which is less heavy than cold air) rises, displacing colder, more dense air in the building). You can see one of these old under-floor convection furnaces in our photos at left.
Popularly called a "gravity furnace" (cold air falls by gravity, and warm air defies gravity by rising), you will see only two "pipes" or ducts on the unit.
A flue gas exhaust flue (the smaller diameter steel "pipe" that exits near the bottom left of the gravity furnace and connects to a brick chimney in our photo - and that larger diameter round duct at the top center of the gravity furnace.
That large round warm air supply plenum or duct delivered warm air into the building through a large floor grate in the first floor above. Our arrows show the direction of air flow through this gravity furnace.
In this photo we can't see the cool air intake but almost certainly it's at the bottom of the unit and is in this horribly inefficient unit, is taking cold wet basement air and heating it up before sending it upstairs. We do see a little of this furnace's repair history - that abandoned motor on the floor in the bottom center tells us that an oil burner was installed and had to have a motor replacement.
Warm air rose from this gravity furnace upwards from the first floor grate into the rest of the building also by convection (or "gravity") flowing up a stairwell, or upstairs through registers cut in the first floor ceilings.
How to Spy on your Parents Through the Gravity Heat Registers
But now let's be honest - we don't know for sure if the air register at left is connected to ductwork or if it's just letting warm air rise by "gravity" (we say "convection") from a floor below.
It's easy to figure out however. Just open the louvers and look through the grating. If you find yourself looking into a duct, typically full of trash and debris in an older home, it's connected to ductwork.
If you find that your can see right into a room below, this is a simple gravity or convection register. When the author (DF) was a boy, we used to spy on our parents and their friends by peering down at them through a register like this after we had been sent upstairs to bed. They were not up to much besides drinking, talking about stuff we couldn't understand, and playing canasta.
Illustration of Octopus Furnaces that Added Warm Air Ducts
Octopus furnaces (illustrated below) added warm air ducts that conducted air directly to different areas of the building. You can see at below left why the heater was called an "octopus" furnace. It may not have always had the eight arms of an octopus but it sure looks like one, with those ducts waving all around. At below left you can also see the stack relay (black rectangular control) on the exhaust flue of this octopus furnace.
At in both of the above photos and enlarged at above right we illustrate that white paper-like duct wrap that was just about always an asbestos paper material. On the octopus furnace at above right you can also see a rectangular heating control switch near the top of the furnace. Those black Tee-shaped controls visible on the ducts themselves are manual duct dampers that allowed the occupants to balance warm air flow among different areas of the building.
Examples of Modern Forced Warm Air Furnaces
Modern warm air furnaces classed as central heating include a heat source (oil burner, gas burner, electricity, solar,etc.), a blower assembly, a cool return air plenum, a warm air supply plenum, and connections to supply and return ducts that bring cool air from the occupied space, pass it through the air handler, and move it as warmed air back into the building.
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Depending on their physical position a furnace may be an upright unit or a horizontal unit suited for low basements or crawl space installation.
Heating system air flow direction determines whether we call the vertical air handler an upflow unit (cool air enters at the bottom) or a downflow unit (cool air enters at the top of the unit).
Our upflow/downflow furnace illustration at left was provided courtesy of Carson Dunlop Associates.
Below at left is an up-flow high efficiency gas fired furnace that we installed.
Air enters that silver plenum at the base of the unit and flows upwards through an air filter, blower assembly, heat exchanger and supply plenum into supply ductwork. Our arrows indicate the direction of air movement through the upflow furnace.
At below right is a horizontal furnace. We guess that return air is entering at the right end of the unit because those flex ducts are larger than the smaller flex duct headed up from a (not visible) connection to the supply plenum at the left end of the unit. A look inside through the blower compartment access door on the other side of this unit would answer the question.
A furnace is not a boiler, and vice versa. A boiler should not be called a furnace, or your heating technician will know she or he can charge you extra because you have no idea what is going on, and what they do will seem more mysterious (and expensive) than ever.
Except for one trouble making company who messed everyone up by making a hot water heating boiler and calling it an "Iron Furnace". Oh well. Boilers heat water and furnaces heat air. Usually.
Do you see ductwork, or water pipes?
If the heat in your building is provided by warm or hot metal radiators, heating baseboards containing finned copper tubing, or wall convectors that look like a radiator but contain finned copper tubing, or if heat is provided by flexible rubber, plastic, or metal tubing run in building floors or ceilings, then the warm or hot water circulating in those devices is probably being delivered by piping circulating water heated by a heating boiler, or possibly by a steam boiler or a heat pump or geo-thermal system.
If your heating radiators have valves which hiss and let air escape as heat is coming on your heat is probably being delivered in pipes which circulate steam from the steam boiler up through radiators in the occupied space.
See STEAM HEATING SYSTEMS .
The Building Thermostat senses temperature, turns on the furnace burner. The oil or gas burner will continue to run (usually) until the call for heat is satisfied at the thermostat.
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.
The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of the heat exchanger, sending the now-warmed air onwards into 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.
Sketch courtesy of Carson Dunlop Associates.
Details of just how a warm air heating system works and how its controls function as well as how these components are inspected, tested, set, or re-set are provided at FURNACE OPERATION DETAILS
Continue with Warm Air Heating Furnace System Diagnosis & Repair
If you have no heat,
Reader Question: My furnace gas burners go on but the blower does not start and then the flame goes off. What's wrong?
Reply: as a safety feature on furnaces, the fan limit switch turns off the burner if the blower does not start
Robert, if the furnace blower fan will not start, the limit switch on your furnace will turn off the burner in order to avoid overheating and damaging the heat exchanger.
You'll need to inspect and fix the blower fan problem. If your furnace blower uses a drive belt to connect the fan to the driving motor check that the belt is in place and not slipping. Other blower fans use a direct drive motor whose shaft spins the fan assembly. IN either case check that the motor is starting;
Continue reading at FURNACE CONTROLS & SWITCHESor select a topic from the More Reading links shown below.
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