What Kind of Heat do I Have?
Heating System Type Identification
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How to identify the type of heat provided at a building: warm air, hot water, steam, heating boilers, heating furnaces, steam boilers, heat pumps
Heating System Types: Boilers, Steam Boilers, Furnaces, Mixed Systems
Types Heating System Fuels: oil, gas, electricity, solar, etc.
Types of Heat Distribution Methods in buildings: air, hot water, radiant heating
What are the basic components of heating systems?
Determining heating boiler or furnace capacity & energy efficiency; how is heater efficiency measured, what is the AFUE rating?
Questions & answers about how to identify heating systems and how to inspect their condition

This article explains how to tell what kind of heating system you have installed. We illustrate and describe every type of residential & light commercial building heating system, including boilers, furnaces, steam boilers, and high efficiency units. We compare hot water heating systems (hydronic heat or hot water boilers), warm air furnaces, and steam systems. We explain AFUE or heating system efficiency ratings and numbers. 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.

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How to determine what type of heating equipment or is installed: boilers vs furnaces, hydronic, steam, hot air (or forced warm air), etc.

If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE.

Don't confuse the type of heating system you have installed since their means of making and distributing heat, their controls, and their equipment are mostly different. We illustrate the basic types of residential heating systems just below. There are of course more sophisticated versions of furnaces and boilers such as high efficiency furnaces or boilers and condensing boilers. Those systems too are described at this website - see article links at page left or just search InspectAPedia using the search box at page top or bottom.

Each of the following basic types of heating system is illustrated and described in this article:

A "heating boiler" (also called hydronic heating) heats the building using hot water. A hot water heating boiler is shown in our photo at left and in a sketch provided later in this article.
A steam heating boiler actually boils water and sends steam through building heating pipes, steam radiators or convectors to deliver heat.
A "furnace" heats a building using hot air or "warm air".
Hybrid or mixed 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.

Details and sketches of different types of heating systems are provided below.

Types Heating System Fuels: oil, gas, electricity, solar, etc

Heating furnaces, boilers, and steam boilers may make use of different energy sources. Your heating system may use

gas (LP or bottled gas or piped-in natural gas) (GAS PIPING, VALVES, CONTROLS),
oil (typically No. 2 home heating oil or a kerosene and oil mix), (OIL TANKS and OIL TANK GAUGES and OIL TANK PIPING & PIPING DEFECTS)
electricity (ELECTRIC HEAT),
solar energy (SOLAR ENERGY SYSTEMS),
geothermal heat source (ground water) (GEOTHERMAL HEATING SYSTEMS)
or a combination of these.

Types of Heat Distribution Methods in buildings: air, hot water, radiant heating

Before we go on to illustrate the types of equipment that actually produce the heat to be used in a building we list the basic methods of heat distribution within a building

Air heat distribution by air ducts: for warm air or hot air heating systems, a modern air duct system includes one or more return ducts that bring cool air from the occupied space back tothe furnace, and one or more supply ducts and registers that deliver warm air to the occuped space. Most buildings use conventionally sized ducts and a blower unit to move air through the heating system. Some buildings may use smaller high-velocity air ducts and supply registers. These are explained in detail at DUCT SYSTEM & DUCT DEFECTS.
Hot water heat distribution is made by hot water baseboards, convectors, radiators, or radiant heating. Cool water returns from the radiating devices to the boiler where it is heated and then circulated (by convection or by one or more circulator pumps) back to the radiating devices. Additional levels of heat distribution control to different building areas may be obtained by individual heat piping loops and individual circulator pumps or zone valves that control each heating loop. These are explained in detail at RADIATORS
Radiant heating distributes heat to the occupied space using electric or hydronic in-floor or in-ceiling radiant heating panels or tubing. See RADIANT HEAT.

Air heating systems blow warm air into the room to warm the room and its occupants. Hot water heating systems radiate warmth into the room by both radiant heating from hot radiators or baseboards and by warming air that circulates through the radiator, convector, or baseboard by convection. Radiant heating systems warm room surfaces and occupants without directly warming the room air itself.

Heating System Types: Boilers, Steam Boilers, Furnaces, Mixed Systems

Warm Air or hot air heating Furnaces

Warm Air Heating Systems - Furnaces: If the heat in your building is provided by warm air that flows out of ceiling, wall, or floor air supply registers into the occupied space, or if your heating system uses a water-to-air heating system 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, and the air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system.

Modern warm air heat furnaces use "forced air" - that is, a blower that circulates building air from return air registers through the heating furnace and pushes it into the occupied space through ductwork and supply registers.

In modern furnaces the heating air flow direction may be "upflow" (cool air enters at the bottom of the furnace and warm air exits at the top, or "downflow" (cool air enters at the top and flows out of the furnace side or bottom such as in our sketch at left).

Older warm air heating systems used gravity (warm air rises by convection) to deliver heat to the occupied space without a blower fan. See AGE of HEATERS, BOILERS, FURNACES for an illustration of a gravity furnace.

For a detailed guide to inspecting and maintaining warm air heating systems or furnaces, see FURNACES, HEATING and FURNACE CONTROLS & SWITCHES and FURNACE OPERATION DETAILS

Sketch of a typical warm air heating system (above left) is provided courtesy of Carson Dunlop Associates.

Hot Water "hydronic" Heating Boilers

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.

Modern hydronic heating systems use one or more circulator pumps to move hot water from the boiler through radiators or baseboards that deliver heat to the occupied space. Cooler water returns to the boiler for re-heating.

Older hot water heating systems may have omitted the circulator pump(s), relying simply on gravity (warm water rises in the pipes by convection) to move hot water through the building.

See BOILERS, HEATING and RADIANT HEAT Floor Mistakes to Avoid.

A sketch of a typical hydronic heating system is shown at left. (Hot water radiators or baseboards are not shown in this sketch.)

What is a Hydronic or "hot water" Heating System?

What are we looking at when we're talking about oil-fired hot water heat? It's helpful to form a simple working definition that helps understand the system. An oil-fired forced hot water or "hydronic" heating system is a collection of components which heats a building by heating and then circulating hot water through heat-radiating devices located in the occupied space.

A "heating boiler" is a steel, copper, or cast iron "box" of hot water, connected to a loop of pipe (and radiators or baseboards) which runs around through the living area.

The same physical water stays in the boiler and is circulated by a pump so that heat is delivered to the living area. Burning oil makes hot gases which are used to heat the water before being exhausted outside. Pumps move fluids through the system. Safety controls of various types are installed at various points protect against a number of potential hazards.

How does a "hot water" Heating Boiler Work?

We discuss how heating boilers work in step-by-step detail at BOILER OPERATION DETAILS

What's the difference between a hydronic (hot water) boiler and a steam boiler?

A "steam boiler" delivers heat to the occupied space in the form of steam: the boiler literally "boils" water and sends steam rising up through steam riser pipes (steam, hotter and less dense than air, rises by convection in the piping system) and through steam radiators in the occupied space.

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.

At the steam boiler controls monitor the boiler pressure (less than 0.5 psi in most residential steam boilers), the boiler water level, and the burner operation (gas or oil).

Additional heating controls unique to steam heat include the air vent or "steam vent" found on each radiator (see sketch at left) allows steam to enter and air to escape from the radiator.

For a detailed guide to inspecting, diagnosing, maintaining & repairing steam heat systems see STEAM HEATING SYSTEMS.

Hybrid or Mixed Heating Systems: Water to Air combination systems & Dual Hot Water & Warm Air Heating Systems

Mixed heating systems typically convert hot water to warm air heat by using a heating boiler to produce the hot water and a fan coil unit that blows room air across that hot water as it circulates through the coil.

Mixed heating systems are used both in individual residential homes, often as backup heat or to make use of air ducts and blowers, or they may be used in a large apartment building or office building to permit installation of a large commercial boiler to provide heat to multiple rooms where both heating and cooling will be provided by the same in-room unit.

The fan coil heating unit shown at left is in a New York City apartment. This unit also provides cooling in summer months. This mixed heating system has had its cover removed and the floor below opened during a mold remediation project: the unit's A/C component had leaked condensate into the floor structure. The heating boiler for the apartment building serves all of the units.

Finally, water to air heat is often used to provide concentrated or spot heat to areas of a building that is otherwise heated with hot water radiators or baseboards.

An example of "spot heat", illustrated in our fan convector article cited below is the provision of heat for a garage work area using an overhead water to air fan convector unit.

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

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.

Comparing large vs. small Warm Air Heating Duct Systems: direct air vs. water-to-air heating designs

If the heat in your building is provided by warm air that flows out of ceiling, wall, or floor air supply registers into the occupied space, or if your heating system uses a water-to-air heating system 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, and the air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system.

But in a direct warm air heating system the air being circulated through the building ductwork is itself directly heated by a heat exchanger located inside of the furnace. Building air enters the furnace air intake, flows through the heat exchanger interior, enters the warm air plenum, and flows from the plenum into the building air duct system.

In a water to air heating system, a heating boiler (oil, gas, electric, solar, other) heats water that is then circulated through a heating coil (it looks a lot like a car radiator). Building air is warmed by being blown across the heating coil. The air heating coil can be quite distant from the heating boiler, connected to it by hot water piping.

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.

Make sure your heating inspection is complete: how to Inspect any Heating System using Physical Location of Components

This approach broadens the scope of the heating system inspection and it may aid in heating system defect recognition or problem diagnosis, for example by observing that a heating boiler is located in a small, air-tight room (possible combustion air problems), or that the furnace is quite close to the oil storage tank.

Below we give the basics of heating system inspection using the physical location "map" of components to assure completeness. See full details of heating system inspection procedures provided at HEATING SYSTEM INSPECT DIAGNOSE REPAIR

Heating components: Identify the heating system components in each building area.
Heating fuel & fuel delivery: At the heating boiler or furnace, identify the fuel source and follow fuel supply piping to its source (an oil tank, LP gas tank, or gas meter, or electricity for example).
Heat distribution: At the boiler or furnace, identify the heat distribution method and follow the delivery of heat (warm air or hot water) leaving the furnace or boiler, and returning to it. Failure to consider this whole path logically risks failing to notice potential problems such as return air taken at a furnace itself or the absence of adequate return air.
Heating controls: At the boiler or furnace identify each of the controls and safety devices and observe their condition. Those devices that are intended for normal operation by the home owner are usually also operated and tested by the inspector. Other devices such as temperature/pressure relief valves are not normally operated but are visually inspected for evidence of a problem.

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BOILERS, HEATING
ELECTRIC HEAT
FURNACES, HEATING
GEOTHERMAL HEATING SYSTEMS
HEAT PUMP SYSTEMS
PULSE COMBUSTION HEATERS

RADIANT HEAT
SOLAR ENERGY SYSTEMS
STEAM HEATING SYSTEMS
WATER HEATERS

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 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.)
Links to our list of additional information on heating system inspection, repair, maintenance
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Ask a Question or Search InspectAPedia

How to determine what type of heating equipment or is installed: boilers vs furnaces, hydronic, steam, hot air (or forced warm air), etc.

Each of the following basic types of heating system is illustrated and described in this article:

Types Heating System Fuels: oil, gas, electricity, solar, etc

Types of Heat Distribution Methods in buildings: air, hot water, radiant heating

Heating System Types: Boilers, Steam Boilers, Furnaces, Mixed Systems

Warm Air or hot air heating Furnaces

Hot Water "hydronic" Heating Boilers

What is a Hydronic or "hot water" Heating System?

How does a "hot water" Heating Boiler Work?

What's the difference between a hydronic (hot water) boiler and a steam boiler?

Hybrid or Mixed Heating Systems: Water to Air combination systems & Dual Hot Water & Warm Air Heating Systems

Definition of Water to Air Heat Exchanger Heating Systems

Comparing large vs. small Warm Air Heating Duct Systems: direct air vs. water-to-air heating designs

Definition of Dual Water and Air Heating Systems

Make sure your heating inspection is complete: how to Inspect any Heating System using Physical Location of Components

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

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