Photograph of  a modern oil-fired heating boiler What Kind of Heat do I Have?
Heating System Type Identification

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How to identify the type of heating equipment & system in a building:

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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

Hot water heating boilerIf your heating system is not working properly,


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.

[Click to enlarge any image]

Those systems too are described at this website - see article links listed at the "More Reading" links at the bottom of this article 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:

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

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 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

Mid efficiency gas furnace (C) Carson Dunlop AssociatesWarm 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,



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

Hot Water "hydronic" Heating Boilers

Photograph of  a modern oil-fired heating boiler

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.



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

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

Steam boiler schematic (C) Carson Dunlop AssociatesA "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

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

Fan convector heating unit © D Friedman at 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

Furnace sketchIf 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.

Reader Question: how does a Gas fired boiler- PTAC unit work?

Fan convector hydronic unit in NYC apartment  (C) Daniel Friedman(Oct 23, 2014) Bob said:

We have a gas fired boiler which sends heat through the ptac unit - what is the mechanics of how it works?


For other readers, the definition of PTAC "Packaged Terminal Air Conditioner" for which PTAC is an abbreviation that can be misleading since PTAC systems also are used to provide heat.

PTAC units are individual, self-contained heating or cooling systems or heat pumps typically used in large multi-occupant buildings such as hotels, hospitals, apartments, and condominiums. PTACs may also be installed to bring heating or cooling to an add-on or addition to a single family home, particularly where an A/C or heat pump s needed for the addition for which extending the building's existing heating or cooling system is not feasible.

For Bob's PTAC system we are seeing a combination system: a gas fired hot water (hydronic) heating boiler, probably already installed to heat the primary building areas, includes an additional hot water heating loop or zone whose piping, instead of running through hot water baseboards or radiators, sends hot water through a heat-exchanging coil inside of a small packaged air handler or blower unit.

Such heaters (or air conditioners in similar designs) may be built into a wall, wall-mounted, ceiling-mounted, or may be suspenced as a ceiling-hung fan convector unit.


Reader Question: what if my heating system is not like the ones on this page?

(Nov 11, 2012) HH said:
I have read over your page, yet none of these systems are like the one we have, we have a hot water boiler that stands alone, next to it is a cupboard that house's copper pipes, it has two different setting for summer and winter, in the summer we switch it so it just proved hot water.

In the winter when we put it to provide heat it does great on the heating side but alas we don't get any hot water. It seem that boiler sends all the heat to the heating system and none to keep the water hot. Its been like this 6 years and no one seems to know how to fix it, the house is ten years old and the first two owners sold within two years of purchase.

I would dearly love to figure out why we can't have heating and hot water.


We'd like to description does not give us enough information to identify your heating system try sending us a sketch or some photos using the Contact Us link found at page top or page bottom. We will research and document your system for you at no charge.

(Jan 23, 2013) MechFIX said:
There a several things this could be. It sounds like it is a hot water cogeneration or combination boiler.

There is one boiler, through which your heating water is circulated. This water is then either used to heat the house and/or used to heat your domestic hot water, usually in an on-demand way via a water-to-water heat exchanger (unless you still have a hot water tank with the heater loop running through it). Typically these systems have two circulation pumps on the heater loop, a small one for the domestic hot water heat exchanger and a large one for the house system.

These systems also rely on valves being set properly such that flow is allowed to go where it is required, which is adjusted by season.

And finally, these systems sometimes work as modulating boilers, meaning they run somewhere between full on and off as required to keep a steady heat. So the issues could be that your domestic hot water heat exchanger circulation pump is not working and so this section is getting very little heat when the house is using most of the capacity, the valves are being closed off or not correctly balanced to ensure sufficient flow to the heat exchanger, or the modulated boiler temperature is staying too low to keep your domestic hot water heat output high enough, which may be correctable by throttling down the house heat valve or by electronic control settings

- Thanks MecFix - Ed.


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


Continue reading at DIAGNOSE & FIX HEATING PROBLEMS-BOILER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



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