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Boilers and Furnaces

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Published
July 10, 2009
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What is the difference between a furnace and a boiler?

Furnaces and boilers are both heating appliances, but each heats a home in a different way. Boilers heat water, which then circulates through a piping arrangement in the form of either hot water or steam. The hot water or steam transfers some of its heat to the air in the occupied space. We call hot water and steam systems "hydronic systems." Furnaces heat air in the occupied space directly.Air from the occupied space passes through the furnace, where the air gets warm and then returns to the heated space.

How do I know if I have a furnace or a boiler?

Here are some easy ways to determine if you have a hydronic system (boiler) or a furnace. Look at your heating appliance and then evaluate the following items.
Here are some indications that you have a boiler:
  • You have baseboard heating

  • There are a lot of copper and/or steel pipes connected to the appliance

  • Your heating appliance looks something like this:



Here are some indications that you have a furnace:

There are air ducts connected to the appliance.

  • There is a humidifier mounted on the ductwork.

  • Your home is heated by warm air entering the space through ceiling-, floor- or wall-mounted registers or grilles.

  • You have air filters in the appliance, or in a common area or hallway.

  • Your appliance looks something like this:



What are the main parts of my furnace?

We can break down a furnace, or forced-air heating system, into two main components: the air-distribution system, and the heat source. The air-distribution system includes a blower, a duct system to carry air from the space to the heating appliance, and a duct system to carry the air back to the occupied space once it passes through the appliance. The heat source is the portion ofthe system that actually generates heat. Heat is transferred from the appliance to the air passing through it at the heat exchanger. The heat exchanger is in the path of flowing air. As air from the conditioned space passes through it, it becomes warm.



What types of fuels are used in a warm/hot-air furnace?

Normally, gas and oil. Gas-fired furnaces run on either natural gas or liquefied petroleum. Liquefied petroleum is more commonly known as LP. Liquefied petroleum can be liquefied propane,liquefied butane or a mixture of both propane and butane. Oil-fired systems found in residential applications operate with #2 fuel oil.Number 2 fuel oil is a hydrocarbon, where each molecule is made up of five carbon atoms and twelve hydrogen atoms. This molecule is commonly referred to as pentane, and is in the same family of highly flammable substances as methane, ethane, propane (which you use in your barbeque grill) and butane (common to cigarette lighters).
Furnaces can also use electric strip heaters as their heat source. Furnaces that use electric strip heaters are often very expensive to operate, especially when these heaters are the only source of heat. Sometimes, we use electric heaters as a back-up heat source, in case the main heat source is unavailable. When using electric strip heaters, the furnace blower must start and remain on before the elements can energize. Energizing the heaters without having the blower operating can cause overheating, and might even start afire. There are electrical interlocks on every electric furnace that prevent this.


How can I tell if my furnace is electric, gas or oil-fired?

There are some obvious ways to find out. If you get a monthly bill from the local oil company, your furnace is likely oil fired. If you get a monthly bill from the gas company, your furnace is gas fired. And if you get astronomical electric bills every month, and do not receive bills from the oil and gas companies, you are likely heating your home with electricity.
All kidding aside, you can determine the source of heat by simply inspecting the unit. A typical oil-fired appliance will have an oil burner located at the front that looks like this:


A typical gas-fired furnace will look something like this:


An electric furnace will not have any of the piping that you see in the previous two images. Since the heat source will be electricity, there will be no fuel lines connected to the appliance.